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<title>The design and implementation of Valgrind</title>
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<a name="mc-techdocs">&nbsp;</a>
<h1 align=center>The design and implementation of Valgrind</h1>
<center>
Detailed technical notes for hackers, maintainers and the
overly-curious<br>
These notes pertain to snapshot 20020306<br>
<p>
<a href="mailto:jseward@acm.org">jseward@acm.org</a><br>
<a href="http://developer.kde.org/~sewardj">http://developer.kde.org/~sewardj</a><br>
Copyright &copy; 2000-2003 Julian Seward
<p>
Valgrind is licensed under the GNU General Public License,
version 2<br>
An open-source tool for finding memory-management problems in
x86 GNU/Linux executables.
</center>
<p>
<hr width="100%">
<h2>Introduction</h2>
This document contains a detailed, highly-technical description of the
internals of Valgrind. This is not the user manual; if you are an
end-user of Valgrind, you do not want to read this. Conversely, if
you really are a hacker-type and want to know how it works, I assume
that you have read the user manual thoroughly.
<p>
You may need to read this document several times, and carefully. Some
important things, I only say once.
<h3>History</h3>
Valgrind came into public view in late Feb 2002. However, it has been
under contemplation for a very long time, perhaps seriously for about
five years. Somewhat over two years ago, I started working on the x86
code generator for the Glasgow Haskell Compiler
(http://www.haskell.org/ghc), gaining familiarity with x86 internals
on the way. I then did Cacheprof (http://www.cacheprof.org), gaining
further x86 experience. Some time around Feb 2000 I started
experimenting with a user-space x86 interpreter for x86-Linux. This
worked, but it was clear that a JIT-based scheme would be necessary to
give reasonable performance for Valgrind. Design work for the JITter
started in earnest in Oct 2000, and by early 2001 I had an x86-to-x86
dynamic translator which could run quite large programs. This
translator was in a sense pointless, since it did not do any
instrumentation or checking.
<p>
Most of the rest of 2001 was taken up designing and implementing the
instrumentation scheme. The main difficulty, which consumed a lot
of effort, was to design a scheme which did not generate large numbers
of false uninitialised-value warnings. By late 2001 a satisfactory
scheme had been arrived at, and I started to test it on ever-larger
programs, with an eventual eye to making it work well enough so that
it was helpful to folks debugging the upcoming version 3 of KDE. I've
used KDE since before version 1.0, and wanted to Valgrind to be an
indirect contribution to the KDE 3 development effort. At the start of
Feb 02 the kde-core-devel crew started using it, and gave a huge
amount of helpful feedback and patches in the space of three weeks.
Snapshot 20020306 is the result.
<p>
In the best Unix tradition, or perhaps in the spirit of Fred Brooks'
depressing-but-completely-accurate epitaph "build one to throw away;
you will anyway", much of Valgrind is a second or third rendition of
the initial idea. The instrumentation machinery
(<code>vg_translate.c</code>, <code>vg_memory.c</code>) and core CPU
simulation (<code>vg_to_ucode.c</code>, <code>vg_from_ucode.c</code>)
have had three redesigns and rewrites; the register allocator,
low-level memory manager (<code>vg_malloc2.c</code>) and symbol table
reader (<code>vg_symtab2.c</code>) are on the second rewrite. In a
sense, this document serves to record some of the knowledge gained as
a result.
<h3>Design overview</h3>
Valgrind is compiled into a Linux shared object,
<code>valgrind.so</code>, and also a dummy one,
<code>valgrinq.so</code>, of which more later. The
<code>valgrind</code> shell script adds <code>valgrind.so</code> to
the <code>LD_PRELOAD</code> list of extra libraries to be
loaded with any dynamically linked library. This is a standard trick,
one which I assume the <code>LD_PRELOAD</code> mechanism was developed
to support.
<p>
<code>valgrind.so</code>
is linked with the <code>-z initfirst</code> flag, which requests that
its initialisation code is run before that of any other object in the
executable image. When this happens, valgrind gains control. The
real CPU becomes "trapped" in <code>valgrind.so</code> and the
translations it generates. The synthetic CPU provided by Valgrind
does, however, return from this initialisation function. So the
normal startup actions, orchestrated by the dynamic linker
<code>ld.so</code>, continue as usual, except on the synthetic CPU,
not the real one. Eventually <code>main</code> is run and returns,
and then the finalisation code of the shared objects is run,
presumably in inverse order to which they were initialised. Remember,
this is still all happening on the simulated CPU. Eventually
<code>valgrind.so</code>'s own finalisation code is called. It spots
this event, shuts down the simulated CPU, prints any error summaries
and/or does leak detection, and returns from the initialisation code
on the real CPU. At this point, in effect the real and synthetic CPUs
have merged back into one, Valgrind has lost control of the program,
and the program finally <code>exit()s</code> back to the kernel in the
usual way.
<p>
The normal course of activity, once Valgrind has started up, is as
follows. Valgrind never runs any part of your program (usually
referred to as the "client"), not a single byte of it, directly.
Instead it uses function <code>VG_(translate)</code> to translate
basic blocks (BBs, straight-line sequences of code) into instrumented
translations, and those are run instead. The translations are stored
in the translation cache (TC), <code>vg_tc</code>, with the
translation table (TT), <code>vg_tt</code> supplying the
original-to-translation code address mapping. Auxiliary array
<code>VG_(tt_fast)</code> is used as a direct-map cache for fast
lookups in TT; it usually achieves a hit rate of around 98% and
facilitates an orig-to-trans lookup in 4 x86 insns, which is not bad.
<p>
Function <code>VG_(dispatch)</code> in <code>vg_dispatch.S</code> is
the heart of the JIT dispatcher. Once a translated code address has
been found, it is executed simply by an x86 <code>call</code>
to the translation. At the end of the translation, the next
original code addr is loaded into <code>%eax</code>, and the
translation then does a <code>ret</code>, taking it back to the
dispatch loop, with, interestingly, zero branch mispredictions.
The address requested in <code>%eax</code> is looked up first in
<code>VG_(tt_fast)</code>, and, if not found, by calling C helper
<code>VG_(search_transtab)</code>. If there is still no translation
available, <code>VG_(dispatch)</code> exits back to the top-level
C dispatcher <code>VG_(toploop)</code>, which arranges for
<code>VG_(translate)</code> to make a new translation. All fairly
unsurprising, really. There are various complexities described below.
<p>
The translator, orchestrated by <code>VG_(translate)</code>, is
complicated but entirely self-contained. It is described in great
detail in subsequent sections. Translations are stored in TC, with TT
tracking administrative information. The translations are subject to
an approximate LRU-based management scheme. With the current
settings, the TC can hold at most about 15MB of translations, and LRU
passes prune it to about 13.5MB. Given that the
orig-to-translation expansion ratio is about 13:1 to 14:1, this means
TC holds translations for more or less a megabyte of original code,
which generally comes to about 70000 basic blocks for C++ compiled
with optimisation on. Generating new translations is expensive, so it
is worth having a large TC to minimise the (capacity) miss rate.
<p>
The dispatcher, <code>VG_(dispatch)</code>, receives hints from
the translations which allow it to cheaply spot all control
transfers corresponding to x86 <code>call</code> and <code>ret</code>
instructions. It has to do this in order to spot some special events:
<ul>
<li>Calls to <code>VG_(shutdown)</code>. This is Valgrind's cue to
exit. NOTE: actually this is done a different way; it should be
cleaned up.
<p>
<li>Returns of system call handlers, to the return address
<code>VG_(signalreturn_bogusRA)</code>. The signal simulator
needs to know when a signal handler is returning, so we spot
jumps (returns) to this address.
<p>
<li>Calls to <code>vg_trap_here</code>. All <code>malloc</code>,
<code>free</code>, etc calls that the client program makes are
eventually routed to a call to <code>vg_trap_here</code>,
and Valgrind does its own special thing with these calls.
In effect this provides a trapdoor, by which Valgrind can
intercept certain calls on the simulated CPU, run the call as it
sees fit itself (on the real CPU), and return the result to
the simulated CPU, quite transparently to the client program.
</ul>
Valgrind intercepts the client's <code>malloc</code>,
<code>free</code>, etc,
calls, so that it can store additional information. Each block
<code>malloc</code>'d by the client gives rise to a shadow block
in which Valgrind stores the call stack at the time of the
<code>malloc</code>
call. When the client calls <code>free</code>, Valgrind tries to
find the shadow block corresponding to the address passed to
<code>free</code>, and emits an error message if none can be found.
If it is found, the block is placed on the freed blocks queue
<code>vg_freed_list</code>, it is marked as inaccessible, and
its shadow block now records the call stack at the time of the
<code>free</code> call. Keeping <code>free</code>'d blocks in
this queue allows Valgrind to spot all (presumably invalid) accesses
to them. However, once the volume of blocks in the free queue
exceeds <code>VG_(clo_freelist_vol)</code>, blocks are finally
removed from the queue.
<p>
Keeping track of A and V bits (note: if you don't know what these are,
you haven't read the user guide carefully enough) for memory is done
in <code>vg_memory.c</code>. This implements a sparse array structure
which covers the entire 4G address space in a way which is reasonably
fast and reasonably space efficient. The 4G address space is divided
up into 64K sections, each covering 64Kb of address space. Given a
32-bit address, the top 16 bits are used to select one of the 65536
entries in <code>VG_(primary_map)</code>. The resulting "secondary"
(<code>SecMap</code>) holds A and V bits for the 64k of address space
chunk corresponding to the lower 16 bits of the address.
<h3>Design decisions</h3>
Some design decisions were motivated by the need to make Valgrind
debuggable. Imagine you are writing a CPU simulator. It works fairly
well. However, you run some large program, like Netscape, and after
tens of millions of instructions, it crashes. How can you figure out
where in your simulator the bug is?
<p>
Valgrind's answer is: cheat. Valgrind is designed so that it is
possible to switch back to running the client program on the real
CPU at any point. Using the <code>--stop-after= </code> flag, you can
ask Valgrind to run just some number of basic blocks, and then
run the rest of the way on the real CPU. If you are searching for
a bug in the simulated CPU, you can use this to do a binary search,
which quickly leads you to the specific basic block which is
causing the problem.
<p>
This is all very handy. It does constrain the design in certain
unimportant ways. Firstly, the layout of memory, when viewed from the
client's point of view, must be identical regardless of whether it is
running on the real or simulated CPU. This means that Valgrind can't
do pointer swizzling -- well, no great loss -- and it can't run on
the same stack as the client -- again, no great loss.
Valgrind operates on its own stack, <code>VG_(stack)</code>, which
it switches to at startup, temporarily switching back to the client's
stack when doing system calls for the client.
<p>
Valgrind also receives signals on its own stack,
<code>VG_(sigstack)</code>, but for different gruesome reasons
discussed below.
<p>
This nice clean switch-back-to-the-real-CPU-whenever-you-like story
is muddied by signals. Problem is that signals arrive at arbitrary
times and tend to slightly perturb the basic block count, with the
result that you can get close to the basic block causing a problem but
can't home in on it exactly. My kludgey hack is to define
<code>SIGNAL_SIMULATION</code> to 1 towards the bottom of
<code>vg_syscall_mem.c</code>, so that signal handlers are run on the
real CPU and don't change the BB counts.
<p>
A second hole in the switch-back-to-real-CPU story is that Valgrind's
way of delivering signals to the client is different from that of the
kernel. Specifically, the layout of the signal delivery frame, and
the mechanism used to detect a sighandler returning, are different.
So you can't expect to make the transition inside a sighandler and
still have things working, but in practice that's not much of a
restriction.
<p>
Valgrind's implementation of <code>malloc</code>, <code>free</code>,
etc, (in <code>vg_clientmalloc.c</code>, not the low-level stuff in
<code>vg_malloc2.c</code>) is somewhat complicated by the need to
handle switching back at arbitrary points. It does work tho.
<h3>Correctness</h3>
There's only one of me, and I have a Real Life (tm) as well as hacking
Valgrind [allegedly :-]. That means I don't have time to waste
chasing endless bugs in Valgrind. My emphasis is therefore on doing
everything as simply as possible, with correctness, stability and
robustness being the number one priority, more important than
performance or functionality. As a result:
<ul>
<li>The code is absolutely loaded with assertions, and these are
<b>permanently enabled.</b> I have no plan to remove or disable
them later. Over the past couple of months, as valgrind has
become more widely used, they have shown their worth, pulling
up various bugs which would otherwise have appeared as
hard-to-find segmentation faults.
<p>
I am of the view that it's acceptable to spend 5% of the total
running time of your valgrindified program doing assertion checks
and other internal sanity checks.
<p>
<li>Aside from the assertions, valgrind contains various sets of
internal sanity checks, which get run at varying frequencies
during normal operation. <code>VG_(do_sanity_checks)</code>
runs every 1000 basic blocks, which means 500 to 2000 times/second
for typical machines at present. It checks that Valgrind hasn't
overrun its private stack, and does some simple checks on the
memory permissions maps. Once every 25 calls it does some more
extensive checks on those maps. Etc, etc.
<p>
The following components also have sanity check code, which can
be enabled to aid debugging:
<ul>
<li>The low-level memory-manager
(<code>VG_(mallocSanityCheckArena)</code>). This does a
complete check of all blocks and chains in an arena, which
is very slow. Is not engaged by default.
<p>
<li>The symbol table reader(s): various checks to ensure
uniqueness of mappings; see <code>VG_(read_symbols)</code>
for a start. Is permanently engaged.
<p>
<li>The A and V bit tracking stuff in <code>vg_memory.c</code>.
This can be compiled with cpp symbol
<code>VG_DEBUG_MEMORY</code> defined, which removes all the
fast, optimised cases, and uses simple-but-slow fallbacks
instead. Not engaged by default.
<p>
<li>Ditto <code>VG_DEBUG_LEAKCHECK</code>.
<p>
<li>The JITter parses x86 basic blocks into sequences of
UCode instructions. It then sanity checks each one with
<code>VG_(saneUInstr)</code> and sanity checks the sequence
as a whole with <code>VG_(saneUCodeBlock)</code>. This stuff
is engaged by default, and has caught some way-obscure bugs
in the simulated CPU machinery in its time.
<p>
<li>The system call wrapper does
<code>VG_(first_and_last_secondaries_look_plausible)</code> after
every syscall; this is known to pick up bugs in the syscall
wrappers. Engaged by default.
<p>
<li>The main dispatch loop, in <code>VG_(dispatch)</code>, checks
that translations do not set <code>%ebp</code> to any value
different from <code>VG_EBP_DISPATCH_CHECKED</code> or
<code>&amp; VG_(baseBlock)</code>. In effect this test is free,
and is permanently engaged.
<p>
<li>There are a couple of ifdefed-out consistency checks I
inserted whilst debugging the new register allocater,
<code>vg_do_register_allocation</code>.
</ul>
<p>
<li>I try to avoid techniques, algorithms, mechanisms, etc, for which
I can supply neither a convincing argument that they are correct,
nor sanity-check code which might pick up bugs in my
implementation. I don't always succeed in this, but I try.
Basically the idea is: avoid techniques which are, in practice,
unverifiable, in some sense. When doing anything, always have in
mind: "how can I verify that this is correct?"
</ul>
<p>
Some more specific things are:
<ul>
<li>Valgrind runs in the same namespace as the client, at least from
<code>ld.so</code>'s point of view, and it therefore absolutely
had better not export any symbol with a name which could clash
with that of the client or any of its libraries. Therefore, all
globally visible symbols exported from <code>valgrind.so</code>
are defined using the <code>VG_</code> CPP macro. As you'll see
from <code>vg_constants.h</code>, this appends some arbitrary
prefix to the symbol, in order that it be, we hope, globally
unique. Currently the prefix is <code>vgPlain_</code>. For
convenience there are also <code>VGM_</code>, <code>VGP_</code>
and <code>VGOFF_</code>. All locally defined symbols are declared
<code>static</code> and do not appear in the final shared object.
<p>
To check this, I periodically do
<code>nm valgrind.so | grep " T "</code>,
which shows you all the globally exported text symbols.
They should all have an approved prefix, except for those like
<code>malloc</code>, <code>free</code>, etc, which we deliberately
want to shadow and take precedence over the same names exported
from <code>glibc.so</code>, so that valgrind can intercept those
calls easily. Similarly, <code>nm valgrind.so | grep " D "</code>
allows you to find any rogue data-segment symbol names.
<p>
<li>Valgrind tries, and almost succeeds, in being completely
independent of all other shared objects, in particular of
<code>glibc.so</code>. For example, we have our own low-level
memory manager in <code>vg_malloc2.c</code>, which is a fairly
standard malloc/free scheme augmented with arenas, and
<code>vg_mylibc.c</code> exports reimplementations of various bits
and pieces you'd normally get from the C library.
<p>
Why all the hassle? Because imagine the potential chaos of both
the simulated and real CPUs executing in <code>glibc.so</code>.
It just seems simpler and cleaner to be completely self-contained,
so that only the simulated CPU visits <code>glibc.so</code>. In
practice it's not much hassle anyway. Also, valgrind starts up
before glibc has a chance to initialise itself, and who knows what
difficulties that could lead to. Finally, glibc has definitions
for some types, specifically <code>sigset_t</code>, which conflict
(are different from) the Linux kernel's idea of same. When
Valgrind wants to fiddle around with signal stuff, it wants to
use the kernel's definitions, not glibc's definitions. So it's
simplest just to keep glibc out of the picture entirely.
<p>
To find out which glibc symbols are used by Valgrind, reinstate
the link flags <code>-nostdlib -Wl,-no-undefined</code>. This
causes linking to fail, but will tell you what you depend on.
I have mostly, but not entirely, got rid of the glibc
dependencies; what remains is, IMO, fairly harmless. AFAIK the
current dependencies are: <code>memset</code>,
<code>memcmp</code>, <code>stat</code>, <code>system</code>,
<code>sbrk</code>, <code>setjmp</code> and <code>longjmp</code>.
<p>
<li>Similarly, valgrind should not really import any headers other
than the Linux kernel headers, since it knows of no API other than
the kernel interface to talk to. At the moment this is really not
in a good state, and <code>vg_syscall_mem</code> imports, via
<code>vg_unsafe.h</code>, a significant number of C-library
headers so as to know the sizes of various structs passed across
the kernel boundary. This is of course completely bogus, since
there is no guarantee that the C library's definitions of these
structs matches those of the kernel. I have started to sort this
out using <code>vg_kerneliface.h</code>, into which I had intended
to copy all kernel definitions which valgrind could need, but this
has not gotten very far. At the moment it mostly contains
definitions for <code>sigset_t</code> and <code>struct
sigaction</code>, since the kernel's definition for these really
does clash with glibc's. I plan to use a <code>vki_</code> prefix
on all these types and constants, to denote the fact that they
pertain to <b>V</b>algrind's <b>K</b>ernel <b>I</b>nterface.
<p>
Another advantage of having a <code>vg_kerneliface.h</code> file
is that it makes it simpler to interface to a different kernel.
Once can, for example, easily imagine writing a new
<code>vg_kerneliface.h</code> for FreeBSD, or x86 NetBSD.
</ul>
<h3>Current limitations</h3>
No threads. I think fixing this is close to a research-grade problem.
<p>
No MMX. Fixing this should be relatively easy, using the same giant
trick used for x86 FPU instructions. See below.
<p>
Support for weird (non-POSIX) signal stuff is patchy. Does anybody
care?
<p>
<hr width="100%">
<h2>The instrumenting JITter</h2>
This really is the heart of the matter. We begin with various side
issues.
<h3>Run-time storage, and the use of host registers</h3>
Valgrind translates client (original) basic blocks into instrumented
basic blocks, which live in the translation cache TC, until either the
client finishes or the translations are ejected from TC to make room
for newer ones.
<p>
Since it generates x86 code in memory, Valgrind has complete control
of the use of registers in the translations. Now pay attention. I
shall say this only once, and it is important you understand this. In
what follows I will refer to registers in the host (real) cpu using
their standard names, <code>%eax</code>, <code>%edi</code>, etc. I
refer to registers in the simulated CPU by capitalising them:
<code>%EAX</code>, <code>%EDI</code>, etc. These two sets of
registers usually bear no direct relationship to each other; there is
no fixed mapping between them. This naming scheme is used fairly
consistently in the comments in the sources.
<p>
Host registers, once things are up and running, are used as follows:
<ul>
<li><code>%esp</code>, the real stack pointer, points
somewhere in Valgrind's private stack area,
<code>VG_(stack)</code> or, transiently, into its signal delivery
stack, <code>VG_(sigstack)</code>.
<p>
<li><code>%edi</code> is used as a temporary in code generation; it
is almost always dead, except when used for the <code>Left</code>
value-tag operations.
<p>
<li><code>%eax</code>, <code>%ebx</code>, <code>%ecx</code>,
<code>%edx</code> and <code>%esi</code> are available to
Valgrind's register allocator. They are dead (carry unimportant
values) in between translations, and are live only in
translations. The one exception to this is <code>%eax</code>,
which, as mentioned far above, has a special significance to the
dispatch loop <code>VG_(dispatch)</code>: when a translation
returns to the dispatch loop, <code>%eax</code> is expected to
contain the original-code-address of the next translation to run.
The register allocator is so good at minimising spill code that
using five regs and not having to save/restore <code>%edi</code>
actually gives better code than allocating to <code>%edi</code>
as well, but then having to push/pop it around special uses.
<p>
<li><code>%ebp</code> points permanently at
<code>VG_(baseBlock)</code>. Valgrind's translations are
position-independent, partly because this is convenient, but also
because translations get moved around in TC as part of the LRUing
activity. <b>All</b> static entities which need to be referred to
from generated code, whether data or helper functions, are stored
starting at <code>VG_(baseBlock)</code> and are therefore reached
by indexing from <code>%ebp</code>. There is but one exception,
which is that by placing the value
<code>VG_EBP_DISPATCH_CHECKED</code>
in <code>%ebp</code> just before a return to the dispatcher,
the dispatcher is informed that the next address to run,
in <code>%eax</code>, requires special treatment.
<p>
<li>The real machine's FPU state is pretty much unimportant, for
reasons which will become obvious. Ditto its <code>%eflags</code>
register.
</ul>
<p>
The state of the simulated CPU is stored in memory, in
<code>VG_(baseBlock)</code>, which is a block of 200 words IIRC.
Recall that <code>%ebp</code> points permanently at the start of this
block. Function <code>vg_init_baseBlock</code> decides what the
offsets of various entities in <code>VG_(baseBlock)</code> are to be,
and allocates word offsets for them. The code generator then emits
<code>%ebp</code> relative addresses to get at those things. The
sequence in which entities are allocated has been carefully chosen so
that the 32 most popular entities come first, because this means 8-bit
offsets can be used in the generated code.
<p>
If I was clever, I could make <code>%ebp</code> point 32 words along
<code>VG_(baseBlock)</code>, so that I'd have another 32 words of
short-form offsets available, but that's just complicated, and it's
not important -- the first 32 words take 99% (or whatever) of the
traffic.
<p>
Currently, the sequence of stuff in <code>VG_(baseBlock)</code> is as
follows:
<ul>
<li>9 words, holding the simulated integer registers,
<code>%EAX</code> .. <code>%EDI</code>, and the simulated flags,
<code>%EFLAGS</code>.
<p>
<li>Another 9 words, holding the V bit "shadows" for the above 9 regs.
<p>
<li>The <b>addresses</b> of various helper routines called from
generated code:
<code>VG_(helper_value_check4_fail)</code>,
<code>VG_(helper_value_check0_fail)</code>,
which register V-check failures,
<code>VG_(helperc_STOREV4)</code>,
<code>VG_(helperc_STOREV1)</code>,
<code>VG_(helperc_LOADV4)</code>,
<code>VG_(helperc_LOADV1)</code>,
which do stores and loads of V bits to/from the
sparse array which keeps track of V bits in memory,
and
<code>VGM_(handle_esp_assignment)</code>, which messes with
memory addressibility resulting from changes in <code>%ESP</code>.
<p>
<li>The simulated <code>%EIP</code>.
<p>
<li>24 spill words, for when the register allocator can't make it work
with 5 measly registers.
<p>
<li>Addresses of helpers <code>VG_(helperc_STOREV2)</code>,
<code>VG_(helperc_LOADV2)</code>. These are here because 2-byte
loads and stores are relatively rare, so are placed above the
magic 32-word offset boundary.
<p>
<li>For similar reasons, addresses of helper functions
<code>VGM_(fpu_write_check)</code> and
<code>VGM_(fpu_read_check)</code>, which handle the A/V maps
testing and changes required by FPU writes/reads.
<p>
<li>Some other boring helper addresses:
<code>VG_(helper_value_check2_fail)</code> and
<code>VG_(helper_value_check1_fail)</code>. These are probably
never emitted now, and should be removed.
<p>
<li>The entire state of the simulated FPU, which I believe to be
108 bytes long.
<p>
<li>Finally, the addresses of various other helper functions in
<code>vg_helpers.S</code>, which deal with rare situations which
are tedious or difficult to generate code in-line for.
</ul>
<p>
As a general rule, the simulated machine's state lives permanently in
memory at <code>VG_(baseBlock)</code>. However, the JITter does some
optimisations which allow the simulated integer registers to be
cached in real registers over multiple simulated instructions within
the same basic block. These are always flushed back into memory at
the end of every basic block, so that the in-memory state is
up-to-date between basic blocks. (This flushing is implied by the
statement above that the real machine's allocatable registers are
dead in between simulated blocks).
<h3>Startup, shutdown, and system calls</h3>
Getting into of Valgrind (<code>VG_(startup)</code>, called from
<code>valgrind.so</code>'s initialisation section), really means
copying the real CPU's state into <code>VG_(baseBlock)</code>, and
then installing our own stack pointer, etc, into the real CPU, and
then starting up the JITter. Exiting valgrind involves copying the
simulated state back to the real state.
<p>
Unfortunately, there's a complication at startup time. Problem is
that at the point where we need to take a snapshot of the real CPU's
state, the offsets in <code>VG_(baseBlock)</code> are not set up yet,
because to do so would involve disrupting the real machine's state
significantly. The way round this is to dump the real machine's state
into a temporary, static block of memory,
<code>VG_(m_state_static)</code>. We can then set up the
<code>VG_(baseBlock)</code> offsets at our leisure, and copy into it
from <code>VG_(m_state_static)</code> at some convenient later time.
This copying is done by
<code>VG_(copy_m_state_static_to_baseBlock)</code>.
<p>
On exit, the inverse transformation is (rather unnecessarily) used:
stuff in <code>VG_(baseBlock)</code> is copied to
<code>VG_(m_state_static)</code>, and the assembly stub then copies
from <code>VG_(m_state_static)</code> into the real machine registers.
<p>
Doing system calls on behalf of the client (<code>vg_syscall.S</code>)
is something of a half-way house. We have to make the world look
sufficiently like that which the client would normally have to make
the syscall actually work properly, but we can't afford to lose
control. So the trick is to copy all of the client's state, <b>except
its program counter</b>, into the real CPU, do the system call, and
copy the state back out. Note that the client's state includes its
stack pointer register, so one effect of this partial restoration is
to cause the system call to be run on the client's stack, as it should
be.
<p>
As ever there are complications. We have to save some of our own state
somewhere when restoring the client's state into the CPU, so that we
can keep going sensibly afterwards. In fact the only thing which is
important is our own stack pointer, but for paranoia reasons I save
and restore our own FPU state as well, even though that's probably
pointless.
<p>
The complication on the above complication is, that for horrible
reasons to do with signals, we may have to handle a second client
system call whilst the client is blocked inside some other system
call (unbelievable!). That means there's two sets of places to
dump Valgrind's stack pointer and FPU state across the syscall,
and we decide which to use by consulting
<code>VG_(syscall_depth)</code>, which is in turn maintained by
<code>VG_(wrap_syscall)</code>.
<h3>Introduction to UCode</h3>
UCode lies at the heart of the x86-to-x86 JITter. The basic premise
is that dealing the the x86 instruction set head-on is just too darn
complicated, so we do the traditional compiler-writer's trick and
translate it into a simpler, easier-to-deal-with form.
<p>
In normal operation, translation proceeds through six stages,
coordinated by <code>VG_(translate)</code>:
<ol>
<li>Parsing of an x86 basic block into a sequence of UCode
instructions (<code>VG_(disBB)</code>).
<p>
<li>UCode optimisation (<code>vg_improve</code>), with the aim of
caching simulated registers in real registers over multiple
simulated instructions, and removing redundant simulated
<code>%EFLAGS</code> saving/restoring.
<p>
<li>UCode instrumentation (<code>vg_instrument</code>), which adds
value and address checking code.
<p>
<li>Post-instrumentation cleanup (<code>vg_cleanup</code>), removing
redundant value-check computations.
<p>
<li>Register allocation (<code>vg_do_register_allocation</code>),
which, note, is done on UCode.
<p>
<li>Emission of final instrumented x86 code
(<code>VG_(emit_code)</code>).
</ol>
<p>
Notice how steps 2, 3, 4 and 5 are simple UCode-to-UCode
transformation passes, all on straight-line blocks of UCode (type
<code>UCodeBlock</code>). Steps 2 and 4 are optimisation passes and
can be disabled for debugging purposes, with
<code>--optimise=no</code> and <code>--cleanup=no</code> respectively.
<p>
Valgrind can also run in a no-instrumentation mode, given
<code>--instrument=no</code>. This is useful for debugging the JITter
quickly without having to deal with the complexity of the
instrumentation mechanism too. In this mode, steps 3 and 4 are
omitted.
<p>
These flags combine, so that <code>--instrument=no</code> together with
<code>--optimise=no</code> means only steps 1, 5 and 6 are used.
<code>--single-step=yes</code> causes each x86 instruction to be
treated as a single basic block. The translations are terrible but
this is sometimes instructive.
<p>
The <code>--stop-after=N</code> flag switches back to the real CPU
after <code>N</code> basic blocks. It also re-JITs the final basic
block executed and prints the debugging info resulting, so this
gives you a way to get a quick snapshot of how a basic block looks as
it passes through the six stages mentioned above. If you want to
see full information for every block translated (probably not, but
still ...) find, in <code>VG_(translate)</code>, the lines
<br><code> dis = True;</code>
<br><code> dis = debugging_translation;</code>
<br>
and comment out the second line. This will spew out debugging
junk faster than you can possibly imagine.
<h3>UCode operand tags: type <code>Tag</code></h3>
UCode is, more or less, a simple two-address RISC-like code. In
keeping with the x86 AT&amp;T assembly syntax, generally speaking the
first operand is the source operand, and the second is the destination
operand, which is modified when the uinstr is notionally executed.
<p>
UCode instructions have up to three operand fields, each of which has
a corresponding <code>Tag</code> describing it. Possible values for
the tag are:
<ul>
<li><code>NoValue</code>: indicates that the field is not in use.
<p>
<li><code>Lit16</code>: the field contains a 16-bit literal.
<p>
<li><code>Literal</code>: the field denotes a 32-bit literal, whose
value is stored in the <code>lit32</code> field of the uinstr
itself. Since there is only one <code>lit32</code> for the whole
uinstr, only one operand field may contain this tag.
<p>
<li><code>SpillNo</code>: the field contains a spill slot number, in
the range 0 to 23 inclusive, denoting one of the spill slots
contained inside <code>VG_(baseBlock)</code>. Such tags only
exist after register allocation.
<p>
<li><code>RealReg</code>: the field contains a number in the range 0
to 7 denoting an integer x86 ("real") register on the host. The
number is the Intel encoding for integer registers. Such tags
only exist after register allocation.
<p>
<li><code>ArchReg</code>: the field contains a number in the range 0
to 7 denoting an integer x86 register on the simulated CPU. In
reality this means a reference to one of the first 8 words of
<code>VG_(baseBlock)</code>. Such tags can exist at any point in
the translation process.
<p>
<li>Last, but not least, <code>TempReg</code>. The field contains the
number of one of an infinite set of virtual (integer)
registers. <code>TempReg</code>s are used everywhere throughout
the translation process; you can have as many as you want. The
register allocator maps as many as it can into
<code>RealReg</code>s and turns the rest into
<code>SpillNo</code>s, so <code>TempReg</code>s should not exist
after the register allocation phase.
<p>
<code>TempReg</code>s are always 32 bits long, even if the data
they hold is logically shorter. In that case the upper unused
bits are required, and, I think, generally assumed, to be zero.
<code>TempReg</code>s holding V bits for quantities shorter than
32 bits are expected to have ones in the unused places, since a
one denotes "undefined".
</ul>
<h3>UCode instructions: type <code>UInstr</code></h3>
<p>
UCode was carefully designed to make it possible to do register
allocation on UCode and then translate the result into x86 code
without needing any extra registers ... well, that was the original
plan, anyway. Things have gotten a little more complicated since
then. In what follows, UCode instructions are referred to as uinstrs,
to distinguish them from x86 instructions. Uinstrs of course have
uopcodes which are (naturally) different from x86 opcodes.
<p>
A uinstr (type <code>UInstr</code>) contains
various fields, not all of which are used by any one uopcode:
<ul>
<li>Three 16-bit operand fields, <code>val1</code>, <code>val2</code>
and <code>val3</code>.
<p>
<li>Three tag fields, <code>tag1</code>, <code>tag2</code>
and <code>tag3</code>. Each of these has a value of type
<code>Tag</code>,
and they describe what the <code>val1</code>, <code>val2</code>
and <code>val3</code> fields contain.
<p>
<li>A 32-bit literal field.
<p>
<li>Two <code>FlagSet</code>s, specifying which x86 condition codes are
read and written by the uinstr.
<p>
<li>An opcode byte, containing a value of type <code>Opcode</code>.
<p>
<li>A size field, indicating the data transfer size (1/2/4/8/10) in
cases where this makes sense, or zero otherwise.
<p>
<li>A condition-code field, which, for jumps, holds a
value of type <code>Condcode</code>, indicating the condition
which applies. The encoding is as it is in the x86 insn stream,
except we add a 17th value <code>CondAlways</code> to indicate
an unconditional transfer.
<p>
<li>Various 1-bit flags, indicating whether this insn pertains to an
x86 CALL or RET instruction, whether a widening is signed or not,
etc.
</ul>
<p>
UOpcodes (type <code>Opcode</code>) are divided into two groups: those
necessary merely to express the functionality of the x86 code, and
extra uopcodes needed to express the instrumentation. The former
group contains:
<ul>
<li><code>GET</code> and <code>PUT</code>, which move values from the
simulated CPU's integer registers (<code>ArchReg</code>s) into
<code>TempReg</code>s, and back. <code>GETF</code> and
<code>PUTF</code> do the corresponding thing for the simulated
<code>%EFLAGS</code>. There are no corresponding insns for the
FPU register stack, since we don't explicitly simulate its
registers.
<p>
<li><code>LOAD</code> and <code>STORE</code>, which, in RISC-like
fashion, are the only uinstrs able to interact with memory.
<p>
<li><code>MOV</code> and <code>CMOV</code> allow unconditional and
conditional moves of values between <code>TempReg</code>s.
<p>
<li>ALU operations. Again in RISC-like fashion, these only operate on
<code>TempReg</code>s (before reg-alloc) or <code>RealReg</code>s
(after reg-alloc). These are: <code>ADD</code>, <code>ADC</code>,
<code>AND</code>, <code>OR</code>, <code>XOR</code>,
<code>SUB</code>, <code>SBB</code>, <code>SHL</code>,
<code>SHR</code>, <code>SAR</code>, <code>ROL</code>,
<code>ROR</code>, <code>RCL</code>, <code>RCR</code>,
<code>NOT</code>, <code>NEG</code>, <code>INC</code>,
<code>DEC</code>, <code>BSWAP</code>, <code>CC2VAL</code> and
<code>WIDEN</code>. <code>WIDEN</code> does signed or unsigned
value widening. <code>CC2VAL</code> is used to convert condition
codes into a value, zero or one. The rest are obvious.
<p>
To allow for more efficient code generation, we bend slightly the
restriction at the start of the previous para: for
<code>ADD</code>, <code>ADC</code>, <code>XOR</code>,
<code>SUB</code> and <code>SBB</code>, we allow the first (source)
operand to also be an <code>ArchReg</code>, that is, one of the
simulated machine's registers. Also, many of these ALU ops allow
the source operand to be a literal. See
<code>VG_(saneUInstr)</code> for the final word on the allowable
forms of uinstrs.
<p>
<li><code>LEA1</code> and <code>LEA2</code> are not strictly
necessary, but allow faciliate better translations. They
record the fancy x86 addressing modes in a direct way, which
allows those amodes to be emitted back into the final
instruction stream more or less verbatim.
<p>
<li><code>CALLM</code> calls a machine-code helper, one of the methods
whose address is stored at some <code>VG_(baseBlock)</code>
offset. <code>PUSH</code> and <code>POP</code> move values
to/from <code>TempReg</code> to the real (Valgrind's) stack, and
<code>CLEAR</code> removes values from the stack.
<code>CALLM_S</code> and <code>CALLM_E</code> delimit the
boundaries of call setups and clearings, for the benefit of the
instrumentation passes. Getting this right is critical, and so
<code>VG_(saneUCodeBlock)</code> makes various checks on the use
of these uopcodes.
<p>
It is important to understand that these uopcodes have nothing to
do with the x86 <code>call</code>, <code>return,</code>
<code>push</code> or <code>pop</code> instructions, and are not
used to implement them. Those guys turn into combinations of
<code>GET</code>, <code>PUT</code>, <code>LOAD</code>,
<code>STORE</code>, <code>ADD</code>, <code>SUB</code>, and
<code>JMP</code>. What these uopcodes support is calling of
helper functions such as <code>VG_(helper_imul_32_64)</code>,
which do stuff which is too difficult or tedious to emit inline.
<p>
<li><code>FPU</code>, <code>FPU_R</code> and <code>FPU_W</code>.
Valgrind doesn't attempt to simulate the internal state of the
FPU at all. Consequently it only needs to be able to distinguish
FPU ops which read and write memory from those that don't, and
for those which do, it needs to know the effective address and
data transfer size. This is made easier because the x86 FP
instruction encoding is very regular, basically consisting of
16 bits for a non-memory FPU insn and 11 (IIRC) bits + an address mode
for a memory FPU insn. So our <code>FPU</code> uinstr carries
the 16 bits in its <code>val1</code> field. And
<code>FPU_R</code> and <code>FPU_W</code> carry 11 bits in that
field, together with the identity of a <code>TempReg</code> or
(later) <code>RealReg</code> which contains the address.
<p>
<li><code>JIFZ</code> is unique, in that it allows a control-flow
transfer which is not deemed to end a basic block. It causes a
jump to a literal (original) address if the specified argument
is zero.
<p>
<li>Finally, <code>INCEIP</code> advances the simulated
<code>%EIP</code> by the specified literal amount. This supports
lazy <code>%EIP</code> updating, as described below.
</ul>
<p>
Stages 1 and 2 of the 6-stage translation process mentioned above
deal purely with these uopcodes, and no others. They are
sufficient to express pretty much all the x86 32-bit protected-mode
instruction set, at
least everything understood by a pre-MMX original Pentium (P54C).
<p>
Stages 3, 4, 5 and 6 also deal with the following extra
"instrumentation" uopcodes. They are used to express all the
definedness-tracking and -checking machinery which valgrind does. In
later sections we show how to create checking code for each of the
uopcodes above. Note that these instrumentation uopcodes, although
some appearing complicated, have been carefully chosen so that
efficient x86 code can be generated for them. GNU superopt v2.5 did a
great job helping out here. Anyways, the uopcodes are as follows:
<ul>
<li><code>GETV</code> and <code>PUTV</code> are analogues to
<code>GET</code> and <code>PUT</code> above. They are identical
except that they move the V bits for the specified values back and
forth to <code>TempRegs</code>, rather than moving the values
themselves.
<p>
<li>Similarly, <code>LOADV</code> and <code>STOREV</code> read and
write V bits from the synthesised shadow memory that Valgrind
maintains. In fact they do more than that, since they also do
address-validity checks, and emit complaints if the read/written
addresses are unaddressible.
<p>
<li><code>TESTV</code>, whose parameters are a <code>TempReg</code>
and a size, tests the V bits in the <code>TempReg</code>, at the
specified operation size (0/1/2/4 byte) and emits an error if any
of them indicate undefinedness. This is the only uopcode capable
of doing such tests.
<p>
<li><code>SETV</code>, whose parameters are also <code>TempReg</code>
and a size, makes the V bits in the <code>TempReg</code> indicated
definedness, at the specified operation size. This is usually
used to generate the correct V bits for a literal value, which is
of course fully defined.
<p>
<li><code>GETVF</code> and <code>PUTVF</code> are analogues to
<code>GETF</code> and <code>PUTF</code>. They move the single V
bit used to model definedness of <code>%EFLAGS</code> between its
home in <code>VG_(baseBlock)</code> and the specified
<code>TempReg</code>.
<p>
<li><code>TAG1</code> denotes one of a family of unary operations on
<code>TempReg</code>s containing V bits. Similarly,
<code>TAG2</code> denotes one in a family of binary operations on
V bits.
</ul>
<p>
These 10 uopcodes are sufficient to express Valgrind's entire
definedness-checking semantics. In fact most of the interesting magic
is done by the <code>TAG1</code> and <code>TAG2</code>
suboperations.
<p>
First, however, I need to explain about V-vector operation sizes.
There are 4 sizes: 1, 2 and 4, which operate on groups of 8, 16 and 32
V bits at a time, supporting the usual 1, 2 and 4 byte x86 operations.
However there is also the mysterious size 0, which really means a
single V bit. Single V bits are used in various circumstances; in
particular, the definedness of <code>%EFLAGS</code> is modelled with a
single V bit. Now might be a good time to also point out that for
V bits, 1 means "undefined" and 0 means "defined". Similarly, for A
bits, 1 means "invalid address" and 0 means "valid address". This
seems counterintuitive (and so it is), but testing against zero on
x86s saves instructions compared to testing against all 1s, because
many ALU operations set the Z flag for free, so to speak.
<p>
With that in mind, the tag ops are:
<ul>
<li><b>(UNARY) Pessimising casts</b>: <code>VgT_PCast40</code>,
<code>VgT_PCast20</code>, <code>VgT_PCast10</code>,
<code>VgT_PCast01</code>, <code>VgT_PCast02</code> and
<code>VgT_PCast04</code>. A "pessimising cast" takes a V-bit
vector at one size, and creates a new one at another size,
pessimised in the sense that if any of the bits in the source
vector indicate undefinedness, then all the bits in the result
indicate undefinedness. In this case the casts are all to or from
a single V bit, so for example <code>VgT_PCast40</code> is a
pessimising cast from 32 bits to 1, whereas
<code>VgT_PCast04</code> simply copies the single source V bit
into all 32 bit positions in the result. Surprisingly, these ops
can all be implemented very efficiently.
<p>
There are also the pessimising casts <code>VgT_PCast14</code>,
from 8 bits to 32, <code>VgT_PCast12</code>, from 8 bits to 16,
and <code>VgT_PCast11</code>, from 8 bits to 8. This last one
seems nonsensical, but in fact it isn't a no-op because, as
mentioned above, any undefined (1) bits in the source infect the
entire result.
<p>
<li><b>(UNARY) Propagating undefinedness upwards in a word</b>:
<code>VgT_Left4</code>, <code>VgT_Left2</code> and
<code>VgT_Left1</code>. These are used to simulate the worst-case
effects of carry propagation in adds and subtracts. They return a
V vector identical to the original, except that if the original
contained any undefined bits, then it and all bits above it are
marked as undefined too. Hence the Left bit in the names.
<p>
<li><b>(UNARY) Signed and unsigned value widening</b>:
<code>VgT_SWiden14</code>, <code>VgT_SWiden24</code>,
<code>VgT_SWiden12</code>, <code>VgT_ZWiden14</code>,
<code>VgT_ZWiden24</code> and <code>VgT_ZWiden12</code>. These
mimic the definedness effects of standard signed and unsigned
integer widening. Unsigned widening creates zero bits in the new
positions, so <code>VgT_ZWiden*</code> accordingly park mark
those parts of their argument as defined. Signed widening copies
the sign bit into the new positions, so <code>VgT_SWiden*</code>
copies the definedness of the sign bit into the new positions.
Because 1 means undefined and 0 means defined, these operations
can (fascinatingly) be done by the same operations which they
mimic. Go figure.
<p>
<li><b>(BINARY) Undefined-if-either-Undefined,
Defined-if-either-Defined</b>: <code>VgT_UifU4</code>,
<code>VgT_UifU2</code>, <code>VgT_UifU1</code>,
<code>VgT_UifU0</code>, <code>VgT_DifD4</code>,
<code>VgT_DifD2</code>, <code>VgT_DifD1</code>. These do simple
bitwise operations on pairs of V-bit vectors, with
<code>UifU</code> giving undefined if either arg bit is
undefined, and <code>DifD</code> giving defined if either arg bit
is defined. Abstract interpretation junkies, if any make it this
far, may like to think of them as meets and joins (or is it joins
and meets) in the definedness lattices.
<p>
<li><b>(BINARY; one value, one V bits) Generate argument improvement
terms for AND and OR</b>: <code>VgT_ImproveAND4_TQ</code>,
<code>VgT_ImproveAND2_TQ</code>, <code>VgT_ImproveAND1_TQ</code>,
<code>VgT_ImproveOR4_TQ</code>, <code>VgT_ImproveOR2_TQ</code>,
<code>VgT_ImproveOR1_TQ</code>. These help out with AND and OR
operations. AND and OR have the inconvenient property that the
definedness of the result depends on the actual values of the
arguments as well as their definedness. At the bit level:
<br><code>1 AND undefined = undefined</code>, but
<br><code>0 AND undefined = 0</code>, and similarly
<br><code>0 OR undefined = undefined</code>, but
<br><code>1 OR undefined = 1</code>.
<br>
<p>
It turns out that gcc (quite legitimately) generates code which
relies on this fact, so we have to model it properly in order to
avoid flooding users with spurious value errors. The ultimate
definedness result of AND and OR is calculated using
<code>UifU</code> on the definedness of the arguments, but we
also <code>DifD</code> in some "improvement" terms which
take into account the above phenomena.
<p>
<code>ImproveAND</code> takes as its first argument the actual
value of an argument to AND (the T) and the definedness of that
argument (the Q), and returns a V-bit vector which is defined (0)
for bits which have value 0 and are defined; this, when
<code>DifD</code> into the final result causes those bits to be
defined even if the corresponding bit in the other argument is undefined.
<p>
The <code>ImproveOR</code> ops do the dual thing for OR
arguments. Note that XOR does not have this property that one
argument can make the other irrelevant, so there is no need for
such complexity for XOR.
</ul>
<p>
That's all the tag ops. If you stare at this long enough, and then
run Valgrind and stare at the pre- and post-instrumented ucode, it
should be fairly obvious how the instrumentation machinery hangs
together.
<p>
One point, if you do this: in order to make it easy to differentiate
<code>TempReg</code>s carrying values from <code>TempReg</code>s
carrying V bit vectors, Valgrind prints the former as (for example)
<code>t28</code> and the latter as <code>q28</code>; the fact that
they carry the same number serves to indicate their relationship.
This is purely for the convenience of the human reader; the register
allocator and code generator don't regard them as different.
<h3>Translation into UCode</h3>
<code>VG_(disBB)</code> allocates a new <code>UCodeBlock</code> and
then uses <code>disInstr</code> to translate x86 instructions one at a
time into UCode, dumping the result in the <code>UCodeBlock</code>.
This goes on until a control-flow transfer instruction is encountered.
<p>
Despite the large size of <code>vg_to_ucode.c</code>, this translation
is really very simple. Each x86 instruction is translated entirely
independently of its neighbours, merrily allocating new
<code>TempReg</code>s as it goes. The idea is to have a simple
translator -- in reality, no more than a macro-expander -- and the --
resulting bad UCode translation is cleaned up by the UCode
optimisation phase which follows. To give you an idea of some x86
instructions and their translations (this is a complete basic block,
as Valgrind sees it):
<pre>
0x40435A50: incl %edx
0: GETL %EDX, t0
1: INCL t0 (-wOSZAP)
2: PUTL t0, %EDX
0x40435A51: movsbl (%edx),%eax
3: GETL %EDX, t2
4: LDB (t2), t2
5: WIDENL_Bs t2
6: PUTL t2, %EAX
0x40435A54: testb $0x20, 1(%ecx,%eax,2)
7: GETL %EAX, t6
8: GETL %ECX, t8
9: LEA2L 1(t8,t6,2), t4
10: LDB (t4), t10
11: MOVB $0x20, t12
12: ANDB t12, t10 (-wOSZACP)
13: INCEIPo $9
0x40435A59: jnz-8 0x40435A50
14: Jnzo $0x40435A50 (-rOSZACP)
15: JMPo $0x40435A5B
</pre>
<p>
Notice how the block always ends with an unconditional jump to the
next block. This is a bit unnecessary, but makes many things simpler.
<p>
Most x86 instructions turn into sequences of <code>GET</code>,
<code>PUT</code>, <code>LEA1</code>, <code>LEA2</code>,
<code>LOAD</code> and <code>STORE</code>. Some complicated ones
however rely on calling helper bits of code in
<code>vg_helpers.S</code>. The ucode instructions <code>PUSH</code>,
<code>POP</code>, <code>CALL</code>, <code>CALLM_S</code> and
<code>CALLM_E</code> support this. The calling convention is somewhat
ad-hoc and is not the C calling convention. The helper routines must
save all integer registers, and the flags, that they use. Args are
passed on the stack underneath the return address, as usual, and if
result(s) are to be returned, it (they) are either placed in dummy arg
slots created by the ucode <code>PUSH</code> sequence, or just
overwrite the incoming args.
<p>
In order that the instrumentation mechanism can handle calls to these
helpers, <code>VG_(saneUCodeBlock)</code> enforces the following
restrictions on calls to helpers:
<ul>
<li>Each <code>CALL</code> uinstr must be bracketed by a preceding
<code>CALLM_S</code> marker (dummy uinstr) and a trailing
<code>CALLM_E</code> marker. These markers are used by the
instrumentation mechanism later to establish the boundaries of the
<code>PUSH</code>, <code>POP</code> and <code>CLEAR</code>
sequences for the call.
<p>
<li><code>PUSH</code>, <code>POP</code> and <code>CLEAR</code>
may only appear inside sections bracketed by <code>CALLM_S</code>
and <code>CALLM_E</code>, and nowhere else.
<p>
<li>In any such bracketed section, no two <code>PUSH</code> insns may
push the same <code>TempReg</code>. Dually, no two two
<code>POP</code>s may pop the same <code>TempReg</code>.
<p>
<li>Finally, although this is not checked, args should be removed from
the stack with <code>CLEAR</code>, rather than <code>POP</code>s
into a <code>TempReg</code> which is not subsequently used. This
is because the instrumentation mechanism assumes that all values
<code>POP</code>ped from the stack are actually used.
</ul>
Some of the translations may appear to have redundant
<code>TempReg</code>-to-<code>TempReg</code> moves. This helps the
next phase, UCode optimisation, to generate better code.
<h3>UCode optimisation</h3>
UCode is then subjected to an improvement pass
(<code>vg_improve()</code>), which blurs the boundaries between the
translations of the original x86 instructions. It's pretty
straightforward. Three transformations are done:
<ul>
<li>Redundant <code>GET</code> elimination. Actually, more general
than that -- eliminates redundant fetches of ArchRegs. In our
running example, uinstr 3 <code>GET</code>s <code>%EDX</code> into
<code>t2</code> despite the fact that, by looking at the previous
uinstr, it is already in <code>t0</code>. The <code>GET</code> is
therefore removed, and <code>t2</code> renamed to <code>t0</code>.
Assuming <code>t0</code> is allocated to a host register, it means
the simulated <code>%EDX</code> will exist in a host CPU register
for more than one simulated x86 instruction, which seems to me to
be a highly desirable property.
<p>
There is some mucking around to do with subregisters;
<code>%AL</code> vs <code>%AH</code> <code>%AX</code> vs
<code>%EAX</code> etc. I can't remember how it works, but in
general we are very conservative, and these tend to invalidate the
caching.
<p>
<li>Redundant <code>PUT</code> elimination. This annuls
<code>PUT</code>s of values back to simulated CPU registers if a
later <code>PUT</code> would overwrite the earlier
<code>PUT</code> value, and there is no intervening reads of the
simulated register (<code>ArchReg</code>).
<p>
As before, we are paranoid when faced with subregister references.
Also, <code>PUT</code>s of <code>%ESP</code> are never annulled,
because it is vital the instrumenter always has an up-to-date
<code>%ESP</code> value available, <code>%ESP</code> changes
affect addressibility of the memory around the simulated stack
pointer.
<p>
The implication of the above paragraph is that the simulated
machine's registers are only lazily updated once the above two
optimisation phases have run, with the exception of
<code>%ESP</code>. <code>TempReg</code>s go dead at the end of
every basic block, from which is is inferrable that any
<code>TempReg</code> caching a simulated CPU reg is flushed (back
into the relevant <code>VG_(baseBlock)</code> slot) at the end of
every basic block. The further implication is that the simulated
registers are only up-to-date at in between basic blocks, and not
at arbitrary points inside basic blocks. And the consequence of
that is that we can only deliver signals to the client in between
basic blocks. None of this seems any problem in practice.
<p>
<li>Finally there is a simple def-use thing for condition codes. If
an earlier uinstr writes the condition codes, and the next uinsn
along which actually cares about the condition codes writes the
same or larger set of them, but does not read any, the earlier
uinsn is marked as not writing any condition codes. This saves
a lot of redundant cond-code saving and restoring.
</ul>
The effect of these transformations on our short block is rather
unexciting, and shown below. On longer basic blocks they can
dramatically improve code quality.
<pre>
at 3: delete GET, rename t2 to t0 in (4 .. 6)
at 7: delete GET, rename t6 to t0 in (8 .. 9)
at 1: annul flag write OSZAP due to later OSZACP
Improved code:
0: GETL %EDX, t0
1: INCL t0
2: PUTL t0, %EDX
4: LDB (t0), t0
5: WIDENL_Bs t0
6: PUTL t0, %EAX
8: GETL %ECX, t8
9: LEA2L 1(t8,t0,2), t4
10: LDB (t4), t10
11: MOVB $0x20, t12
12: ANDB t12, t10 (-wOSZACP)
13: INCEIPo $9
14: Jnzo $0x40435A50 (-rOSZACP)
15: JMPo $0x40435A5B
</pre>
<h3>UCode instrumentation</h3>
Once you understand the meaning of the instrumentation uinstrs,
discussed in detail above, the instrumentation scheme is fairly
straightforward. Each uinstr is instrumented in isolation, and the
instrumentation uinstrs are placed before the original uinstr.
Our running example continues below. I have placed a blank line
after every original ucode, to make it easier to see which
instrumentation uinstrs correspond to which originals.
<p>
As mentioned somewhere above, <code>TempReg</code>s carrying values
have names like <code>t28</code>, and each one has a shadow carrying
its V bits, with names like <code>q28</code>. This pairing aids in
reading instrumented ucode.
<p>
One decision about all this is where to have "observation points",
that is, where to check that V bits are valid. I use a minimalistic
scheme, only checking where a failure of validity could cause the
original program to (seg)fault. So the use of values as memory
addresses causes a check, as do conditional jumps (these cause a check
on the definedness of the condition codes). And arguments
<code>PUSH</code>ed for helper calls are checked, hence the weird
restrictions on help call preambles described above.
<p>
Another decision is that once a value is tested, it is thereafter
regarded as defined, so that we do not emit multiple undefined-value
errors for the same undefined value. That means that
<code>TESTV</code> uinstrs are always followed by <code>SETV</code>
on the same (shadow) <code>TempReg</code>s. Most of these
<code>SETV</code>s are redundant and are removed by the
post-instrumentation cleanup phase.
<p>
The instrumentation for calling helper functions deserves further
comment. The definedness of results from a helper is modelled using
just one V bit. So, in short, we do pessimising casts of the
definedness of all the args, down to a single bit, and then
<code>UifU</code> these bits together. So this single V bit will say
"undefined" if any part of any arg is undefined. This V bit is then
pessimally cast back up to the result(s) sizes, as needed. If, by
seeing that all the args are got rid of with <code>CLEAR</code> and
none with <code>POP</code>, Valgrind sees that the result of the call
is not actually used, it immediately examines the result V bit with a
<code>TESTV</code> -- <code>SETV</code> pair. If it did not do this,
there would be no observation point to detect that the some of the
args to the helper were undefined. Of course, if the helper's results
are indeed used, we don't do this, since the result usage will
presumably cause the result definedness to be checked at some suitable
future point.
<p>
In general Valgrind tries to track definedness on a bit-for-bit basis,
but as the above para shows, for calls to helpers we throw in the
towel and approximate down to a single bit. This is because it's too
complex and difficult to track bit-level definedness through complex
ops such as integer multiply and divide, and in any case there is no
reasonable code fragments which attempt to (eg) multiply two
partially-defined values and end up with something meaningful, so
there seems little point in modelling multiplies, divides, etc, in
that level of detail.
<p>
Integer loads and stores are instrumented with firstly a test of the
definedness of the address, followed by a <code>LOADV</code> or
<code>STOREV</code> respectively. These turn into calls to
(for example) <code>VG_(helperc_LOADV4)</code>. These helpers do two
things: they perform an address-valid check, and they load or store V
bits from/to the relevant address in the (simulated V-bit) memory.
<p>
FPU loads and stores are different. As above the definedness of the
address is first tested. However, the helper routine for FPU loads
(<code>VGM_(fpu_read_check)</code>) emits an error if either the
address is invalid or the referenced area contains undefined values.
It has to do this because we do not simulate the FPU at all, and so
cannot track definedness of values loaded into it from memory, so we
have to check them as soon as they are loaded into the FPU, ie, at
this point. We notionally assume that everything in the FPU is
defined.
<p>
It follows therefore that FPU writes first check the definedness of
the address, then the validity of the address, and finally mark the
written bytes as well-defined.
<p>
If anyone is inspired to extend Valgrind to MMX/SSE insns, I suggest
you use the same trick. It works provided that the FPU/MMX unit is
not used to merely as a conduit to copy partially undefined data from
one place in memory to another. Unfortunately the integer CPU is used
like that (when copying C structs with holes, for example) and this is
the cause of much of the elaborateness of the instrumentation here
described.
<p>
<code>vg_instrument()</code> in <code>vg_translate.c</code> actually
does the instrumentation. There are comments explaining how each
uinstr is handled, so we do not repeat that here. As explained
already, it is bit-accurate, except for calls to helper functions.
Unfortunately the x86 insns <code>bt/bts/btc/btr</code> are done by
helper fns, so bit-level accuracy is lost there. This should be fixed
by doing them inline; it will probably require adding a couple new
uinstrs. Also, left and right rotates through the carry flag (x86
<code>rcl</code> and <code>rcr</code>) are approximated via a single
V bit; so far this has not caused anyone to complain. The
non-carry rotates, <code>rol</code> and <code>ror</code>, are much
more common and are done exactly. Re-visiting the instrumentation for
AND and OR, they seem rather verbose, and I wonder if it could be done
more concisely now.
<p>
The lowercase <code>o</code> on many of the uopcodes in the running
example indicates that the size field is zero, usually meaning a
single-bit operation.
<p>
Anyroads, the post-instrumented version of our running example looks
like this:
<pre>
Instrumented code:
0: GETVL %EDX, q0
1: GETL %EDX, t0
2: TAG1o q0 = Left4 ( q0 )
3: INCL t0
4: PUTVL q0, %EDX
5: PUTL t0, %EDX
6: TESTVL q0
7: SETVL q0
8: LOADVB (t0), q0
9: LDB (t0), t0
10: TAG1o q0 = SWiden14 ( q0 )
11: WIDENL_Bs t0
12: PUTVL q0, %EAX
13: PUTL t0, %EAX
14: GETVL %ECX, q8
15: GETL %ECX, t8
16: MOVL q0, q4
17: SHLL $0x1, q4
18: TAG2o q4 = UifU4 ( q8, q4 )
19: TAG1o q4 = Left4 ( q4 )
20: LEA2L 1(t8,t0,2), t4
21: TESTVL q4
22: SETVL q4
23: LOADVB (t4), q10
24: LDB (t4), t10
25: SETVB q12
26: MOVB $0x20, t12
27: MOVL q10, q14
28: TAG2o q14 = ImproveAND1_TQ ( t10, q14 )
29: TAG2o q10 = UifU1 ( q12, q10 )
30: TAG2o q10 = DifD1 ( q14, q10 )
31: MOVL q12, q14
32: TAG2o q14 = ImproveAND1_TQ ( t12, q14 )
33: TAG2o q10 = DifD1 ( q14, q10 )
34: MOVL q10, q16
35: TAG1o q16 = PCast10 ( q16 )
36: PUTVFo q16
37: ANDB t12, t10 (-wOSZACP)
38: INCEIPo $9
39: GETVFo q18
40: TESTVo q18
41: SETVo q18
42: Jnzo $0x40435A50 (-rOSZACP)
43: JMPo $0x40435A5B
</pre>
<h3>UCode post-instrumentation cleanup</h3>
<p>
This pass, coordinated by <code>vg_cleanup()</code>, removes redundant
definedness computation created by the simplistic instrumentation
pass. It consists of two passes,
<code>vg_propagate_definedness()</code> followed by
<code>vg_delete_redundant_SETVs</code>.
<p>
<code>vg_propagate_definedness()</code> is a simple
constant-propagation and constant-folding pass. It tries to determine
which <code>TempReg</code>s containing V bits will always indicate
"fully defined", and it propagates this information as far as it can,
and folds out as many operations as possible. For example, the
instrumentation for an ADD of a literal to a variable quantity will be
reduced down so that the definedness of the result is simply the
definedness of the variable quantity, since the literal is by
definition fully defined.
<p>
<code>vg_delete_redundant_SETVs</code> removes <code>SETV</code>s on
shadow <code>TempReg</code>s for which the next action is a write.
I don't think there's anything else worth saying about this; it is
simple. Read the sources for details.
<p>
So the cleaned-up running example looks like this. As above, I have
inserted line breaks after every original (non-instrumentation) uinstr
to aid readability. As with straightforward ucode optimisation, the
results in this block are undramatic because it is so short; longer
blocks benefit more because they have more redundancy which gets
eliminated.
<pre>
at 29: delete UifU1 due to defd arg1
at 32: change ImproveAND1_TQ to MOV due to defd arg2
at 41: delete SETV
at 31: delete MOV
at 25: delete SETV
at 22: delete SETV
at 7: delete SETV
0: GETVL %EDX, q0
1: GETL %EDX, t0
2: TAG1o q0 = Left4 ( q0 )
3: INCL t0
4: PUTVL q0, %EDX
5: PUTL t0, %EDX
6: TESTVL q0
8: LOADVB (t0), q0
9: LDB (t0), t0
10: TAG1o q0 = SWiden14 ( q0 )
11: WIDENL_Bs t0
12: PUTVL q0, %EAX
13: PUTL t0, %EAX
14: GETVL %ECX, q8
15: GETL %ECX, t8
16: MOVL q0, q4
17: SHLL $0x1, q4
18: TAG2o q4 = UifU4 ( q8, q4 )
19: TAG1o q4 = Left4 ( q4 )
20: LEA2L 1(t8,t0,2), t4
21: TESTVL q4
23: LOADVB (t4), q10
24: LDB (t4), t10
26: MOVB $0x20, t12
27: MOVL q10, q14
28: TAG2o q14 = ImproveAND1_TQ ( t10, q14 )
30: TAG2o q10 = DifD1 ( q14, q10 )
32: MOVL t12, q14
33: TAG2o q10 = DifD1 ( q14, q10 )
34: MOVL q10, q16
35: TAG1o q16 = PCast10 ( q16 )
36: PUTVFo q16
37: ANDB t12, t10 (-wOSZACP)
38: INCEIPo $9
39: GETVFo q18
40: TESTVo q18
42: Jnzo $0x40435A50 (-rOSZACP)
43: JMPo $0x40435A5B
</pre>
<h3>Translation from UCode</h3>
This is all very simple, even though <code>vg_from_ucode.c</code>
is a big file. Position-independent x86 code is generated into
a dynamically allocated array <code>emitted_code</code>; this is
doubled in size when it overflows. Eventually the array is handed
back to the caller of <code>VG_(translate)</code>, who must copy
the result into TC and TT, and free the array.
<p>
This file is structured into four layers of abstraction, which,
thankfully, are glued back together with extensive
<code>__inline__</code> directives. From the bottom upwards:
<ul>
<li>Address-mode emitters, <code>emit_amode_regmem_reg</code> et al.
<p>
<li>Emitters for specific x86 instructions. There are quite a lot of
these, with names such as <code>emit_movv_offregmem_reg</code>.
The <code>v</code> suffix is Intel parlance for a 16/32 bit insn;
there are also <code>b</code> suffixes for 8 bit insns.
<p>
<li>The next level up are the <code>synth_*</code> functions, which
synthesise possibly a sequence of raw x86 instructions to do some
simple task. Some of these are quite complex because they have to
work around Intel's silly restrictions on subregister naming. See
<code>synth_nonshiftop_reg_reg</code> for example.
<p>
<li>Finally, at the top of the heap, we have
<code>emitUInstr()</code>,
which emits code for a single uinstr.
</ul>
<p>
Some comments:
<ul>
<li>The hack for FPU instructions becomes apparent here. To do a
<code>FPU</code> ucode instruction, we load the simulated FPU's
state into from its <code>VG_(baseBlock)</code> into the real FPU
using an x86 <code>frstor</code> insn, do the ucode
<code>FPU</code> insn on the real CPU, and write the updated FPU
state back into <code>VG_(baseBlock)</code> using an
<code>fnsave</code> instruction. This is pretty brutal, but is
simple and it works, and even seems tolerably efficient. There is
no attempt to cache the simulated FPU state in the real FPU over
multiple back-to-back ucode FPU instructions.
<p>
<code>FPU_R</code> and <code>FPU_W</code> are also done this way,
with the minor complication that we need to patch in some
addressing mode bits so the resulting insn knows the effective
address to use. This is easy because of the regularity of the x86
FPU instruction encodings.
<p>
<li>An analogous trick is done with ucode insns which claim, in their
<code>flags_r</code> and <code>flags_w</code> fields, that they
read or write the simulated <code>%EFLAGS</code>. For such cases
we first copy the simulated <code>%EFLAGS</code> into the real
<code>%eflags</code>, then do the insn, then, if the insn says it
writes the flags, copy back to <code>%EFLAGS</code>. This is a
bit expensive, which is why the ucode optimisation pass goes to
some effort to remove redundant flag-update annotations.
</ul>
<p>
And so ... that's the end of the documentation for the instrumentating
translator! It's really not that complex, because it's composed as a
sequence of simple(ish) self-contained transformations on
straight-line blocks of code.
<h3>Top-level dispatch loop</h3>
Urk. In <code>VG_(toploop)</code>. This is basically boring and
unsurprising, not to mention fiddly and fragile. It needs to be
cleaned up.
<p>
The only perhaps surprise is that the whole thing is run
on top of a <code>setjmp</code>-installed exception handler, because,
supposing a translation got a segfault, we have to bail out of the
Valgrind-supplied exception handler <code>VG_(oursignalhandler)</code>
and immediately start running the client's segfault handler, if it has
one. In particular we can't finish the current basic block and then
deliver the signal at some convenient future point, because signals
like SIGILL, SIGSEGV and SIGBUS mean that the faulting insn should not
simply be re-tried. (I'm sure there is a clearer way to explain this).
<h3>Exceptions, creating new translations</h3>
<h3>Self-modifying code</h3>
<h3>Lazy updates of the simulated program counter</h3>
Simulated <code>%EIP</code> is not updated after every simulated x86
insn as this was regarded as too expensive. Instead ucode
<code>INCEIP</code> insns move it along as and when necessary.
Currently we don't allow it to fall more than 4 bytes behind reality
(see <code>VG_(disBB)</code> for the way this works).
<p>
Note that <code>%EIP</code> is always brought up to date by the inner
dispatch loop in <code>VG_(dispatch)</code>, so that if the client
takes a fault we know at least which basic block this happened in.
<h3>The translation cache and translation table</h3>
<h3>Signals</h3>
Horrible, horrible. <code>vg_signals.c</code>.
Basically, since we have to intercept all system
calls anyway, we can see when the client tries to install a signal
handler. If it does so, we make a note of what the client asked to
happen, and ask the kernel to route the signal to our own signal
handler, <code>VG_(oursignalhandler)</code>. This simply notes the
delivery of signals, and returns.
<p>
Every 1000 basic blocks, we see if more signals have arrived. If so,
<code>VG_(deliver_signals)</code> builds signal delivery frames on the
client's stack, and allows their handlers to be run. Valgrind places
in these signal delivery frames a bogus return address,
<code>VG_(signalreturn_bogusRA)</code>, and checks all jumps to see
if any jump to it. If so, this is a sign that a signal handler is
returning, and if so Valgrind removes the relevant signal frame from
the client's stack, restores the from the signal frame the simulated
state before the signal was delivered, and allows the client to run
onwards. We have to do it this way because some signal handlers never
return, they just <code>longjmp()</code>, which nukes the signal
delivery frame.
<p>
The Linux kernel has a different but equally horrible hack for
detecting signal handler returns. Discovering it is left as an
exercise for the reader.
<h3>Errors, error contexts, error reporting, suppressions</h3>
<h3>Client malloc/free</h3>
<h3>Low-level memory management</h3>
<h3>A and V bitmaps</h3>
<h3>Symbol table management</h3>
<h3>Dealing with system calls</h3>
<h3>Namespace management</h3>
<h3>GDB attaching</h3>
<h3>Non-dependence on glibc or anything else</h3>
<h3>The leak detector</h3>
<h3>Performance problems</h3>
<h3>Continuous sanity checking</h3>
<h3>Tracing, or not tracing, child processes</h3>
<h3>Assembly glue for syscalls</h3>
<hr width="100%">
<h2>Extensions</h2>
Some comments about Stuff To Do.
<h3>Bugs</h3>
Stephan Kulow and Marc Mutz report problems with kmail in KDE 3 CVS
(RC2 ish) when run on Valgrind. Stephan has it deadlocking; Marc has
it looping at startup. I can't repro either behaviour. Needs
repro-ing and fixing.
<h3>Threads</h3>
Doing a good job of thread support strikes me as almost a
research-level problem. The central issues are how to do fast cheap
locking of the <code>VG_(primary_map)</code> structure, whether or not
accesses to the individual secondary maps need locking, what
race-condition issues result, and whether the already-nasty mess that
is the signal simulator needs further hackery.
<p>
I realise that threads are the most-frequently-requested feature, and
I am thinking about it all. If you have guru-level understanding of
fast mutual exclusion mechanisms and race conditions, I would be
interested in hearing from you.
<h3>Verification suite</h3>
Directory <code>tests/</code> contains various ad-hoc tests for
Valgrind. However, there is no systematic verification or regression
suite, that, for example, exercises all the stuff in
<code>vg_memory.c</code>, to ensure that illegal memory accesses and
undefined value uses are detected as they should be. It would be good
to have such a suite.
<h3>Porting to other platforms</h3>
It would be great if Valgrind was ported to FreeBSD and x86 NetBSD,
and to x86 OpenBSD, if it's possible (doesn't OpenBSD use a.out-style
executables, not ELF ?)
<p>
The main difficulties, for an x86-ELF platform, seem to be:
<ul>
<li>You'd need to rewrite the <code>/proc/self/maps</code> parser
(<code>vg_procselfmaps.c</code>).
Easy.
<p>
<li>You'd need to rewrite <code>vg_syscall_mem.c</code>, or, more
specifically, provide one for your OS. This is tedious, but you
can implement syscalls on demand, and the Linux kernel interface
is, for the most part, going to look very similar to the *BSD
interfaces, so it's really a copy-paste-and-modify-on-demand job.
As part of this, you'd need to supply a new
<code>vg_kerneliface.h</code> file.
<p>
<li>You'd also need to change the syscall wrappers for Valgrind's
internal use, in <code>vg_mylibc.c</code>.
</ul>
All in all, I think a port to x86-ELF *BSDs is not really very
difficult, and in some ways I would like to see it happen, because
that would force a more clear factoring of Valgrind into platform
dependent and independent pieces. Not to mention, *BSD folks also
deserve to use Valgrind just as much as the Linux crew do.
<p>
<hr width="100%">
<h2>Easy stuff which ought to be done</h2>
<h3>MMX instructions</h3>
MMX insns should be supported, using the same trick as for FPU insns.
If the MMX registers are not used to copy uninitialised junk from one
place to another in memory, this means we don't have to actually
simulate the internal MMX unit state, so the FPU hack applies. This
should be fairly easy.
<h3>Fix stabs-info reader</h3>
The machinery in <code>vg_symtab2.c</code> which reads "stabs" style
debugging info is pretty weak. It usually correctly translates
simulated program counter values into line numbers and procedure
names, but the file name is often completely wrong. I think the
logic used to parse "stabs" entries is weak. It should be fixed.
The simplest solution, IMO, is to copy either the logic or simply the
code out of GNU binutils which does this; since GDB can clearly get it
right, binutils (or GDB?) must have code to do this somewhere.
<h3>BT/BTC/BTS/BTR</h3>
These are x86 instructions which test, complement, set, or reset, a
single bit in a word. At the moment they are both incorrectly
implemented and incorrectly instrumented.
<p>
The incorrect instrumentation is due to use of helper functions. This
means we lose bit-level definedness tracking, which could wind up
giving spurious uninitialised-value use errors. The Right Thing to do
is to invent a couple of new UOpcodes, I think <code>GET_BIT</code>
and <code>SET_BIT</code>, which can be used to implement all 4 x86
insns, get rid of the helpers, and give bit-accurate instrumentation
rules for the two new UOpcodes.
<p>
I realised the other day that they are mis-implemented too. The x86
insns take a bit-index and a register or memory location to access.
For registers the bit index clearly can only be in the range zero to
register-width minus 1, and I assumed the same applied to memory
locations too. But evidently not; for memory locations the index can
be arbitrary, and the processor will index arbitrarily into memory as
a result. This too should be fixed. Sigh. Presumably indexing
outside the immediate word is not actually used by any programs yet
tested on Valgrind, for otherwise they (presumably) would simply not
work at all. If you plan to hack on this, first check the Intel docs
to make sure my understanding is really correct.
<h3>Using PREFETCH instructions</h3>
Here's a small but potentially interesting project for performance
junkies. Experiments with valgrind's code generator and optimiser(s)
suggest that reducing the number of instructions executed in the
translations and mem-check helpers gives disappointingly small
performance improvements. Perhaps this is because performance of
Valgrindified code is limited by cache misses. After all, each read
in the original program now gives rise to at least three reads, one
for the <code>VG_(primary_map)</code>, one of the resulting
secondary, and the original. Not to mention, the instrumented
translations are 13 to 14 times larger than the originals. All in all
one would expect the memory system to be hammered to hell and then
some.
<p>
So here's an idea. An x86 insn involving a read from memory, after
instrumentation, will turn into ucode of the following form:
<pre>
... calculate effective addr, into ta and qa ...
TESTVL qa -- is the addr defined?
LOADV (ta), qloaded -- fetch V bits for the addr
LOAD (ta), tloaded -- do the original load
</pre>
At the point where the <code>LOADV</code> is done, we know the actual
address (<code>ta</code>) from which the real <code>LOAD</code> will
be done. We also know that the <code>LOADV</code> will take around
20 x86 insns to do. So it seems plausible that doing a prefetch of
<code>ta</code> just before the <code>LOADV</code> might just avoid a
miss at the <code>LOAD</code> point, and that might be a significant
performance win.
<p>
Prefetch insns are notoriously tempermental, more often than not
making things worse rather than better, so this would require
considerable fiddling around. It's complicated because Intels and
AMDs have different prefetch insns with different semantics, so that
too needs to be taken into account. As a general rule, even placing
the prefetches before the <code>LOADV</code> insn is too near the
<code>LOAD</code>; the ideal distance is apparently circa 200 CPU
cycles. So it might be worth having another analysis/transformation
pass which pushes prefetches as far back as possible, hopefully
immediately after the effective address becomes available.
<p>
Doing too many prefetches is also bad because they soak up bus
bandwidth / cpu resources, so some cleverness in deciding which loads
to prefetch and which to not might be helpful. One can imagine not
prefetching client-stack-relative (<code>%EBP</code> or
<code>%ESP</code>) accesses, since the stack in general tends to show
good locality anyway.
<p>
There's quite a lot of experimentation to do here, but I think it
might make an interesting week's work for someone.
<p>
As of 15-ish March 2002, I've started to experiment with this, using
the AMD <code>prefetch/prefetchw</code> insns.
<h3>User-defined permission ranges</h3>
This is quite a large project -- perhaps a month's hacking for a
capable hacker to do a good job -- but it's potentially very
interesting. The outcome would be that Valgrind could detect a
whole class of bugs which it currently cannot.
<p>
The presentation falls into two pieces.
<p>
<b>Part 1: user-defined address-range permission setting</b>
<p>
Valgrind intercepts the client's <code>malloc</code>,
<code>free</code>, etc calls, watches system calls, and watches the
stack pointer move. This is currently the only way it knows about
which addresses are valid and which not. Sometimes the client program
knows extra information about its memory areas. For example, the
client could at some point know that all elements of an array are
out-of-date. We would like to be able to convey to Valgrind this
information that the array is now addressable-but-uninitialised, so
that Valgrind can then warn if elements are used before they get new
values.
<p>
What I would like are some macros like this:
<pre>
VALGRIND_MAKE_NOACCESS(addr, len)
VALGRIND_MAKE_WRITABLE(addr, len)
VALGRIND_MAKE_READABLE(addr, len)
</pre>
and also, to check that memory is addressible/initialised,
<pre>
VALGRIND_CHECK_ADDRESSIBLE(addr, len)
VALGRIND_CHECK_INITIALISED(addr, len)
</pre>
<p>
I then include in my sources a header defining these macros, rebuild
my app, run under Valgrind, and get user-defined checks.
<p>
Now here's a neat trick. It's a nuisance to have to re-link the app
with some new library which implements the above macros. So the idea
is to define the macros so that the resulting executable is still
completely stand-alone, and can be run without Valgrind, in which case
the macros do nothing, but when run on Valgrind, the Right Thing
happens. How to do this? The idea is for these macros to turn into a
piece of inline assembly code, which (1) has no effect when run on the
real CPU, (2) is easily spotted by Valgrind's JITter, and (3) no sane
person would ever write, which is important for avoiding false matches
in (2). So here's a suggestion:
<pre>
VALGRIND_MAKE_NOACCESS(addr, len)
</pre>
becomes (roughly speaking)
<pre>
movl addr, %eax
movl len, %ebx
movl $1, %ecx -- 1 describes the action; MAKE_WRITABLE might be
-- 2, etc
rorl $13, %ecx
rorl $19, %ecx
rorl $11, %eax
rorl $21, %eax
</pre>
The rotate sequences have no effect, and it's unlikely they would
appear for any other reason, but they define a unique byte-sequence
which the JITter can easily spot. Using the operand constraints
section at the end of a gcc inline-assembly statement, we can tell gcc
that the assembly fragment kills <code>%eax</code>, <code>%ebx</code>,
<code>%ecx</code> and the condition codes, so this fragment is made
harmless when not running on Valgrind, runs quickly when not on
Valgrind, and does not require any other library support.
<p>
<b>Part 2: using it to detect interference between stack variables</b>
<p>
Currently Valgrind cannot detect errors of the following form:
<pre>
void fooble ( void )
{
int a[10];
int b[10];
a[10] = 99;
}
</pre>
Now imagine rewriting this as
<pre>
void fooble ( void )
{
int spacer0;
int a[10];
int spacer1;
int b[10];
int spacer2;
VALGRIND_MAKE_NOACCESS(&amp;spacer0, sizeof(int));
VALGRIND_MAKE_NOACCESS(&amp;spacer1, sizeof(int));
VALGRIND_MAKE_NOACCESS(&amp;spacer2, sizeof(int));
a[10] = 99;
}
</pre>
Now the invalid write is certain to hit <code>spacer0</code> or
<code>spacer1</code>, so Valgrind will spot the error.
<p>
There are two complications.
<p>
The first is that we don't want to annotate sources by hand, so the
Right Thing to do is to write a C/C++ parser, annotator, prettyprinter
which does this automatically, and run it on post-CPP'd C/C++ source.
See http://www.cacheprof.org for an example of a system which
transparently inserts another phase into the gcc/g++ compilation
route. The parser/prettyprinter is probably not as hard as it sounds;
I would write it in Haskell, a powerful functional language well
suited to doing symbolic computation, with which I am intimately
familar. There is already a C parser written in Haskell by someone in
the Haskell community, and that would probably be a good starting
point.
<p>
The second complication is how to get rid of these
<code>NOACCESS</code> records inside Valgrind when the instrumented
function exits; after all, these refer to stack addresses and will
make no sense whatever when some other function happens to re-use the
same stack address range, probably shortly afterwards. I think I
would be inclined to define a special stack-specific macro
<pre>
VALGRIND_MAKE_NOACCESS_STACK(addr, len)
</pre>
which causes Valgrind to record the client's <code>%ESP</code> at the
time it is executed. Valgrind will then watch for changes in
<code>%ESP</code> and discard such records as soon as the protected
area is uncovered by an increase in <code>%ESP</code>. I hesitate
with this scheme only because it is potentially expensive, if there
are hundreds of such records, and considering that changes in
<code>%ESP</code> already require expensive messing with stack access
permissions.
<p>
This is probably easier and more robust than for the instrumenter
program to try and spot all exit points for the procedure and place
suitable deallocation annotations there. Plus C++ procedures can
bomb out at any point if they get an exception, so spotting return
points at the source level just won't work at all.
<p>
Although some work, it's all eminently doable, and it would make
Valgrind into an even-more-useful tool.
<p>
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