This patch adds a function that allows to directly properly size an xarray

when the size is known in advance.

3 places identified where this function can be used trivially.

The result is a reduction of 'realloc' operations in core
arena, and a small reduction in ttaux arena
(it is the nr of operations that decreases, the memory usage itself
stays the same (ignoring some 'rounding' effects).

E.g. for perf/bigcode 0, we change from
  core 1085742/ 216745904 totalloc-blocks/bytes,     1085733 searches
  ttaux 5348/   6732560 totalloc-blocks/bytes,        5326 searches
to
  core 712666/ 190998592 totalloc-blocks/bytes,      712657 searches
  ttaux 5319/   6731808 totalloc-blocks/bytes,        5296 searches

For bz2, we switch from
  core 50285/  32383664 totalloc-blocks/bytes,       50256 searches
  ttaux 670/    245160 totalloc-blocks/bytes,         669 searches
to
  core 32564/  29971984 totalloc-blocks/bytes,       32535 searches
  ttaux 605/    243280 totalloc-blocks/bytes,         604 searches

Performance wise, on amd64, this improves memcheck performance
on perf tests by 0.0, 0.1 or 0.2 seconds depending on the test.



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@15173

include/pub_tool_xarray.h

@@ -104,6 +104,13 @@ extern Bool VG_(lookupXA_UNSAFE) ( const XArray* xao, const void* key,
 /* How elements are there in this XArray now? */
 extern Word VG_(sizeXA) ( const XArray* );
 
+/* If you know how many elements an XArray will have, you can
+   optimise memory usage and number of reallocation needed
+   to insert these elements. The call to VG_(hintSizeXA) must be
+   done just after the call to VG_(newXA), before any element
+   has been inserted. */
+extern void VG_(hintSizeXA) ( XArray*, Word);
+
 /* Index into the XArray.  Checks bounds and bombs if the index is
    invalid.  What this returns is the address of the specified element
    in the array, not (of course) the element itself.  Note that the