diff --git a/coregrind/m_xarray.c b/coregrind/m_xarray.c
index cdcf97861..4dc8ebdb7 100644
--- a/coregrind/m_xarray.c
+++ b/coregrind/m_xarray.c
@@ -44,6 +44,7 @@ struct _XArray {
    Int   (*cmpFn) ( void*, void* ); /* cmp fn (may be NULL) */
    Word  elemSzB;   /* element size in bytes */
    void* arr;       /* pointer to elements */
+   Word  initsizeE; /* HINT only: initial size, 0 if no hint */
    Word  usedsizeE; /* # used elements in arr */
    Word  totsizeE;  /* max size of arr, in elements */
    Bool  sorted;    /* is it sorted? */
@@ -72,6 +73,7 @@ XArray* VG_(newXA) ( void*(*alloc_fn)(HChar*,SizeT),
    xa->free      = free_fn;
    xa->cmpFn     = NULL;
    xa->elemSzB   = elemSzB;
+   xa->initsizeE = 0;
    xa->usedsizeE = 0;
    xa->totsizeE  = 0;
    xa->sorted    = False;
@@ -79,6 +81,19 @@ XArray* VG_(newXA) ( void*(*alloc_fn)(HChar*,SizeT),
    return xa;
 }
 
+XArray* VG_(newSizedXA) ( void*(*alloc_fn)(HChar*,SizeT), 
+                          HChar* cc,
+                          void(*free_fn)(void*),
+                          Word elemSzB,
+                          Word nInitialElems )
+{
+   XArray* xa;
+   tl_assert(nInitialElems >= 0);
+   xa = VG_(newXA)( alloc_fn, cc, free_fn, elemSzB );
+   xa->initsizeE = nInitialElems;
+   return xa;
+}
+
 XArray* VG_(cloneXA)( HChar* cc, XArray* xao )
 {
    struct _XArray* xa = (struct _XArray*)xao;
@@ -145,7 +160,7 @@ inline void* VG_(indexXA) ( XArray* xao, Word n )
 
 static inline void ensureSpaceXA ( struct _XArray* xa )
 {
-   if (xa->usedsizeE == xa->totsizeE) {
+   if (UNLIKELY(xa->usedsizeE == xa->totsizeE)) {
       void* tmp;
       Word  newsz;
       if (xa->totsizeE == 0)
@@ -158,7 +173,11 @@ static inline void ensureSpaceXA ( struct _XArray* xa )
             Hence increase the initial array size for tiny elements in
             an attempt to avoid reallocations of size 2, 4, 8 if the
             array does start to fill up. */
-         if (xa->elemSzB == 1) newsz = 8;
+         /* Also, if there's a hinted initial size, use that instead of
+            the logic in the preceding comment. */
+         tl_assert(xa->initsizeE >= 0);
+         if (xa->initsizeE > 0) newsz = xa->initsizeE;
+         else if (xa->elemSzB == 1) newsz = 8;
          else if (xa->elemSzB == 2) newsz = 4;
          else newsz = 2;
       } else {
diff --git a/include/pub_tool_xarray.h b/include/pub_tool_xarray.h
index cd1b02e43..ff8dfb344 100644
--- a/include/pub_tool_xarray.h
+++ b/include/pub_tool_xarray.h
@@ -54,6 +54,17 @@ extern XArray* VG_(newXA) ( void*(*alloc_fn)(HChar*,SizeT),
                             void(*free_fn)(void*),
                             Word elemSzB );
 
+/* Same as VG_(newXA), except allows specification of an initial
+   number of elements for the array, so as to avoid a potentially
+   large wasted cost of repeatedly resizing the array when the caller
+   knows something about what the expected final size is going to
+   be. */
+extern XArray* VG_(newSizedXA) ( void*(*alloc_fn)(HChar*,SizeT), 
+                                 HChar* cc,
+                                 void(*free_fn)(void*),
+                                 Word elemSzB,
+                                 Word nInitialElems );
+
 /* Free all memory associated with an XArray. */
 extern void VG_(deleteXA) ( XArray* );