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memmove: Don't call memcpy if regions overlap

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memmove() unconditionally calls memcpy() if "dst" < "src". For
example, in the code below, memmove() would end up calling memcpy(),
even though the regions of memory overlap.

int main() {
  char buf3[0x800];
  char *dst  = &buf3[1];
  char *src = &buf3[0x400];
  memset(buf3, 0, sizeof(buf3));
  memmove(dst, src, 0x400);
  printf("1: %s\n", buf3);
  return 0;
}

Calling memcpy() on overlaping regions only works if you assume
that memcpy() copies from start to finish. On some architectures,
it's more efficient to call memcpy() from finish to start.

This is also triggering a failure in some of my code.

More reading:
* http://lwn.net/Articles/414467/
* https://bugzilla.redhat.com/show_bug.cgi?id=638477#c31 (comment 31)

Change-Id: I65a51ae3a52dd4af335fe5c278056b8c2cbd8948
This commit is contained in:
Nick Kralevich 2012-07-11 17:34:04 -07:00
parent 6334c662ca
commit e64259e860
1 changed files with 4 additions and 3 deletions
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7
libc/string/memmove.c
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@ -32,10 +32,11 @@ void *memmove(void *dst, const void *src, size_t n)
{
const char *p = src;
char *q = dst;
/* We can use the optimized memcpy if the destination is below the
* source (i.e. q < p), or if it is completely over it (i.e. q >= p+n).
/* We can use the optimized memcpy if the source and destination
* don't overlap.
*/
if (__builtin_expect((q < p) || ((size_t)(q - p) >= n), 1)) {
if (__builtin_expect(((q < p) && ((size_t)(p - q) >= n))
|| ((p < q) && ((size_t)(q - p) >= n)), 1)) {
return memcpy(dst, src, n);
} else {
bcopy(src, dst, n);