[Bug middle-end/21602] New: builtin memmove could be memcpy is src and dst don't alias

["ghazi at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

The current builtin memmove optimizes to memcpy if it can prove that the source 
pointer is in readonly memory, under the assumption that the destination 
pointer must be writable and therefore couldn't overlap.  However we could 
generalize this such that if we can prove the source and dest don't alias, then 
we can do the transformation.  E.g. given the following code, we should 
optimize the memmove call in both foo() and bar(), however we only do foo() in 
mainline.

typedef __SIZE_TYPE__ size_t;
extern void *malloc (size_t);
extern void *memmove (void *, const void *, size_t);

void *foo (void)
{
  void *dst = malloc (13);
  return memmove (dst, "hello world\n", 13);
}

void *bar (void *src)
{
  void *dst = malloc(13);
  return memmove (dst, src, 13);
}

-- 
           Summary: builtin memmove could be memcpy is src and dst don't
                    alias
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P2
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: ghazi at gcc dot gnu dot org
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21602