[PATCH] Don't invalidate string length cache when not needed

[Jakub Jelinek]

On Thu, May 16, 2013 at 04:18:27PM +0200, Jakub Jelinek wrote:
> As q could point to p, if we didn't do what your patch does on the p[0] = 'X';
> store, then we'd need to invalidate the recorded length of the q string.
> Similarly if there is p[0] = '\0' or p[0] = var.

Ah, another thing while we are at it.
For p[0] = '\0'; case when p[0] is known to be '\0' already, we
remove it if:
              /* When storing '\0', the store can be removed
                 if we know it has been stored in the current function.  */
              if (!stmt_could_throw_p (stmt) && si->writable)
(and in that case don't invalidate anything either).  But the above
condition is false, we set si->writable (correct) and si->dont_invalidate,
while we could do instead of that the same what you do for non-zero store to
non-zero location, i.e. gsi_next (gsi); return false;.
Perhaps a testcase for that is:
size_t
bar (char *p, char *r)
{
  size_t len1 = strlen (r);
  char *q = strchr (p, '\0');
  *q = '\0';
  return len1 - strlen (r); // This strlen should be optimized into len1.
}

strlen (q) should be known to be zero at that point, but si->writable should
be false, we don't know if p doesn't point say into .rodata, and
stmt_could_throw_p probably should return true too.

	Jakub