On Thu, May 16, 2013 at 03:38:47PM +0200, Jakub Jelinek wrote:
> Please add here a comment what it does and why, that if si->length
> is non-zero constant, we know that the character at that spot is
> not '\0' and when storing non-'\0' to that location, we can't affect
> size of any strings at all. Therefore we do the gsi_next + return false
> to signal caller that it shouldn't invalidate anything.
>
> Ok with that change, thanks.
Thanks, this what I'll commit later today if no objections.
2013-05-16 Marek Polacek <pola...@redhat.com>
* tree-ssa-strlen.c (handle_char_store): Don't invalidate
cached length when doing non-zero store.
* gcc.dg/strlenopt-25.c: New test.
--- gcc/tree-ssa-strlen.c.mp 2013-05-15 14:11:20.079707492 +0200
+++ gcc/tree-ssa-strlen.c 2013-05-16 16:03:50.373504796 +0200
@@ -1717,6 +1717,27 @@ handle_char_store (gimple_stmt_iterator
si->endptr = ssaname;
si->dont_invalidate = true;
}
+ /* If si->length is non-zero constant, we aren't overwriting '\0',
+ and if we aren't storing '\0', we know that the length of the
+ string remains the same. In that case we move to the next
+ gimple statement and return to signal the caller that it shouldn't
+ invalidate anything.
+
+ This is benefical for cases like:
+
+ char p[] = "foobar";
+ size_t len = strlen (p);
+ p[0] = 'X'
+ size_t len2 = strlen (p);
+
+ where we should be able to optimize away the second strlen call. */
+ else if (si != NULL && si->length != NULL_TREE
+ && TREE_CODE (si->length) == INTEGER_CST
+ && integer_nonzerop (gimple_assign_rhs1 (stmt)))
+ {
+ gsi_next (gsi);
+ return false;
+ }
}
else if (idx == 0 && initializer_zerop (gimple_assign_rhs1 (stmt)))
{
--- gcc/testsuite/gcc.dg/strlenopt-25.c.mp 2013-05-15 17:15:18.702118637
+0200
+++ gcc/testsuite/gcc.dg/strlenopt-25.c 2013-05-15 18:26:27.881030317 +0200
@@ -0,0 +1,18 @@
+/* { dg-do run } */
+/* { dg-options "-O2 -fdump-tree-strlen" } */
+
+#include "strlenopt.h"
+
+int
+main ()
+{
+ char p[] = "foobar";
+ int len, len2;
+ len = strlen (p);
+ p[0] = 'O';
+ len2 = strlen (p);
+ return len - len2;
+}
+
+/* { dg-final { scan-tree-dump-times "strlen \\(" 0 "strlen" } } */
+/* { dg-final { cleanup-tree-dump "strlen" } } */
Marek
