gimple-fold: Improve optimize_memcpy_to_memset by walking back until aliasing says the ref is a may clobber. [PR118947]

The case here is we have:
```
    char buf[32] = {};
    void* ret = aaa();
    __builtin_memcpy(ret, buf, 32);
```

And buf does not escape.  But we don't prop the zeroing from buf to the memcpy statement
because optimize_memcpy_to_memset only looks back one statement. This can be fixed to look back
until we get an statement that may clobber the reference.  If we get a phi node, then we don't do
anything.

Bootstrapped and tested on x86_64-linux-gnu.

	PR tree-optimization/118947

gcc/ChangeLog:

	* gimple-fold.cc (optimize_memcpy_to_memset): Walk back until we get a
	statement that may clobber the read.

gcc/testsuite/ChangeLog:

	* gcc.dg/pr118947-1.c: New test.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>

1 files changed, 15 insertions, 0 deletions

diff --git a/gcc/testsuite/gcc.dg/pr118947-1.c b/gcc/testsuite/gcc.dg/pr118947-1.c
new file mode 100644
index 0000000..70b7f80
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr118947-1.c
@@ -0,0 +1,15 @@
+/* PR tree-optimization/118947 */
+/* { dg-do compile { target size32plus } } */
+/* { dg-options "-O2 -fdump-tree-forwprop1-details" } */
+/* { dg-final { scan-tree-dump-times "after previous" 1 "forwprop1" } } */
+
+void* aaa();
+void* bbb()
+{
+    char buf[32] = {};
+    /*  Tha call to aaa should not matter and clobber buf. */
+    void* ret = aaa();
+    __builtin_memcpy(ret, buf, 32);
+    return ret;
+}
+