Re: [RFA][PATCH] Minor fix to aliasing machinery

[Jeff Law]

On 10/30/13 03:34, Richard Biener wrote:

         * tree-ssa-alias.c (stmt_kills_ref_p_1): Handle case where
         ao_ref_base returns a MEM_REF.

         * gcc.dg/tree-ssa/alias-26.c: New test.

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/alias-26.c
b/gcc/testsuite/gcc.dg/tree-ssa/alias-26.c
new file mode 100644
index 0000000..b5625b8
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/alias-26.c
@@ -0,0 +1,12 @@
+/* { dg-do compile } */
+/* { dg-options "-O1 -fdump-tree-optimized" } */
+
+void f (long *p) {
+  *p = 42;
+  p[4] = 42;
+  __builtin_memset (p, 0, 100);
+}
+
+/* { dg-final { scan-tree-dump-not "= 42" "optimized" } } */
+/* { dg-final { cleanup-tree-dump "optimized" } } */
+
diff --git a/gcc/tree-ssa-alias.c b/gcc/tree-ssa-alias.c
index 4db83bd..5120e72 100644
--- a/gcc/tree-ssa-alias.c
+++ b/gcc/tree-ssa-alias.c
@@ -2079,6 +2079,7 @@ stmt_kills_ref_p_1 (gimple stmt, ao_ref *ref)
               tree dest = gimple_call_arg (stmt, 0);
               tree len = gimple_call_arg (stmt, 2);
               tree base = NULL_TREE;
+             tree ref_base;
               HOST_WIDE_INT offset = 0;
               if (!host_integerp (len, 0))
                 return false;
@@ -2087,8 +2088,11 @@ stmt_kills_ref_p_1 (gimple stmt, ao_ref *ref)
                                                       &offset);
               else if (TREE_CODE (dest) == SSA_NAME)
                 base = dest;
+             ref_base = ao_ref_base (ref);
               if (base
-                 && base == ao_ref_base (ref))
+                 && ((TREE_CODE (ref_base) == MEM_REF
+                      && base == TREE_OPERAND (ref_base, 0))

That's not sufficient - ref_base may have an offset, so for correctness
you have to check that integer_zerop (TREE_OPERAND (ref_base, 0)).
But this now looks convoluted and somewhat backward, and still
does not catch all cases (including the def-stmt lookup recently
added to ao_ref_from_ptr_and_size).
So how do you want to proceed?  I'm not really up for burning through 
this code right now and trying to sort out how it ought to work.

Perhaps checkin the test (xfailed) and wait for someone with the 
interest and time to push this through to completion?

jeff