[PATCH][v2] tree-optimization/120043 - bogus conditional store elimination

[Richard Biener]

The following fixes conditional store elimination to properly
check for conditional stores to readonly memory which we can
obviously not store to unconditionally.  The tree_could_trap_p
predicate used is only considering rvalues and the chosen
approach mimics that of loop store motion.

Bootstrapped and tested on x86_64-unknown-linux-gnu, pushed.

The previous idea of changing tree_could_trap_p had too much fallout.

Richard.

        PR tree-optimization/120043
        * tree-ssa-phiopt.cc (cond_store_replacement): Check
        whether the store is to readonly memory.

        * gcc.dg/torture/pr120043.c: New testcase.
---
 gcc/testsuite/gcc.dg/torture/pr120043.c | 10 ++++++++++
 gcc/tree-ssa-phiopt.cc                  |  8 +++++++-
 2 files changed, 17 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/gcc.dg/torture/pr120043.c

diff --git a/gcc/testsuite/gcc.dg/torture/pr120043.c 
b/gcc/testsuite/gcc.dg/torture/pr120043.c
new file mode 100644
index 00000000000..ae27468d86d
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr120043.c
@@ -0,0 +1,10 @@
+/* { dg-do run } */
+/* { dg-additional-options "-fallow-store-data-races" } */
+
+const int a;
+int *b;
+int main()
+{
+  &a != b || (*b = 1);
+  return 0;
+}
diff --git a/gcc/tree-ssa-phiopt.cc b/gcc/tree-ssa-phiopt.cc
index 54ecd93495a..4e96cf33a52 100644
--- a/gcc/tree-ssa-phiopt.cc
+++ b/gcc/tree-ssa-phiopt.cc
@@ -3546,8 +3546,14 @@ cond_store_replacement (basic_block middle_bb, 
basic_block join_bb,
       /* If LHS is an access to a local variable without address-taken
         (or when we allow data races) and known not to trap, we could
         always safely move down the store.  */
+      tree base;
       if (ref_can_have_store_data_races (lhs)
-         || tree_could_trap_p (lhs))
+         || tree_could_trap_p (lhs)
+         /* tree_could_trap_p is a predicate for rvalues, so check
+            for readonly memory explicitly.  */
+         || ((base = get_base_address (lhs))
+             && DECL_P (base)
+             && TREE_READONLY (base)))
        return false;
     }
 
-- 
2.43.0