[PATCH] Fix PR82129

[Richard Biener]

The following "fixes" PR82129 (hides the issue).  It folds
writes from uninitialized data to CLOBBERs:

-  *h5_26(D) = tv_24(D);
+  *h5_26(D) ={v} {CLOBBER};

Bootstrap & regtest running on x86_64-unknown-linux-gnu.

Richard.

2017-09-12  Richard Biener  <rguent...@suse.de>

        PR tree-optimization/82129
        * gimple-fold.c (fold_gimple_assign): Fold stores from undefined
        values to clobbers.

        * gcc.dg/torture/pr82129.c: New testcase.

Index: gcc/gimple-fold.c
===================================================================
--- gcc/gimple-fold.c   (revision 252002)
+++ gcc/gimple-fold.c   (working copy)
@@ -410,6 +410,20 @@ fold_gimple_assign (gimple_stmt_iterator
 
        else if (DECL_P (rhs))
          return get_symbol_constant_value (rhs);
+
+       else if (TREE_CODE (rhs) == SSA_NAME
+                && SSA_NAME_IS_DEFAULT_DEF (rhs)
+                && ! ssa_defined_default_def_p (rhs)
+                && gimple_store_p (stmt)
+                && ! gimple_has_volatile_ops (stmt))
+         {
+           /* Replace a store from an undefined value with a clobber
+              which will not generate code, thus avoid doing this when
+              the store is volatile.  */
+           tree clobber = build_constructor (TREE_TYPE (rhs), NULL);
+           TREE_THIS_VOLATILE (clobber) = true;
+           return clobber;
+         }
       }
       break;
 
Index: gcc/testsuite/gcc.dg/torture/pr82129.c
===================================================================
--- gcc/testsuite/gcc.dg/torture/pr82129.c      (nonexistent)
+++ gcc/testsuite/gcc.dg/torture/pr82129.c      (working copy)
@@ -0,0 +1,51 @@
+/* { dg-do compile } */
+
+int pj;
+
+void
+g4 (unsigned long int *bc, unsigned long int *h5)
+{
+  if (pj != 0)
+    {
+      int ib = 0;
+
+      while (bc != 0)
+       {
+m6:
+         for (pj = 0; pj < 2; ++pj)
+           pj = 0;
+
+         while (pj != 0)
+           {
+             for (;;)
+               {
+               }
+
+             while (ib != 0)
+               {
+                 unsigned long int tv = *bc;
+                 unsigned long int n7;
+
+                 *bc = 1;
+                 while (*bc != 0)
+                   {
+                   }
+
+ut:
+                 if (pj == 0)
+                   n7 = *h5 > 0;
+                 else
+                   {
+                     *h5 = tv;
+                     n7 = *h5;
+                   }
+                 ib += n7;
+               }
+           }
+       }
+
+      goto ut;
+    }
+
+  goto m6;
+}