Re: [PATCH] Fix pattern causing C_MAYBE_CONST_EXPRs leak into gimplifier (PR c/68513)

[Marek Polacek <polacek at redhat dot com>]

On Thu, Nov 26, 2015 at 12:34:55PM +0000, Joseph Myers wrote:
> On Thu, 26 Nov 2015, Marek Polacek wrote:
> 
> > I had a go at this, but I'm now skeptical about removing c_save_expr.
> > save_expr calls fold (), so we need to ensure that we don't pass any
> > C_MAYBE_CONST_EXPRs into it, meaning that we'd need to call c_fully_fold before
> > save_expr anyway...
> > 
> > So maybe go the "remove C_MAYBE_CONST_EXPRs in SAVE_EXPRs in c_gimplify_expr"
> > way?
> 
> I believe it should be safe for gimplification to process 
> C_MAYBE_CONST_EXPR in the same way c_fully_fold_internal does.  That is, 
> this should not affect correctness.  If a C_MAYBE_CONST_EXPR got through 
> to gimplification, in some cases it may mean that something did not get 
> properly folded with c_fully_fold as it should have done - but if the move 
> to match.pd means all optimizations currently done with fold end up 
> working on GIMPLE as well, any missed optimizations from this should 
> disappear (and if we can solve the diagnostics issues, eventually fewer 
> calls to c_fully_fold should be needed and they should be more about 
> checking what can occur in constant expressions and less about folding for 
> optimization).
 
So here's an attempt to strip C_MAYBE_CONST_EXPRs, only for SAVE_EXPRs, because
c_fully_fold in c_process_stmt_expr should deal with other expressions.

My worry was of course C_MAYBE_CONST_EXPR_PRE.  But it seems we'll never have
any at that point, since it's already been processed and transformed to a
COMPOUND_EXPR.  But do I like this patch?  No.

> The general principle of delaying folding also means that we should move 
> away from convert_* folding things.

Yep, I tried using _nofold variants, but it had soem fallout.  Anyway,
something for next stage1.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2015-11-26  Marek Polacek  <polacek@redhat.com>

	PR c/68513
	* c-gimplify.c (strip_c_maybe_const_expr_r): New.
	(c_gimplify_expr): Call it.

	* gcc.dg/torture/pr68513.c: New test.

diff --git gcc/c-family/c-gimplify.c gcc/c-family/c-gimplify.c
index fc4a44a..c096575 100644
--- gcc/c-family/c-gimplify.c
+++ gcc/c-family/c-gimplify.c
@@ -212,6 +212,21 @@ c_build_bind_expr (location_t loc, tree block, tree body)
 
 /* Gimplification of expression trees.  */
 
+/* Callback for c_gimplify_expr.  Strip C_MAYBE_CONST_EXPRs in TP so that
+   they don't leak into the middle end.  */
+
+static tree
+strip_c_maybe_const_expr_r (tree *tp, int *walk_subtrees, void *)
+{
+  if (TREE_CODE (*tp) == C_MAYBE_CONST_EXPR)
+    {
+      gcc_assert (C_MAYBE_CONST_EXPR_PRE (*tp) == NULL_TREE);
+      *tp = C_MAYBE_CONST_EXPR_EXPR (*tp);
+      *walk_subtrees = 0;
+    }
+  return NULL_TREE;
+}
+
 /* Do C-specific gimplification on *EXPR_P.  PRE_P and POST_P are as in
    gimplify_expr.  */
 
@@ -296,6 +311,10 @@ c_gimplify_expr (tree *expr_p, gimple_seq *pre_p ATTRIBUTE_UNUSED,
 	  return (enum gimplify_status) gimplify_cilk_spawn (expr_p);
 	}
 
+    case SAVE_EXPR:
+      walk_tree_without_duplicates (expr_p, strip_c_maybe_const_expr_r, NULL);
+      break;
+
     default:;
     }
 
diff --git gcc/testsuite/gcc.dg/torture/pr68513.c gcc/testsuite/gcc.dg/torture/pr68513.c
index e69de29..4e08b29 100644
--- gcc/testsuite/gcc.dg/torture/pr68513.c
+++ gcc/testsuite/gcc.dg/torture/pr68513.c
@@ -0,0 +1,13 @@
+/* PR c/68513 */
+/* { dg-do compile } */
+
+int i;
+unsigned u;
+volatile unsigned int *e;
+
+void
+fn1 (void)
+{
+  (short) ((i ? *e : 0) & ~u | i & u);
+  (short) (((0, 0) ? *e : 0) & ~u | i & u);
+}

	Marek