C++ PATCH to constexpr loops and SAVE_EXPR

C++ PATCH to constexpr loops and SAVE_EXPR

[Jason Merrill]

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This patch addresses a significant wrong-code issue with constexpr 
loops: the value of a SAVE_EXPR was being cached across iterations, so 
an increment would only happen once.

Tested x86_64-pc-linux-gnu, applying to trunk.

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commit b4fbc938b1ce031d9b082f2d3e062976ac431977
Author: Jason Merrill <jason@redhat.com>
Date:   Wed Mar 2 16:10:07 2016 -0500

    	Fix constexpr handling of SAVE_EXPR in loops.
    
    	* constexpr.c (struct constexpr_ctx): Add save_exprs field.
    	(cxx_eval_loop_expr): Discard SAVE_EXPR values before looping.
    	(cxx_eval_constant_expression) [SAVE_EXPR]: Add it to the set.
    	(cxx_eval_outermost_constant_expr, is_sub_constant_expr): Initialize.

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 5a81469..4fadc0f 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -897,6 +897,9 @@ struct constexpr_ctx {
   /* Values for any temporaries or local variables within the
      constant-expression. */
   hash_map<tree,tree> *values;
+  /* SAVE_EXPRs that we've seen within the current LOOP_EXPR.  NULL if we
+     aren't inside a loop.  */
+  hash_set<tree> *save_exprs;
   /* The CONSTRUCTOR we're currently building up for an aggregate
      initializer.  */
   tree ctor;
@@ -3161,16 +3164,27 @@ cxx_eval_loop_expr (const constexpr_ctx *ctx, tree t,
 		    bool *non_constant_p, bool *overflow_p,
 		    tree *jump_target)
 {
+  constexpr_ctx new_ctx = *ctx;
+
   tree body = TREE_OPERAND (t, 0);
   while (true)
     {
-      cxx_eval_statement_list (ctx, body,
+      hash_set<tree> save_exprs;
+      new_ctx.save_exprs = &save_exprs;
+
+      cxx_eval_statement_list (&new_ctx, body,
 			       non_constant_p, overflow_p, jump_target);
       if (returns (jump_target) || breaks (jump_target) || *non_constant_p)
 	break;
+
+      /* Forget saved values of SAVE_EXPRs.  */
+      for (hash_set<tree>::iterator iter = save_exprs.begin();
+	   iter != save_exprs.end(); ++iter)
+	new_ctx.values->remove (*iter);
     }
   if (breaks (jump_target))
     *jump_target = NULL_TREE;
+
   return NULL_TREE;
 }
 
@@ -3452,6 +3466,8 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	  r = cxx_eval_constant_expression (ctx, TREE_OPERAND (t, 0), false,
 					    non_constant_p, overflow_p);
 	  ctx->values->put (t, r);
+	  if (ctx->save_exprs)
+	    ctx->save_exprs->add (t);
 	}
       break;
 
@@ -3875,7 +3891,10 @@ cxx_eval_outermost_constant_expr (tree t, bool allow_non_constant,
   bool non_constant_p = false;
   bool overflow_p = false;
   hash_map<tree,tree> map;
-  constexpr_ctx ctx = { NULL, &map, NULL, NULL, allow_non_constant, strict };
+
+  constexpr_ctx ctx = { NULL, &map, NULL, NULL, NULL,
+			allow_non_constant, strict };
+
   tree type = initialized_type (t);
   tree r = t;
   if (AGGREGATE_TYPE_P (type) || VECTOR_TYPE_P (type))
@@ -3983,7 +4002,9 @@ is_sub_constant_expr (tree t)
   bool non_constant_p = false;
   bool overflow_p = false;
   hash_map <tree, tree> map;
-  constexpr_ctx ctx = { NULL, &map, NULL, NULL, true, true };
+
+  constexpr_ctx ctx = { NULL, &map, NULL, NULL, NULL, true, true };
+
   cxx_eval_constant_expression (&ctx, t, false, &non_constant_p,
 				&overflow_p);
   return !non_constant_p && !overflow_p;
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-loop3.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-loop3.C
new file mode 100644
index 0000000..5e7c3c9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-loop3.C
@@ -0,0 +1,23 @@
+// { dg-do compile { target c++14 } }
+
+struct A
+{
+  int i;
+};
+
+constexpr bool f()
+{
+  A ar[4] = { 1, 2, 3, 4 };
+  A *ap = ar;
+  int i = 0;
+  do
+    *ap++ = A{i};
+  while (++i < 3);
+  return (ar[0].i == 0
+	  && ar[1].i == 1
+	  && ar[2].i == 2
+	  && ar[3].i == 4);
+}
+
+#define SA(X) static_assert((X),#X)
+SA(f());