PR c++/85746: Don't fold __builtin_constant_p prematurely

2019-10-22  Marc Glisse  <marc.glisse@inria.fr>

gcc/cp/
	* constexpr.c (cxx_eval_builtin_function_call): Only set
	force_folding_builtin_constant_p if manifestly_const_eval.

gcc/testsuite/
	* g++.dg/pr85746.C: New file.

From-SVN: r277292

gcc/cp/ChangeLog

+2019-10-22  Marc Glisse  <marc.glisse@inria.fr>
+
+	PR c++/85746
+	* constexpr.c (cxx_eval_builtin_function_call): Only set
+	force_folding_builtin_constant_p if manifestly_const_eval.
+
 2019-10-22  Richard Sandiford  <richard.sandiford@arm.com>
 
 	* cp-tree.h (STF_USER_VISIBLE): New constant.

gcc/cp/constexpr.c

@@ -1260,7 +1260,7 @@ cxx_eval_builtin_function_call (const constexpr_ctx *ctx, tree t, tree fun,
     }
 
   bool save_ffbcp = force_folding_builtin_constant_p;
-  force_folding_builtin_constant_p = true;
+  force_folding_builtin_constant_p |= ctx->manifestly_const_eval;
   tree save_cur_fn = current_function_decl;
   /* Return name of ctx->call->fundef->decl for __builtin_FUNCTION ().  */
   if (fndecl_built_in_p (fun, BUILT_IN_FUNCTION)

gcc/testsuite/ChangeLog

+2019-10-22  Marc Glisse  <marc.glisse@inria.fr>
+
+	PR c++/85746
+	* g++.dg/pr85746.C: New file.
+
 2019-10-22  Richard Biener  <rguenther@suse.de>
 
 	PR tree-optimization/92173

gcc/testsuite/g++.dg/pr85746.C

+/* { dg-do compile } */
+/* { dg-options "-O -fdump-tree-gimple" } */
+
+int f(int a,int b){
+  // The front-end should not fold this to 0.
+  int c = __builtin_constant_p(a < b);
+  return c;
+}
+
+/* { dg-final { scan-tree-dump "__builtin_constant_p" "gimple" } } */