[C++ PATCH] Fix up __builtin_is_constant_evaluated handling in array type sizes (PR c++/88446)

Jason Merrill jason@redhat.com
Thu Dec 13 20:05:00 GMT 2018


On 12/11/18 3:35 PM, Marek Polacek wrote:
> On Tue, Dec 11, 2018 at 05:39:25PM +0100, Jakub Jelinek wrote:
>> Hi!
>>
>> As mentioned in the PR, while we allow VLAs in some contexts in C++ as
>> an extension, they aren't standard and the standard requires in those spots
>> constant expressions, thus __builtin_is_constant_evaluated () needs to be
>> true if those sizes are indeed constant expressions.
>>
>> Fixed by calling cxx_eval_outermost_constant_expr with
>> pretend_const_required too in that case.
>>
>> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
>>
>> 2018-12-11  Jakub Jelinek  <jakub@redhat.com>
>>
>> 	PR c++/88446
>> 	* cp-tree.h (maybe_constant_value): Add pretend_const_required
>> 	argument.
>> 	* constexpr.c (maybe_constant_value): Likewise.  If true, don't
>> 	cache and call cxx_eval_outermost_constant_expr with true as
>> 	pretend_const_required.
>> 	* decl.c (compute_array_index_type_loc): Call maybe_constant_value
>> 	with true as pretend_const_required.
>>
>> 	* g++.dg/cpp2a/is-constant-evaluated3.C: New test.
>>
>> --- gcc/cp/cp-tree.h.jj	2018-12-07 00:23:15.024998595 +0100
>> +++ gcc/cp/cp-tree.h	2018-12-11 13:55:51.933845503 +0100
>> @@ -7663,7 +7663,7 @@ extern bool require_rvalue_constant_expr
>>   extern bool require_potential_rvalue_constant_expression (tree);
>>   extern tree cxx_constant_value			(tree, tree = NULL_TREE);
>>   extern tree cxx_constant_init			(tree, tree = NULL_TREE);
>> -extern tree maybe_constant_value		(tree, tree = NULL_TREE);
>> +extern tree maybe_constant_value		(tree, tree = NULL_TREE, bool = false);
>>   extern tree maybe_constant_init			(tree, tree = NULL_TREE, bool = false);
>>   extern tree fold_non_dependent_expr		(tree, tsubst_flags_t = tf_warning_or_error);
>>   extern tree fold_simple				(tree);
>> --- gcc/cp/constexpr.c.jj	2018-12-11 12:01:27.968941683 +0100
>> +++ gcc/cp/constexpr.c	2018-12-11 13:56:50.382890876 +0100
>> @@ -5244,7 +5244,7 @@ fold_simple (tree t)
>>   static GTY((deletable)) hash_map<tree, tree> *cv_cache;
>>   
>>   tree
>> -maybe_constant_value (tree t, tree decl)
>> +maybe_constant_value (tree t, tree decl, bool pretend_const_required)
>>   {
>>     tree r;
>>   
> 
> Could you please describe the new param in the comment?
> 
>> --- gcc/cp/decl.c.jj	2018-12-07 00:23:15.000000000 +0100
>> +++ gcc/cp/decl.c	2018-12-11 13:57:30.779231098 +0100
>> @@ -9645,7 +9645,10 @@ compute_array_index_type_loc (location_t
>>   	{
>>   	  size = instantiate_non_dependent_expr_sfinae (size, complain);
>>   	  size = build_converted_constant_expr (size_type_node, size, complain);
>> -	  size = maybe_constant_value (size);
>> +	  /* Pedantically a constant expression is required here and so
>> +	     __builtin_is_constant_evaluated () should fold to true if it
>> +	     is successfully folded into a constant.  */
>> +	  size = maybe_constant_value (size, NULL_TREE, true);
> 
> Perhaps use /*pretend_const_required=*/true?

Please.  And the name of the parameter should change to match the other 
patches.  OK with those changes.

Jason



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