From: Jason Merrill <jason@redhat.com>
To: Jakub Jelinek <jakub@redhat.com>
Cc: gcc-patches@gcc.gnu.org
Subject: Re: [C++ PATCH] Fix __builtin_{is_constant_evaluated,constant_p} handling in static_assert (PR c++/86524, PR c++/88446)
Date: Fri, 21 Dec 2018 02:51:00 -0000 [thread overview]
Message-ID: <4abcfe78-7e84-5188-6616-ddba47d9a5bd@redhat.com> (raw)
In-Reply-To: <20181220222719.GS23305@tucnak>
On 12/20/18 5:27 PM, Jakub Jelinek wrote:
> On Thu, Dec 20, 2018 at 04:47:29PM -0500, Jason Merrill wrote:
>>> So are you ok with what is in the patch below, i.e.
>>> {
>>> bool non_cst_p = false, ovf_p = false;
>>> tree a = cxx_eval_constant_expression (&new_ctx, args[i], false,
>>> &non_cst_p, &ovf_p);
>>> if ((!non_cst_p && !ovf_p) || !ctx->manifestly_const_eval)
>>> args[i] = a;
>>> }
>>> , or perhaps without the || !ctx->manifestly_const_eval?
>>
>> I don't see how that makes a difference from what was there before; if the
>> argument to cxx_eval_constant_expression is non-constant, it returns the
>> argument unchanged.
>
> If that is guaranteed, then it is ok to keep it as is I guess.
> Will change it then.
>
>>> So, if the
>>> argument is a constant expression, fold to that, if it is not, just do
>>> cp_fully_fold on it if it is __builtin_constant_p, otherwise nothing?
>>
>> Hmm, cp_fully_fold probably also needs to add a manifestly_const_eval
>> parameter to pass along to maybe_constant_value.
>
> But if we need cp_fully_fold, doesn't that mean that the earlier
> cxx_eval_constant_expression failed and thus the argument is not a constant
> expression? Should __builtin_is_constant_evaluated () evaluate to true
> even if the argument is not a constant expression?
Ah, no, good point.
> Say if there is
> int v;
> constexpr int foo (void)
> {
> return __builtin_constant_p (v * (__builtin_is_constant_evaluated () ? 1 : 0));
> }
> Because v is not a constant expression,
> v * (__builtin_is_constant_evaluated () ? 1 : 0) shouldn't be either.
>
> cp_fully_fold does:
> /* FIXME cp_fold ought to be a superset of maybe_constant_value so we don't
> have to call both. */
> if (cxx_dialect >= cxx11)
> {
> x = maybe_constant_value (x);
> /* Sometimes we are given a CONSTRUCTOR but the call above wraps it into
> a TARGET_EXPR; undo that here. */
> if (TREE_CODE (x) == TARGET_EXPR)
> x = TARGET_EXPR_INITIAL (x);
> else if (TREE_CODE (x) == VIEW_CONVERT_EXPR
> && TREE_CODE (TREE_OPERAND (x, 0)) == CONSTRUCTOR
> && TREE_TYPE (TREE_OPERAND (x, 0)) == TREE_TYPE (x))
> x = TREE_OPERAND (x, 0);
> }
> return cp_fold_rvalue (x);
> Is there a reason to call that maybe_constant_value at all when we've called
> cxx_eval_constant_expression first? Wouldn't cp_fold_rvalue (or
> c_fully_fold with false as last argument) be sufficient there?
I think that would be better, yes.
Jason
next prev parent reply other threads:[~2018-12-21 2:49 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-12-12 22:30 Jakub Jelinek
2018-12-19 10:19 ` Patch ping (Re: [C++ PATCH] Fix __builtin_{is_constant_evaluated,constant_p} handling in static_assert (PR c++/86524, PR c++/88446)) Jakub Jelinek
2018-12-20 19:49 ` [C++ PATCH] Fix __builtin_{is_constant_evaluated,constant_p} handling in static_assert (PR c++/86524, PR c++/88446) Jason Merrill
2018-12-20 21:26 ` Jakub Jelinek
2018-12-20 21:28 ` Jason Merrill
2018-12-20 21:43 ` Jakub Jelinek
2018-12-20 21:47 ` Jason Merrill
2018-12-20 22:41 ` Jakub Jelinek
2018-12-21 2:51 ` Jason Merrill [this message]
2018-12-21 9:05 ` [C++ PATCH] Fix __builtin_{is_constant_evaluated,constant_p} handling in static_assert (PR c++/86524, PR c++/88446, take 2) Jakub Jelinek
2018-12-21 19:31 ` Jason Merrill
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