From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
	id 649FF3858C50; Mon, 25 Mar 2024 11:52:49 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 649FF3858C50
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
	s=default; t=1711367569;
	bh=avAA6e/uPEVkj5f6jo9k9YyZfAJrTYCDlK67VUzOfS8=;
	h=From:To:Subject:Date:In-Reply-To:References:From;
	b=m7OZhAuEtHF8Fs0a2Jn/CHU4E4HxhTVEIQeZE4X/JagJJmNNK5lmQnqPX56qHyba8
	 poevW5LLx58I+jG0VSWCGuGG6X/FtRDd1LsdV6X9kViSGpZG9lQBQgd6z449g1kUiN
	 lnusypAuy4w3Ca31F6MXOyjOH1sKxohhkOL302U0=
From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/114462] [C++26] P2809R3 - Trivial infinite loops are not
 undefined behavior
Date: Mon, 25 Mar 2024 11:52:48 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 14.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: jakub at gcc dot gnu.org
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: cc
Message-ID: <bug-114462-4-QbdGoDvgb9@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-114462-4@http.gcc.gnu.org/bugzilla/>
References: <bug-114462-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
List-Id: <gcc-bugs.sourceware.org>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114462

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
I know, but I think this will need changes in the C++ FE.
I was asking recently Jason what the "when interpreted as a
constant-expression"
in the paper actually means, whether such expression should be evaluated as
manifestly constant evaluation or not.
If yes, it seems like an incompatible change in a DR, because say
#include <type_traits>
void
foo ()
{
  while (std::is_constant_evaluated ())
    ;
}
changing behavior from previous doing nothing to an infinite loop.
If it is not manifestly constant evaluation, still, the FE would need to (at
least for the cases listed in the paper) try to silently constant evaluate =
the
condition (with
mce_false such that std::is_constant_evaluated () is folded to false in the=
re;
but perhaps only during cp_fold_function?) and if that evaluates to non-zero
replace the condition with true, such that the middle-end would then really
consider it as infinite loop regardless of -ffinite-loops.
Because if one has
constexpr bool
bar ()
{
  return true;
}
void
baz ()
{
  while (bar ())
    ;
}
we want middle-end to see while (true) ; rather than while (bar ()) ; becau=
se
the latter would be under -ffinite-loops decision.=