From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 48) id 6C67B3844B2B; Fri, 16 Dec 2022 21:14:37 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 6C67B3844B2B DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org; s=default; t=1671225277; bh=yvBGv9hNxpAVQJdL0Zf5VWMuel/eGUn8rjEB4quFF/0=; h=From:To:Subject:Date:In-Reply-To:References:From; b=U8dQwvcQbafb0oJyUdsL1lUzkgDl4fEhuEEZpN1IruUDva6FBeZcnvMh3TOP7+39/ 4dO/MgIlICrLnsU87bkAuqNngoTzDrCNVnjl5rlRtWzDBVjJBQ6wjAEqZcfVvItJar S6NcyNDWpipJ/1CN9RgrWFp6iDqi2F4jLDMt2N5U= From: "cvs-commit at gcc dot gnu.org" To: gcc-bugs@gcc.gnu.org Subject: [Bug c++/102780] Checking constraints using large fold expression is slow Date: Fri, 16 Dec 2022 21:14:37 +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: 12.0 X-Bugzilla-Keywords: compile-time-hog X-Bugzilla-Severity: normal X-Bugzilla-Who: cvs-commit at gcc dot gnu.org X-Bugzilla-Status: ASSIGNED X-Bugzilla-Resolution: X-Bugzilla-Priority: P3 X-Bugzilla-Assigned-To: ppalka at gcc dot gnu.org X-Bugzilla-Target-Milestone: --- X-Bugzilla-Flags: X-Bugzilla-Changed-Fields: Message-ID: In-Reply-To: References: 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: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102780 --- Comment #6 from CVS Commits --- The releases/gcc-10 branch has been updated by Patrick Palka : https://gcc.gnu.org/g:16605ad9b1e1a6c5aab2c98b5b0f995285584f81 commit r10-11124-g16605ad9b1e1a6c5aab2c98b5b0f995285584f81 Author: Patrick Palka Date: Thu Oct 28 10:05:14 2021 -0400 c++: quadratic constexpr behavior for left-assoc logical exprs [PR10278= 0] In the testcase below the two left fold expressions each expand into a constant logical expression with 1024 terms, for which potential_const_= expr takes more than a minute to return true. This happens because p_c_e_1 performs trial evaluation of the first operand of a &&/|| in order to determine whether to consider the potentiality of the second operand. And because the expanded expression is left-associated, this trial evaluation causes p_c_e_1 to be quadratic in the number of terms of the expression. This patch fixes this quadratic behavior by making p_c_e_1 preemptively compute potentiality of the second operand of a &&/||, and perform trial evaluation of the first operand only if the second operand isn't potentially constant. We must be careful to avoid emitting bogus diagnostics during the preemptive computation; to that end, we perform this shortcut only when tf_error is cleared, and when tf_error is set we now first check potentiality of the whole expression quietly and replay the check noisily for diagnostics. Apart from fixing the quadraticness for left-associated logical exprs, this change also reduces compile time for the libstdc++ testcase 20_util/variant/87619.cc by about 15% even though our uses right folds instead of left folds. Likewise for the testcase in the PR, for which compile time is reduced by 30%. The reason for these speedups is that p_c_e_1 no longer performs expensive trial evaluation of each t= erm of large constant logical expressions when determining their potentiali= ty. PR c++/102780 PR c++/108138 gcc/cp/ChangeLog: * constexpr.c (potential_constant_expression_1) : When tf_error isn't set, preemptively check potentiality of the second operand before performing trial evaluation of the first operand. (potential_constant_expression_1): When tf_error is set, first check potentiality quietly and return true if successful, otherwise proceed noisily to give errors. gcc/testsuite/ChangeLog: * g++.dg/cpp1z/fold13.C: New test. (cherry picked from commit 9927ecbb42d5be48fa933adc26f8601fab5007ca)=