From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [63.128.21.124]) by sourceware.org (Postfix) with ESMTP id 893103945C06 for ; Wed, 2 Sep 2020 20:55:18 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 893103945C06 Received: from mail-qk1-f197.google.com (mail-qk1-f197.google.com [209.85.222.197]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-16-2e4IhE4jPLOY3esDoX40oA-1; Wed, 02 Sep 2020 16:55:16 -0400 X-MC-Unique: 2e4IhE4jPLOY3esDoX40oA-1 Received: by mail-qk1-f197.google.com with SMTP id e63so250898qkd.14 for ; Wed, 02 Sep 2020 13:55:16 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=JPuBUZ0Ni0fa7z1D9S2KyO9Kree07brm/X3YKzIuhGs=; b=lW2QTuh8a8c3+ug5G1badDKjxkl9+Zk1ryflIXPfsw+Rrg3ExXqTw119c+yXZi8CsD xsDOMPYIubjC79zfyvfxL0xY3ANXH79P0uUcqrqkJxqxryZqfi8xDfUnNM3UvX4Xm7Ax gY3VqU7kTSaUGK3q0jBgbV+J3W79o3b557M4sDh8Op0wiPj8KQOTZnO49mkn8QYjWScl vlBdcdGo2OAxmIyLUJrnN5hid33aowbMAkS7iNfY/cmshhQ4CHNtLoqsGgnrdNRCn9EQ mL6e8p7ZPh6olBgZ456X3Ld/Aed93O3l0tAjlhdegWSs3S+9lTYZ4TKnY2sjnYQwtRWJ sqag== X-Gm-Message-State: AOAM533JTNKRgjnoUMMqvO6lS5pbcSs1BBeOQtVJ7R2TtuLKU0sINAxm j/AD+uTs+eW7cfDmLcqfl084Thm8tVuoYxTzoRIEyfbNcuEssC3GNr+7q+hXRvvCi3N6eRN7L7q SydhlKw34gWWxO1+KrA== X-Received: by 2002:aed:3bb8:: with SMTP id r53mr67490qte.289.1599080116127; Wed, 02 Sep 2020 13:55:16 -0700 (PDT) X-Google-Smtp-Source: ABdhPJxU0jnazljbqdxwdHDJlDLja5nXjsjvATKXc9dZOxeVpNjlBC+a5J3uQH1G9HAacQ5O7hv6nw== X-Received: by 2002:aed:3bb8:: with SMTP id r53mr67462qte.289.1599080115703; Wed, 02 Sep 2020 13:55:15 -0700 (PDT) Received: from [192.168.1.148] (209-6-216-142.s141.c3-0.smr-cbr1.sbo-smr.ma.cable.rcncustomer.com. [209.6.216.142]) by smtp.gmail.com with ESMTPSA id 196sm623665qkm.49.2020.09.02.13.55.14 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 02 Sep 2020 13:55:15 -0700 (PDT) Subject: Re: [PATCH] c++: Disable -frounding-math during manifestly constant evaluation [PR96862] To: gcc-patches@gcc.gnu.org, Jakub Jelinek References: <20200901074946.GV18149@tucnak> From: Jason Merrill Message-ID: Date: Wed, 2 Sep 2020 16:55:14 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0.002 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Spam-Status: No, score=-10.0 required=5.0 tests=BAYES_00, BODY_8BITS, DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, NICE_REPLY_A, RCVD_IN_DNSWL_NONE, RCVD_IN_MSPIKE_H5, RCVD_IN_MSPIKE_WL, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: gcc-patches@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc-patches mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 02 Sep 2020 20:55:20 -0000 On 9/1/20 6:13 AM, Marc Glisse wrote: > On Tue, 1 Sep 2020, Jakub Jelinek via Gcc-patches wrote: > >> As discussed in the PR, fold-const.c punts on floating point constant >> evaluation if the result is inexact and -frounding-math is turned on. >>      /* Don't constant fold this floating point operation if the >>         result may dependent upon the run-time rounding mode and >>         flag_rounding_math is set, or if GCC's software emulation >>         is unable to accurately represent the result.  */ >>      if ((flag_rounding_math >>           || (MODE_COMPOSITE_P (mode) && >> !flag_unsafe_math_optimizations)) >>          && (inexact || !real_identical (&result, &value))) >>        return NULL_TREE; >> Jonathan said that we should be evaluating them anyway, e.g. conceptually >> as if they are done with the default rounding mode before user had a >> chance >> to change that, and e.g. in C in initializers it is also ignored. >> In fact, fold-const.c for C initializers turns off various other options: >> >> /* Perform constant folding and related simplification of initializer >>   expression EXPR.  These behave identically to "fold_buildN" but ignore >>   potential run-time traps and exceptions that fold must preserve.  */ >> >> #define START_FOLD_INIT \ >>  int saved_signaling_nans = flag_signaling_nans;\ >>  int saved_trapping_math = flag_trapping_math;\ >>  int saved_rounding_math = flag_rounding_math;\ >>  int saved_trapv = flag_trapv;\ >>  int saved_folding_initializer = folding_initializer;\ >>  flag_signaling_nans = 0;\ >>  flag_trapping_math = 0;\ >>  flag_rounding_math = 0;\ >>  flag_trapv = 0;\ >>  folding_initializer = 1; >> >> #define END_FOLD_INIT \ >>  flag_signaling_nans = saved_signaling_nans;\ >>  flag_trapping_math = saved_trapping_math;\ >>  flag_rounding_math = saved_rounding_math;\ >>  flag_trapv = saved_trapv;\ >>  folding_initializer = saved_folding_initializer; >> >> So, shall cxx_eval_outermost_constant_expr instead turn off all those >> options (then warning_sentinel wouldn't be the right thing to use, but >> given >> the 8 or how many return stmts in cxx_eval_outermost_constant_expr, we'd >> need a RAII class for this.  Not sure about the folding_initializer, that >> one is affecting complex multiplication and division constant evaluation >> somehow. > > I don't think we need to turn off flag_signaling_nans or flag_trapv. I > think we want to turn off flag_trapping_math so we can fold 1./0 to inf > (still in a context where folding is mandatory). Setting > folding_initializer seems consistent with that, enabling infinite > results in complex folding (it also forces folding of > __builtin_constant_p, which may be redundant with > force_folding_builtin_constant_p). C++ says that division by zero has undefined behavior, and that an expression with undefined behavior is not constant, so we shouldn't fold 1./0 to inf anyway. The same is true of other trapping operations. So clearing flag_signaling_nans, flag_trapping_math, and flag_trapv seems wrong for C++. And folding_initializer seems to be used for the same sort of thing. >> The following patch has been bootstrapped/regtested on x86_64-linux and >> i686-linux, but see above, maybe we want something else. >> >> 2020-09-01  Jakub Jelinek  >> >>     PR c++/96862 >>     * constexpr.c (cxx_eval_outermost_constant_expr): Temporarily disable >>     flag_rounding_math during manifestly constant evaluation. OK. >>     * g++.dg/cpp1z/constexpr-96862.C: New test. >> >> --- gcc/cp/constexpr.c.jj    2020-08-31 14:10:15.826921458 +0200 >> +++ gcc/cp/constexpr.c    2020-08-31 15:41:26.429964532 +0200 >> @@ -6680,6 +6680,8 @@ cxx_eval_outermost_constant_expr (tree t >>             allow_non_constant, strict, >>             manifestly_const_eval || !allow_non_constant }; >> >> +  /* Turn off -frounding-math for manifestly constant evaluation.  */ >> +  warning_sentinel rm (flag_rounding_math, ctx.manifestly_const_eval); >>   tree type = initialized_type (t); >>   tree r = t; >>   bool is_consteval = false; >> --- gcc/testsuite/g++.dg/cpp1z/constexpr-96862.C.jj    2020-08-31 >> 15:50:07.847473028 +0200 >> +++ gcc/testsuite/g++.dg/cpp1z/constexpr-96862.C    2020-08-31 >> 15:49:40.829861168 +0200 >> @@ -0,0 +1,20 @@ >> +// PR c++/96862 >> +// { dg-do compile { target c++17 } } >> +// { dg-additional-options "-frounding-math" } >> + >> +constexpr double a = 0x1.0p+100 + 0x1.0p-100; >> +const double b = 0x1.0p+100 + 0x1.0p-100; >> +const double &&c = 0x1.0p+100 + 0x1.0p-100; >> +static_assert (0x1.0p+100 + 0x1.0p-100 == 0x1.0p+100, ""); >> + >> +void >> +foo () >> +{ >> +  constexpr double d = 0x1.0p+100 + 0x1.0p-100; >> +  const double e = 0x1.0p+100 + 0x1.0p-100; >> +  const double &&f = 0x1.0p+100 + 0x1.0p-100; >> +  static_assert (0x1.0p+100 + 0x1.0p-100 == 0x1.0p+100, ""); >> +} >> + >> +const double &g = a; >> +const double &h = b; >> >>     Jakub >