[{"id":3680371,"web_url":"http://patchwork.ozlabs.org/comment/3680371/","msgid":"","list_archive_url":null,"date":"2026-04-22T09:23:02","subject":"Re: [PATCH v2] libstdc++: follow std in numeric_limits::traps\n and integral traps","submitter":{"id":48004,"url":"http://patchwork.ozlabs.org/api/people/48004/","name":"Jonathan Wakely","email":"jwakely@redhat.com"},"content":"On Wed, 22 Apr 2026 at 07:12, Alexandre Oliva wrote:\n>\n> On Apr 21, 2026, Jonathan Wakely wrote:\n>\n> > This is strictly speaking an ABI change for most targets, but I doubt\n> > anybody cares about numeric_limits::traps. I hope\n> > nobody is depending on it for anything.\n>\n> > Anybody who needs the old behaviour can compile with\n> > -D__glibcxx_integral_traps=1 if they really need it, so I think this\n> > is OK to fix ion stage 1.\n>\n> Given the ABI change argument, I decided to revert the bool\n> specialization to use the macro, so that -D__glibcxx_integral_traps=1\n> fully restores the earlier ABI properties.\n>\n> >> (should the otherwise-effectively-empty cpu_defines.h headers be deleted\n> >> altogether?)\n>\n> > I had to look into that, but it looks like we don't bother having an\n> > empty cpu_defines.h, and just use the default generic/cpu_defines.h\n> > file. So I think yes, they should be removed.\n>\n> ACK\n>\n> > I don't think we need such a detailed comment here (especially not\n> > referring to non-bool specializations).\n>\n> > We already have this comment above the whole explicit specialization,\n> > which justifies why it's there and what all its values are:\n>\n> > // _GLIBCXX_RESOLVE_LIB_DEFECTS\n> > // 184. numeric_limits wording problems\n>\n> > So I would just remove the comment on traps entirely.\n>\n> Fair enough. I tweaked the opening comment, and dropped the one next to\n> the specialization. Here's what I've just regstrapped on\n> x86_64-linux-gnu and powerpc64le-linux-gnu. Ok for stage1?\n\nOK for stage 1 - thanks!\n\n\n>\n>\n> There's a comment from 2002 suggesting that\n> numeric_limits::traps was in a DR, but C++ standards including\n> 11, 17 and 23 explicitly set it to false, presumably in response to\n> issue 184.\n>\n> Issue 554 clarifies that traps is about values that may trap, rather\n> than operations that may trap, so we were wrong in the interpretation\n> about divide-by-zero operations' trapping on integral types that led\n> to __glibcxx_integral_traps's defaulting to true, and some of its\n> overrides.\n>\n> Align numeric_limits::traps with the standard, default\n> __glibcxx_integral_traps to false, drop the overriders based on the\n> incorrect interpretation, but keep __glibcxx_integral_traps to allow\n> command-line restoring of this ABI fix, and for the admittedly\n> unlikely case of trapping integral values' coming to exist on some\n> architecture.\n>\n>\n> for libstdc++-v3/ChangeLog\n>\n> * include/std/limits (__glibcxx_integral_traps): Set to\n> false. Update comments.\n> (numeric_limits::traps): Drop comments.\n> * config/cpu/arm/cpu_defines.h: Remove.\n> * config/cpu/powerpc/cpu_defines.h: Likewise.\n> * configure.host (cpu_defines_dir): Adjust.\n> ---\n> libstdc++-v3/config/cpu/arm/cpu_defines.h | 40 -------------------------\n> libstdc++-v3/config/cpu/powerpc/cpu_defines.h | 36 -----------------------\n> libstdc++-v3/configure.host | 6 ----\n> libstdc++-v3/include/std/limits | 14 +++++----\n> 4 files changed, 8 insertions(+), 88 deletions(-)\n> delete mode 100644 libstdc++-v3/config/cpu/arm/cpu_defines.h\n> delete mode 100644 libstdc++-v3/config/cpu/powerpc/cpu_defines.h\n>\n> diff --git a/libstdc++-v3/config/cpu/arm/cpu_defines.h b/libstdc++-v3/config/cpu/arm/cpu_defines.h\n> deleted file mode 100644\n> index 57e4cbbe0136b..0000000000000\n> --- a/libstdc++-v3/config/cpu/arm/cpu_defines.h\n> +++ /dev/null\n> @@ -1,40 +0,0 @@\n> -// Specific definitions for generic platforms -*- C++ -*-\n> -\n> -// Copyright (C) 2015-2026 Free Software Foundation, Inc.\n> -//\n> -// This file is part of the GNU ISO C++ Library. This library is free\n> -// software; you can redistribute it and/or modify it under the\n> -// terms of the GNU General Public License as published by the\n> -// Free Software Foundation; either version 3, or (at your option)\n> -// any later version.\n> -\n> -// This library is distributed in the hope that it will be useful,\n> -// but WITHOUT ANY WARRANTY; without even the implied warranty of\n> -// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the\n> -// GNU General Public License for more details.\n> -\n> -// Under Section 7 of GPL version 3, you are granted additional\n> -// permissions described in the GCC Runtime Library Exception, version\n> -// 3.1, as published by the Free Software Foundation.\n> -\n> -// You should have received a copy of the GNU General Public License and\n> -// a copy of the GCC Runtime Library Exception along with this program;\n> -// see the files COPYING3 and COPYING.RUNTIME respectively. If not, see\n> -// .\n> -\n> -/** @file bits/cpu_defines.h\n> - * This is an internal header file, included by other library headers.\n> - * Do not attempt to use it directly. @headername{iosfwd}\n> - */\n> -\n> -#ifndef _GLIBCXX_CPU_DEFINES\n> -#define _GLIBCXX_CPU_DEFINES 1\n> -\n> -// Integer divide instructions don't trap on ARM.\n> -#ifdef __ARM_ARCH_EXT_IDIV__\n> -#define __glibcxx_integral_traps false\n> -#else\n> -#define __glibcxx_integral_traps true\n> -#endif\n> -\n> -#endif\n> diff --git a/libstdc++-v3/config/cpu/powerpc/cpu_defines.h b/libstdc++-v3/config/cpu/powerpc/cpu_defines.h\n> deleted file mode 100644\n> index f4248e0f602a3..0000000000000\n> --- a/libstdc++-v3/config/cpu/powerpc/cpu_defines.h\n> +++ /dev/null\n> @@ -1,36 +0,0 @@\n> -// Specific definitions for generic platforms -*- C++ -*-\n> -\n> -// Copyright (C) 2005-2026 Free Software Foundation, Inc.\n> -//\n> -// This file is part of the GNU ISO C++ Library. This library is free\n> -// software; you can redistribute it and/or modify it under the\n> -// terms of the GNU General Public License as published by the\n> -// Free Software Foundation; either version 3, or (at your option)\n> -// any later version.\n> -\n> -// This library is distributed in the hope that it will be useful,\n> -// but WITHOUT ANY WARRANTY; without even the implied warranty of\n> -// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the\n> -// GNU General Public License for more details.\n> -\n> -// Under Section 7 of GPL version 3, you are granted additional\n> -// permissions described in the GCC Runtime Library Exception, version\n> -// 3.1, as published by the Free Software Foundation.\n> -\n> -// You should have received a copy of the GNU General Public License and\n> -// a copy of the GCC Runtime Library Exception along with this program;\n> -// see the files COPYING3 and COPYING.RUNTIME respectively. If not, see\n> -// .\n> -\n> -/** @file bits/cpu_defines.h\n> - * This is an internal header file, included by other library headers.\n> - * Do not attempt to use it directly. @headername{iosfwd}\n> - */\n> -\n> -#ifndef _GLIBCXX_CPU_DEFINES\n> -#define _GLIBCXX_CPU_DEFINES 1\n> -\n> -// Integer divide instructions don't trap on PowerPC.\n> -#define __glibcxx_integral_traps false\n> -\n> -#endif\n> diff --git a/libstdc++-v3/configure.host b/libstdc++-v3/configure.host\n> index ff97216167dec..e52a32bb4c682 100644\n> --- a/libstdc++-v3/configure.host\n> +++ b/libstdc++-v3/configure.host\n> @@ -150,15 +150,9 @@ case \"${host_cpu}\" in\n> amdgcn)\n> cpu_defines_dir=cpu/gcn\n> ;;\n> - arm*)\n> - cpu_defines_dir=cpu/arm\n> - ;;\n> nvptx)\n> cpu_defines_dir=cpu/nvptx\n> ;;\n> - powerpc* | rs6000)\n> - cpu_defines_dir=cpu/powerpc\n> - ;;\n> esac\n>\n>\n> diff --git a/libstdc++-v3/include/std/limits b/libstdc++-v3/include/std/limits\n> index f0ba2851a0a2e..cfdbe02c844c2 100644\n> --- a/libstdc++-v3/include/std/limits\n> +++ b/libstdc++-v3/include/std/limits\n> @@ -81,10 +81,15 @@\n> // The default values are appropriate for many 32-bit targets.\n>\n> // GCC only intrinsically supports modulo integral types. The only remaining\n> -// integral exceptional values is division by zero. Only targets that do not\n> -// signal division by zero in some \"hard to ignore\" way should use false.\n> +// integral exceptional values is division by zero, but that's an operation,\n> +// not a value, and traps is about values that trap (Issue554), so there aren't\n> +// any for integral types. The standard has numeric_limits::traps\n> +// explicitly set to false. However, we used to set them all to true based on\n> +// the division-by-zero misinterpretation, so we retain this macro, and those\n> +// who relied on the misinterpretation can restore it with a command-line\n> +// define and revert the ABI-changing effects of this fix.\n> #ifndef __glibcxx_integral_traps\n> -# define __glibcxx_integral_traps true\n> +# define __glibcxx_integral_traps false\n> #endif\n>\n> // float\n> @@ -446,9 +451,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n> static _GLIBCXX_USE_CONSTEXPR bool is_bounded = true;\n> static _GLIBCXX_USE_CONSTEXPR bool is_modulo = false;\n>\n> - // It is not clear what it means for a boolean type to trap.\n> - // This is a DR on the LWG issue list. Here, I use integer\n> - // promotion semantics.\n> static _GLIBCXX_USE_CONSTEXPR bool traps = __glibcxx_integral_traps;\n> static _GLIBCXX_USE_CONSTEXPR bool tinyness_before = false;\n> static _GLIBCXX_USE_CONSTEXPR float_round_style round_style\n>\n> --\n> Alexandre Oliva, happy hacker https://blog.lx.oliva.nom.br/\n> Free Software Activist FSFLA co-founder GNU Toolchain Engineer\n> More tolerance and less prejudice are key for inclusion and diversity.\n> Excluding neuro-others for not behaving \"\"normal\"\" is *not* inclusive!\n>","headers":{"Return-Path":"","X-Original-To":["incoming@patchwork.ozlabs.org","gcc-patches@gcc.gnu.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","gcc-patches@gcc.gnu.org"],"Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (1024-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=fc+82ZjR;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=gcc.gnu.org\n (client-ip=2620:52:6:3111::32; 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Wed, 22 Apr 2026\n 02:23:18 -0700 (PDT)"],"MIME-Version":"1.0","References":"\n \n \n \n ","In-Reply-To":"","From":"Jonathan Wakely ","Date":"Wed, 22 Apr 2026 10:23:02 +0100","X-Gm-Features":"AQROBzCmMVI81kjT1dXy22_aU0qe9B7kwZH8aX1fnaez9mpv0VJHd1iG8pqn8io","Message-ID":"\n ","Subject":"Re: [PATCH v2] libstdc++: follow std in numeric_limits::traps\n and integral traps","To":"Alexandre Oliva ","Cc":"gcc-patches@gcc.gnu.org, libstdc++@gcc.gnu.org","X-Mimecast-Spam-Score":"0","X-Mimecast-MFC-PROC-ID":"YYLtu1gmNhsbXO0SKd_80osRI8PKEbWP64Dg59n3rzI_1776849799","X-Mimecast-Originator":"redhat.com","Content-Type":"text/plain; charset=\"UTF-8\"","X-BeenThere":"gcc-patches@gcc.gnu.org","X-Mailman-Version":"2.1.30","Precedence":"list","List-Id":"Gcc-patches mailing list ","List-Unsubscribe":",\n ","List-Archive":"","List-Post":"","List-Help":"","List-Subscribe":",\n ","Errors-To":"gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org"}}]