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Tue, 29 Oct 2024 09:59:10 +0100 Date: Tue, 29 Oct 2024 09:59:10 +0100 From: Jakub Jelinek To: libstdc++@gcc.gnu.org Cc: gcc-patches@gcc.gnu.org Subject: [PATCH] libstdc++: Use if consteval rather than if (std::__is_constant_evaluated()) for {,b}float16_t nextafter [PR117321] Message-ID: MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Disposition: inline X-Spam-Status: No, score=-3.6 required=5.0 tests=BAYES_00, DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, RCVD_IN_DNSWL_NONE, RCVD_IN_MSPIKE_H2, SPF_HELO_NONE, SPF_NONE, TXREP, URI_HEX autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org X-BeenThere: gcc-patches@gcc.gnu.org X-Mailman-Version: 2.1.30 Precedence: list List-Id: Gcc-patches mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Jakub Jelinek Errors-To: gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org Hi! The nextafter_c++23.cc testcase fails to link at -O0. The problem is that eventhough std::__is_constant_evaluated() has always_inline attribute, that at -O0 just means that we inline the call, but its result is still assigned to a temporary which is tested later, nothing at -O0 propagates that false into the if and optimizes away the if body. And the __builtin_nextafterf16{,b} calls are meant to be used solely for constant evaluation, the C libraries don't define nextafterf16 these days. As __STDCPP_FLOAT16_T__ and __STDCPP_BFLOAT16_T__ are predefined right now only by GCC, not by clang which doesn't implement the extended floating point types paper, and as they are predefined in C++23 and later modes only, I think we can just use if consteval which is folded already during the FE and the body isn't included even at -O0. I've added a feature test for that just in case clang implements those and implements those in some weird way. Note, if (__builtin_is_constant_evaluted()) would work correctly too, that is also folded to false at gimplification time and the corresponding if block not emitted at all. But for -O0 it can't be wrapped into a helper inline function. Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk? 2024-10-29 Jakub Jelinek PR libstdc++/117321 * include/c_global/cmath (nextafter(_Float16, _Float16)): Use if consteval rather than if (std::__is_constant_evaluated()) around the __builtin_nextafterf16 call. (nextafter(__gnu_cxx::__bfloat16_t, __gnu_cxx::__bfloat16_t)): Use if consteval rather than if (std::__is_constant_evaluated()) around the __builtin_nextafterf16b call. * testsuite/26_numerics/headers/cmath/117321.cc: New test. Jakub --- libstdc++-v3/include/c_global/cmath.jj 2024-09-25 17:25:07.227360607 +0200 +++ libstdc++-v3/include/c_global/cmath 2024-10-28 13:10:46.348655046 +0100 @@ -2880,8 +2880,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION constexpr _Float16 nextafter(_Float16 __x, _Float16 __y) { - if (std::__is_constant_evaluated()) - return __builtin_nextafterf16(__x, __y); +#if __cpp_if_consteval >= 202106L + // Can't use if (std::__is_constant_evaluated()) here, as it + // doesn't guarantee optimizing the body away at -O0 and + // nothing defines nextafterf16. + if consteval { return __builtin_nextafterf16(__x, __y); } +#endif #ifdef __INT16_TYPE__ using __float16_int_type = __INT16_TYPE__; #else @@ -3598,8 +3602,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION constexpr __gnu_cxx::__bfloat16_t nextafter(__gnu_cxx::__bfloat16_t __x, __gnu_cxx::__bfloat16_t __y) { - if (std::__is_constant_evaluated()) - return __builtin_nextafterf16b(__x, __y); +#if __cpp_if_consteval >= 202106L + // Can't use if (std::__is_constant_evaluated()) here, as it + // doesn't guarantee optimizing the body away at -O0 and + // nothing defines nextafterf16b. + if consteval { return __builtin_nextafterf16b(__x, __y); } +#endif #ifdef __INT16_TYPE__ using __bfloat16_int_type = __INT16_TYPE__; #else --- libstdc++-v3/testsuite/26_numerics/headers/cmath/117321.cc.jj 2024-10-28 12:56:10.699876240 +0100 +++ libstdc++-v3/testsuite/26_numerics/headers/cmath/117321.cc 2024-10-28 12:56:59.875189173 +0100 @@ -0,0 +1,5 @@ +// { dg-do run { target c++23 } } +// { dg-require-cmath "" } +// { dg-additional-options "-O0" } + +#include "nextafter_c++23.cc"