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[2001:8b0:bf73:93f7::51bb:e332]) by smtp.gmail.com with ESMTPSA id s8-20020a7bc388000000b00412260889d9sm2879104wmj.1.2024.02.16.09.22.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 16 Feb 2024 09:22:54 -0800 (PST) Received: from jsm28 (helo=localhost) by digraph.polyomino.org.uk with local-esmtp (Exim 4.95) (envelope-from ) id 1rb1uo-002f3w-3G; Fri, 16 Feb 2024 17:22:22 +0000 Date: Fri, 16 Feb 2024 17:22:22 +0000 (UTC) From: Joseph Myers To: Vincent Lefevre cc: libc-alpha@sourceware.org Subject: Re: document the fact that "Known Maximum Errors" might not be maximal In-Reply-To: <20240216094334.GA3653@qaa.vinc17.org> Message-ID: References: <20240216094334.GA3653@qaa.vinc17.org> MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=US-ASCII X-Spam-Status: No, score=-4.1 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,T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: On Fri, 16 Feb 2024, Vincent Lefevre wrote: > BTW, the manual contains > > fmaf - - - - - > fma - - - - - > fmal - - - - - > fmaf128 - - - - - > fma_ldoublef - - - - - > fma_ldouble - - - - - > fma_ldoublel - - - - - > fma_ldoublef128 - - - - - > > However, correct rounding is required for these functions. IBM long double doesn't have such a thing as correct rounding, so we allow errors of up to 3ulp (the documented accuracy for IBM long double division) for functions expected to be correctly rounded for otehr formats. > And I can't see any reference to f128, ldoublef, etc. in the manual > except in this table. See @deftypefunx _FloatN fmafN (_Float@var{N} @var{x}, _Float@var{N} @var{y}, _Float@var{N} @var{z}) @deftypefunx _FloatNx fmafNx (_Float@var{N}x @var{x}, _Float@var{N}x @var{y}, _Float@var{N}x @var{z}) in the manual for fmaf128 (which should always be correctly rounded, since _Float128 is always IEEE binary128). The "_ldouble" suffix is how libm-test-ulps indicates the argument type for narrowing functions that are parametrized by two types (that is, it's an implementation detail that ideally wouldn't appear in the manual at all). IBM long double arguments (thus, float or double returns - functions {f,d}{add,sub,mul,div,sqrt,fma}l) is the only case where the narrowing functions should ever have results that are not correctly rounded. -- Joseph S. Myers josmyers@redhat.com