From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on inbox.vuxu.org X-Spam-Level: X-Spam-Status: No, score=-3.1 required=5.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,RCVD_IN_DNSWL_MED,RCVD_IN_MSPIKE_H4, RCVD_IN_MSPIKE_WL,T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.4 Received: from second.openwall.net (second.openwall.net [193.110.157.125]) by inbox.vuxu.org (Postfix) with SMTP id E4FC0217F0 for ; Wed, 28 Aug 2024 22:15:28 +0200 (CEST) Received: (qmail 31820 invoked by uid 550); 28 Aug 2024 20:15:24 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Reply-To: musl@lists.openwall.com Received: (qmail 31785 invoked from network); 28 Aug 2024 20:15:23 -0000 Date: Wed, 28 Aug 2024 16:15:14 -0400 From: Rich Felker To: Alexander Monakov Cc: musl@lists.openwall.com, Alex =?utf-8?B?UsO4bm5l?= Petersen Message-ID: <20240828201514.GH10433@brightrain.aerifal.cx> References: <20240828152826.826990-1-alex@alexrp.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Subject: Re: [musl] [PATCH] configure: prevent compilers from turning a * b + c into fma(a, b, c) On Wed, Aug 28, 2024 at 06:53:30PM +0300, Alexander Monakov wrote: > > On Wed, 28 Aug 2024, Alex Rønne Petersen wrote: > > > I've seen Clang do this for expressions in the fma() implementation itself, > > which of course led to infinite recursion. This happened when targeting > > arm-linux-musleabi with full soft float mode and -march=armv8-a. I imagine > > it's possible for GCC to do similar silliness. > > musl passes -std=c99 to the compiler, and in all GCC releases so far* that > disables FMA contraction (as opposed to -std=gnu99 or whichever -std=gnuXX > is enabled by default, where unrestricted contraction is implicitly enabled, > i.e. the non-standard and dangerous -ffp-contract=fast mode). > > Clang respects #pragma STDC FP_CONTRACT OFF, so that is available as > a smaller hammer than disabling fma across the board. Breaking up > contractable expression in fma*.c will work too. > > [*] maybe modulo bugs in old releases where the backend doesn't respect > -ffp-contract=off and which Glibc worked around with -mno-fused-madd. I would rather havee it globally off, as the intent is that the code be fully deterministic for a given ABI (excess precision being the main thing that could differ by ABI), not randomly varying according to how a compiler decides to optimize. With that said, I don't see how the compiler could ever generate calls to fma(), since we're -ffreestanding, but I think it could generate inline fma instructions on targets where they're available, and this is generally undesirable (see above). For gcc, I think -std=c99 already has that covered, but I'm not sure what clang does. Rich