From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from gate.crashing.org (gate.crashing.org [63.228.1.57]) by sourceware.org (Postfix) with ESMTP id 591883856DFB for ; Thu, 4 Aug 2022 20:56:17 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 591883856DFB Authentication-Results: sourceware.org; dmarc=none (p=none dis=none) header.from=kernel.crashing.org Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=kernel.crashing.org Received: from gate.crashing.org (localhost.localdomain [127.0.0.1]) by gate.crashing.org (8.14.1/8.14.1) with ESMTP id 274KrwYK001397; Thu, 4 Aug 2022 15:53:58 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 274Kruhc001396; Thu, 4 Aug 2022 15:53:56 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Thu, 4 Aug 2022 15:53:55 -0500 From: Segher Boessenkool To: Michael Meissner , GCC Development , "Kewen.Lin" , David Edelsohn , Peter Bergner , Will Schmidt , Jason Merrill , Nathan Sidwell , Mike Stump , Iain Sandoe , Joseph Myers , Tulio Magno Quites Machado Filho , Alan Modra , Nick Clifton , Jeff Law , Jakub Jelinek , Richard Biener , "David S. Miller" , "Carlos O'Donell" , libc-alpha@sourceware.org Subject: Re: Resend: Potential upcoming changes in mangling to PowerPC GCC Message-ID: <20220804205355.GO25951@gate.crashing.org> References: Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.4.2.3i X-Spam-Status: No, score=-3.0 required=5.0 tests=BAYES_00, JMQ_SPF_NEUTRAL, KAM_DMARC_STATUS, KAM_MANYTO, SPF_HELO_PASS, SPF_PASS, TXREP autolearn=no 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@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 04 Aug 2022 20:56:19 -0000 Hi! On Thu, Aug 04, 2022 at 01:48:51PM -0400, Michael Meissner wrote: > At the moment, GCC 12 on the server PowerPC systems supports multiple 128-bit > floating point types: > > * _Float128 (in the C language): IEEE 128-bit floating point; > > * __float128 (in the C and C++ languages): IEEE 128-bit floating point; > > * long double: One of IEEE 128-bit floating, IBM 128-bit floating point, > or 64-bit floating point; (and) > > * __ibm128: Explicit IBM 128-bit floating point. And __ieee128, which (unlike __float128) explicitly is IEEE QP float (and as a bonus that is obvious in every context, too). > If a file is compiled when long double uses the IEEE 128-bit floating point > type, then the __float128 type is the long double type and it uses the TFmode > mode. And while the _Float128 type is distinct from long double, it also uses > TFmode. The __ibm128 type is distinct, and it uses IFmode. It would be a lot simpler and less roundabout and inside out if we could do this the other way around: start with the QP float and double-double types and modes, and point the long double type and TFmode at that. But alas. > While things mostly work with this setup, there are some things that don't work > as well. For example, 3 of the tests fail when you are using a system like > Fedora 36 where IEEE 128-bit long double is default. These 3 tests use the > 'nanqs' built-in function, which is mapped to 'nanf128s' and it delivers a > _Float128 signaling NaN. But since __float128 uses a different type, the > signaling NaN is converted and it loses the signaling property. So you are saying __float128 and _Float128 should *not* be separate types? Or, the testcases are buggy, make unwarranted assumptions? > In addition, it would be nice if we could refine the setting of bits in the ELF > header so that if you pass an explicit __float128 or __ibm128 object, it > doesn't set the bits that you used long double of the appropriate type. But > the code that sets these bits is done in the RTL stages, and it only looks at > modes, not at types. So fix that? It is a clear bug. > Now, I'm working on patches to 'do the right thing': > > * Make _Float128 and __float128 always use the same distinct type and > always use KFmode; > > * Make __ibm128 use a distinct type and always use IFmode; (and) It cannot always use IFmode? Generic code uses TFmode for long double (which can be double-double). > Because long double mangles the same as either __float128 or __ibm128, you > cannot write programs like: > > double convert (__ibm128 x) { return x; } > double convert (__float128 x) { return x; } > double convert (long double x) { return x; } I proposed separate mangling for long double, all those years ago, but long double is the same type as either __ibm128 or __ieee128, always. Mangling it differently only causes huge complications and it solves nothing. > At the moment, the mangling rules are: > > * If the type uses the IBM 128-bit encoding, use "g" for mangling; > > * If the type uses the IEEE 128-bit encoding, use "u9__ieee128" for > mangling. > > I would suggest at least adding the rule: > > * If the type is explicitly __ibm128, use "u8__ibm128" for the mangling, > and if it is long double that uses the IBM 128-bit encoding, continue > to use "g" for the mangling. That is wrong. The same type should mangle the same in all cases. > But in changing the mangling, we have the potential to create compatibility > issues, s/potential to/surety this will/ Please open PRs for the broken testcases (one for each, unless of course you are confident they are the same problems: it is much easier to join PRs than to split them). Segher