From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 110693 invoked by alias); 11 Aug 2019 07:20:04 -0000 Mailing-List: contact gcc-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Id: List-Archive: List-Post: List-Help: Sender: gcc-owner@gcc.gnu.org Received: (qmail 110664 invoked by uid 89); 11 Aug 2019 07:20:03 -0000 Authentication-Results: sourceware.org; auth=none X-Spam-SWARE-Status: No, score=-4.7 required=5.0 tests=AWL,BAYES_00,GIT_PATCH_1,RCVD_IN_DNSWL_NONE autolearn=ham version=3.3.1 spammy=UD:builtins.def, faddl, narrower, builtinsdef X-HELO: gate.crashing.org Received: from gate.crashing.org (HELO gate.crashing.org) (63.228.1.57) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with ESMTP; Sun, 11 Aug 2019 07:20:01 +0000 Received: from gate.crashing.org (localhost.localdomain [127.0.0.1]) by gate.crashing.org (8.14.1/8.14.1) with ESMTP id x7B7JvNw025169; Sun, 11 Aug 2019 02:19:57 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id x7B7JuQX025166; Sun, 11 Aug 2019 02:19:56 -0500 Date: Sun, 11 Aug 2019 07:20:00 -0000 From: Segher Boessenkool To: Tejas Joshi Cc: gcc@gcc.gnu.org, Martin Jambor , hubicka@ucw.cz, joseph@codesourcery.com Subject: Re: Expansion of narrowing math built-ins into power instructions Message-ID: <20190811071955.GV31406@gate.crashing.org> References: <87sgqnx4i6.fsf@oldenburg2.str.redhat.com> <20190731144722.GS31406@gate.crashing.org> <20190808200514.GL31406@gate.crashing.org> <20190810164600.GT31406@gate.crashing.org> 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-IsSubscribed: yes X-SW-Source: 2019-08/txt/msg00085.txt.bz2 Hi Tejas, On Sun, Aug 11, 2019 at 10:34:26AM +0530, Tejas Joshi wrote: > > As far as I understand that flag should set the behaviour of the fadd > > function, not the __builtin_fadd one. So I don't know. > > According to ISO/IEC TS 18661, I am supposed to implement the fadd > variants for folding and expand them inline, that take double and long > double as arguments and return > addition in appropriate narrower type, float and double. As far as I > know, we use __builtin_ to call the internal functions? I do not know > which the only fadd function is. See the manual, section "Other Built-in Functions Provided by GCC": @opindex fno-builtin GCC includes built-in versions of many of the functions in the standard C library. These functions come in two forms: one whose names start with the @code{__builtin_} prefix, and the other without. Both forms have the same type (including prototype), the same address (when their address is taken), and the same meaning as the C library functions even if you specify the @option{-fno-builtin} option @pxref{C Dialect Options}). Many of these functions are only optimized in certain cases; if they are not optimized in a particular case, a call to the library function is emitted. > > double precision one. But instead you want to add two double precision > > numbers, producing a single precision one? The fadds instruction fits > > Yes. > > > well to that, but you'll have to check exactly how the fadd() function > > should behave with respect to rounding and exceptions and the like. I read 18661-1 now... and yup, "fadds" will work fine, and there are no complications like this as far as I see. For QP to either DP or SP, you can do round-to-odd followed by one of the conversion instructions. The ISA manual describes this; I can help you with it, but first get DP->SP (fadd) working? For the non-QP long doubles we have... There is the option of using DP for it, which isn't standard-compliant, many other archs have it too, and it is simple anyway, because you have all code for operations already. You can mostly just ignore this option. For double-double... Well firstly, double-double is on the way out, so adding new features to it is pretty useless? Just ignore it unless you have time left, I'd say. > In Joseph's initial mail that describes what should be carried out in > the course of project, about rounding and exceptions. I have strictly > followed this description for my folding patch : > > * The narrowing functions, e.g. fadd, faddl, daddl, are a bit different > from most other built-in math.h functions because the return type is > different from the argument types. You could start by adding them to > builtins.def similarly to roundeven (with new macros to handle adding such > functions for relevant pairs of _FloatN, _FloatNx types). These functions > could be folded for constant arguments only if the result is exact, or if > -fno-rounding-math -fno-trapping-math (and -fno-math-errno if the result > involves overflow / underflow). For Power, all five basic operations (add, sub, mul, div, fma) work fine wrt rounding mode if using the fadds etc. insns, for DP->SP. All exceptions work as expected, except maybe underflow and overflow, but 18661 doesn't require much at all for those anyway :-) Segher