From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mp2 ([2001:41d0:2:4a6f::]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)) by ms0.migadu.com with LMTPS id Qi7zDcM392DphQAAgWs5BA (envelope-from ) for ; Tue, 20 Jul 2021 22:53:23 +0200 Received: from aspmx1.migadu.com ([2001:41d0:2:4a6f::]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)) by mp2 with LMTPS id iHohCcM392CMGQAAB5/wlQ (envelope-from ) for ; Tue, 20 Jul 2021 20:53:23 +0000 Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by aspmx1.migadu.com (Postfix) with ESMTPS id 7D75714A1F for ; Tue, 20 Jul 2021 22:53:22 +0200 (CEST) Received: from localhost ([::1]:49034 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1m5wjx-0001xJ-DF for larch@yhetil.org; Tue, 20 Jul 2021 16:53:21 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:42246) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1m5wje-0001v2-PB for guix-patches@gnu.org; Tue, 20 Jul 2021 16:53:03 -0400 Received: from debbugs.gnu.org ([209.51.188.43]:52201) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1m5wje-000417-Hp for guix-patches@gnu.org; Tue, 20 Jul 2021 16:53:02 -0400 Received: from Debian-debbugs by debbugs.gnu.org with local (Exim 4.84_2) (envelope-from ) id 1m5wje-0002bR-Dk for guix-patches@gnu.org; Tue, 20 Jul 2021 16:53:02 -0400 X-Loop: help-debbugs@gnu.org Subject: [bug#49659] [PATCH core-updates] gnu: guile: Fix failing tests on i686-linux. Resent-From: Ludovic =?UTF-8?Q?Court=C3=A8s?= Original-Sender: "Debbugs-submit" Resent-CC: guix-patches@gnu.org Resent-Date: Tue, 20 Jul 2021 20:53:02 +0000 Resent-Message-ID: Resent-Sender: help-debbugs@gnu.org X-GNU-PR-Message: followup 49659 X-GNU-PR-Package: guix-patches X-GNU-PR-Keywords: patch To: Maxime Devos Cc: 49659@debbugs.gnu.org Received: via spool by 49659-submit@debbugs.gnu.org id=B49659.16268143309945 (code B ref 49659); Tue, 20 Jul 2021 20:53:02 +0000 Received: (at 49659) by debbugs.gnu.org; 20 Jul 2021 20:52:10 +0000 Received: from localhost ([127.0.0.1]:35514 helo=debbugs.gnu.org) by debbugs.gnu.org with esmtp (Exim 4.84_2) (envelope-from ) id 1m5win-0002aL-R1 for submit@debbugs.gnu.org; Tue, 20 Jul 2021 16:52:10 -0400 Received: from eggs.gnu.org ([209.51.188.92]:44228) by debbugs.gnu.org with esmtp (Exim 4.84_2) (envelope-from ) id 1m5wil-0002Zu-8R for 49659@debbugs.gnu.org; Tue, 20 Jul 2021 16:52:08 -0400 Received: from fencepost.gnu.org ([2001:470:142:3::e]:58508) by eggs.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1m5wif-0003PS-35; Tue, 20 Jul 2021 16:52:01 -0400 Received: from [2a01:e0a:1d:7270:af76:b9b:ca24:c465] (port=45238 helo=ribbon) by fencepost.gnu.org with esmtpsa (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1m5wie-0006Um-S0; Tue, 20 Jul 2021 16:52:01 -0400 From: Ludovic =?UTF-8?Q?Court=C3=A8s?= References: <20210720112712.25905-1-maximedevos@telenet.be> <871r7tks2i.fsf@gnu.org> Date: Tue, 20 Jul 2021 22:51:59 +0200 In-Reply-To: (Maxime Devos's message of "Tue, 20 Jul 2021 18:55:52 +0200") Message-ID: <87fsw8k8sw.fsf_-_@gnu.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.2 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-BeenThere: debbugs-submit@debbugs.gnu.org X-Mailman-Version: 2.1.18 Precedence: list X-BeenThere: guix-patches@gnu.org List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: guix-patches-bounces+larch=yhetil.org@gnu.org Sender: "Guix-patches" X-Migadu-Flow: FLOW_IN ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=yhetil.org; s=key1; t=1626814402; h=from:from:sender:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding:resent-cc: resent-from:resent-sender:resent-message-id:in-reply-to:in-reply-to: references:references:list-id:list-help:list-unsubscribe: list-subscribe:list-post; bh=O3QGN7XVtZhga7yHM/+kUawp+rz8V1t+GzCjRR4c8qo=; b=boA6HiHEVkxTpvEoemxOYKCHif3BuM6M5LKWEVKO/oZcmwiDnr7WrZ7l8fXI1aYkE/Mp9C 7iC8lmiPqwFQ/uYxLd+yy6fXCMbkQOuMEz3U44ednHMTFm79Q9G0rVKn0GyR/sOkJnIpsK bcNaJ1EBPpMyHEaGupeoURY76I2dZIKutbRXqsVWadMf8rusONaybHBG7WOdMNwVeTngo/ uen8xBsBH78+ofagGzZjpq47gTShWosZxen5xGtrDej+qcCT+U1iVB9wW2d0y0pLL7Bmpz Ize+AkzVLIq+Vk6cFcwOo7isk4U9BAPjFJeYJuPDwlLxr9+6T5tTvd5ZO4P04Q== ARC-Seal: i=1; s=key1; d=yhetil.org; t=1626814402; a=rsa-sha256; cv=none; b=aNw42XACXesj1vgR8UwObTNORgBgOpYSD3WjThTt+ydOiiN27M0lgOpXTrOCc5CjbAuopg gFfQy9hDcEyhJ98Vrdbf0OitWG7/FCYolXUxGv8BySstSE8hJSiJM2Wj5prM1s9JzH0rko EWSRMBRXyg97PcynZXy9WYdZXKMiHDFcaKrwCcu0wACufG+ILgk1SFbalgiCt1BQWeeiZH bWAO0UU2vFRioW55L24tEHji0Kii3v8ysq2GxH92l8kBtbT7hdLf+y4AbaHzYEzytDFAbZ IDXOu6QBn+3QQokcJbS/liA1qFjKL9b0vB07Z6xpRgjHaduvWp1Lls7jD9B/Eg== ARC-Authentication-Results: i=1; aspmx1.migadu.com; dkim=none; dmarc=pass (policy=none) header.from=gnu.org; spf=pass (aspmx1.migadu.com: domain of guix-patches-bounces@gnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom=guix-patches-bounces@gnu.org X-Migadu-Spam-Score: -2.92 Authentication-Results: aspmx1.migadu.com; dkim=none; dmarc=pass (policy=none) header.from=gnu.org; spf=pass (aspmx1.migadu.com: domain of guix-patches-bounces@gnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom=guix-patches-bounces@gnu.org X-Migadu-Queue-Id: 7D75714A1F X-Spam-Score: -2.92 X-Migadu-Scanner: scn0.migadu.com X-TUID: 2Tpfes3CPsvs Maxime Devos skribis: > Ludovic Court=C3=A8s schreef op di 20-07-2021 om 15:55 [+0200]: [...] >> 1. Should we make it conditional on >> (or (string-prefix? "i686-" %host-type) >> (string-prefix? "i586-" %host-type)) > > Rather, (target-x86-32?). target-x86-32? also recognises "i486-linux-gnu" > even though that's not a =E2=80=98supported=E2=80=99 cross-target. Yes, makes sense. >> ? (I wonder why armhf-linux doesn=E2=80=99t have the same problem.) > > AFAIK floats & doubles on arm don't have excess precision. > > Floating-point numbers are either 32-bit or 64-bit, > unlike in x86, where the floating-point registers are 80-bit > but (sizeof) double=3D=3D8 (64 bits). > > (I'm ignoring MMX, SSE and the like.) > > I don't know any architectures beside x86 which have excess precision. > "-fexcess-precision=3Dstandard" should be harmless on architectures > that don't have excess precision. > > I'd make it unconditional, but conditional on x86-target? should work > for all =E2=80=98supported=E2=80=99 targets in Guix. Alright. I=E2=80=99d still err on the side of making the change only for target-x86-= 32?, because that=E2=80=99s the only case where we know it=E2=80=99s needed. >> 2. Is there any downside to compiling all of libguile with this flag? > > I searched with "git grep -F double" and "git grep -F float". > Floating-point arithmetic doen't seem to be used much outside numbers.c. > > There's vm-engine.c, but the results of the calculations are written > to some (stack?) memory (not a register), so the excess precision > would be thrown away anyway, so I don't expect a slow-down. > > No code appears to be relying on excess precision. OK. >> 3. Do we have a clear explanation of why =E2=80=98-fexcess-precision= =3Dfast=E2=80=99 >> (the default) would lead to failures in =E2=80=98numbers.test=E2=80= =99? > > The problem I think is that the rounding choices made in > scm_i_inexact_floor_divide > must be consistent with those made in > scm_i_inexact_floor_quotient > and=20 > scm_i_inexact_floor_remainder > (There are tests testing whether the results agree.) > > "-fexcess-precision=3Dstandard" reduces the degrees of freedom GCC has > in choosing when to round, so it is more likely the rounding choices > coincide? It's only an untested hypothesis though. > > FWIW, I think this line: > > /* in scm_i_inexact_floor_remainder */ > return scm_i_from_double (x - y * floor (x / y)); > > should be (for less fragility in case GCC changes its optimisations and > register allocation / spilling) > > /* in scm_i_inexact_floor_remainder */ > return scm_i_from_double (x - y * (double) floor (x / y)); > > even then, there's no guarantee the rounding choices for x/y > are the same in scm_i_inexact_floor_divide, scm_i_inexact_floor_remainder > and scm_i_inexact_floor_quotient. Makes sense. Seems to me that this should simply be implemented differently to avoid the inconsistency in the first place (or one could ignore IA32 altogether=E2=80=A6). > I dunno if 'floor' is broken. Let me explain why this output is possible= for a > well-implemented 'floor': > > I want to note that printf("%f", floor(x/y)) > can display 16 different strings: > > GCC can choose to round 'x' and/or 'y' by moving it from a register to = stack memory. > (doesn't apply here I think because SCM values discard the excess preci= sion) > > GCC can choose to round the result of x/y (same reasons) > > GCC can choose to round the result of floor(x/y) (same reasons) > > Likewise, printf("%f", x/y) can display 8 different strings, and the roun= ding > choices may be different from those made for printf("%f", floor(x/y)). > > So this inconsistency (x/y=3D91.00... > 90.00 =3D floor(x/y)) doesn't re= ally > surprise me. A more concrete scenario: suppose the CPU is configured to = round > upwards, and 'floor' is a mapping between extended-precision floating-poi= nt numbers. > > Let 'x' and 'y' be two floating-point numbers, such that: > > (1) the mathematical value of 'x/y' is slightly less than 91 > (2) 'x/y' when calculated in extended precision is slightly less than 91. > Denote this approximation as F1. > (3) 'x/y' when calculated in double precision is 91 (or slightly larger) > (due to the =E2=80=98rounding upwards=E2=80=99 mode, in =E2=80=98rou= nding downwards=E2=80=99 it might > have been slightly less than 91 as in (2)) > Denote this approximation as F2. > > Then [floor(x/y)=3D] floor(F1)=3Dfloor(90.999...)=3D90.0, > and [x/y=3D] F2=3D91.0, thus we seemingly have x/y >=3D 1 + floor(x/y), > even though that's mathematically nonsense. > > Thus, by choosing when to round (in-)appropriately, it is possible (on x8= 6) > that printf("x/y=3D%f, floor(x/y)=3D%f",x/y,floor(x/y)) will output "x/y= =3D91.0 floor(x/y)=3D90.0". I=E2=80=99m no expert but that makes sense to me. Could you send an updated patch? If you think of a way to fix the issue in Guile itself, we can also do that. :-) Thanks for the investigation & explanation! Ludo=E2=80=99.