bug#40406: python-matplotlib fails to build on i686-linux

[Diego Nicola Barbato]

Hey,

Diego Nicola Barbato <dnbarbato@posteo.de> writes:

[...]

> Apparently there is nothing wrong with the slider.  Instead matrix
> multiplication, which is used under the hood for transformations, seems
> to sometimes produce incorrect results on i686-linux.  I have reported
> this as a separate bug (https://debbugs.gnu.org/41665).

I got this wrong: This issue isn't caused by the Numpy bug, since
Matplotlib doesn't use Numpy for transformations.  Both bugs are caused
by the excess precision of the x87 FPU's floating point registers.

I've attached a patch which makes sure that the C and C++ extensions are
compiled with -ffloat-store.  This doesn't get rid of all possible
rounding errors but it's enough for the slider test to pass.

[...]

Regards,

Diego

>From 7c60615e29a1aab7922139183d191cc8cedd5c7f Mon Sep 17 00:00:00 2001
From: Diego Nicola Barbato <dnbarbato@posteo.de>
Date: Mon, 8 Jun 2020 02:31:17 +0200
Subject: [PATCH] gnu: python-matplotlib: Fix rounding errors on x86 CPUs.

Fixes <https://debbugs.gnu.org/40406>.
Reported by Diego Nicola Barbato <dnbarbato@posteo.de>.

* gnu/packages/python-xyz.scm (python-matplotlib)[arguments]: Set the
environment variable CFLAGS to -ffloat-store.
---
 gnu/packages/python-xyz.scm | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/gnu/packages/python-xyz.scm b/gnu/packages/python-xyz.scm
index 94e63d1c74..f0b96c6fb0 100644
--- a/gnu/packages/python-xyz.scm
+++ b/gnu/packages/python-xyz.scm
@@ -81,6 +81,7 @@
 ;;; Copyright © 2020 Josh Holland <josh@inv.alid.pw>
 ;;; Copyright © 2020 Yuval Kogman <nothingmuch@woobling.org>
 ;;; Copyright © 2020 Michael Rohleder <mike@rohleder.de>
+;;; Copyright © 2020 Diego N. Barbato <dnbarbato@posteo.de>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -4744,6 +4745,10 @@ convert between colorspaces like sRGB, XYZ, CIEL*a*b*, 
CIECAM02, CAM02-UCS, etc.
                ;; has not effect.
                (setenv "LD_LIBRARY_PATH" (string-append cairo "/lib"))
                (setenv "HOME" (getcwd))
+               ;; Fix rounding errors when using the x87 FPU.
+               ,@(if (string-prefix? "i686" (%current-system))
+                     '((setenv "CFLAGS" "-ffloat-store"))
+                     '())
                (call-with-output-file "setup.cfg"
                  (lambda (port)
                    (format port "[directories]~%
-- 
2.26.2