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Re: Emacs using SSE2 on x86

From: Sven Joachim
Subject: Re: Emacs using SSE2 on x86
Date: Sat, 04 Jul 2020 20:58:34 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.0.91 (gnu/linux)

On 2020-07-04 09:27 -0700, Paul Eggert wrote:

> Bug#42147 reports that Emacs floating-point rounds differently on x86 than on
> all other platforms, and in its Message #104 I propose a patch to fix this by
> using GCC's -msse2 -mfpmath=sse options. This patch is needed on GNU/Linux
> systems; it should be irrelevant on macOS (where SSE2 has always been 
> required)
> or on MS-Windows development systems (where SSE2 has been the default since 
> 2012).
> The patch cajoles GCC and similar compilers into using 64-bit floating point
> instead of erratically substituting 80-bit floating point. Although using 80
> bits can improve accuracy, it means the numbers disagree with other platforms
> and (as Bug#42147 notes) this is more of a problem for Emacs than any lost 
> accuracy.
> The patch relies on SSE2 instructions on the x86, which were introduced in 
> 2000
> and which have by now become universal. I don't see a downside of requiring 
> SSE2
> for GCC etc., because as far as I can see, in the GNU/Linux world the only
> people running 32-bit Emacs on Intelish platforms nowadays are using AMD64 
> Linux
> kernels and compiling with 'gcc -m32', and AMD64 requires SSE2 anyway. 
> However,
> I thought I'd mention this on emacs-devel in case I'm missing something.

The Debian i386 port still uses older machines without SSE2 as its
baseline, and apparently there are some people who still use Pentium III
or Athlon XP machines, even for web browsing[1].

Whether you want to support them is up to you, but in general Emacs runs
very well even on such old hardware.


1. https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=877445

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