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Date: Sun, 13 Oct 2024 08:14:07 +0900
To: cygwin AT cygwin DOT com
Subject: Re: cygwin 3.5.4-1: signal handling destroys 'long double' values
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From: Takashi Yano via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>
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Hi Brian,

On Tue, 8 Oct 2024 10:37:14 -0600
Brian Inglis wrote:
> On 2024-10-08 10:14, Brian Inglis via Cygwin wrote:
> > On 2024-10-08 05:20, Takashi Yano via Cygwin wrote:
> >> On Mon, 7 Oct 2024 15:11:52 +0200
> >> Christian Franke wrote:
> >>> $ gcc -o sigtest -O2 sigtest.c
> >>>
> >>> $ ./sigtest > out.txt
> >>> (press ^C 42x :-)
> >>>
> >>> $ sort out.txt | uniq -c
> >>>         3 x = 0x1.23456789p+0, y = -nan, d = -nan
> >>>         6 x = 0x1.23456789p+0, y = 0x1.23456789p+0, d = -nan
> >>>        33 x = 0x1.23456789p+0, y = 0x1.23456789p+0, d = 0x0p+0
> >>>
> >>> The problem also occurs if compiled without -O2, but less often. No
> >>> problem occurs if compiled with -DWORKS which suggests that only 'long
> >>> double' is affected.
> >>
> >> Thanks for the report. I looked into this problem and might find the
> >> cause. It seems due to a bug of scripts/gendef. It generates signal
> >> handler caller (sigfe.s) which stores/restores the registers.
> >>
> >> In sigdelayed, control word is stored/restored by fnstcw/fldcw instruction,
> >> however, fninit instruction destroys some status registers in FPU (x87).
> >>
> >> I think we shold use fnstenv/fldenv rather than fnstcw/fldcw and fninit.
> >> However, I'm not familiar with x87 instructions, so I may overlook
> >> something.
> >>
> >> Could anyone expert of x87 instructions and sigfe stuff give some
> >> comments?
> > 
> > AIUI x87 FP handling is outdated and mainly unused on current systems, as 
> > current systems do more and use more than the legacy x87 instructions and stack.
> > 
> > See https://en.cppreference.com/w/c/numeric/fenv and related docs for more 
> > modern approaches.
> > 
> > You would have to look into the AMD/Intel/IEEE docs for lower level details.
> 
> This is basically what ISTR:
> 
> https://beta.boost.org/doc/libs/1_82_0/libs/context/doc/html/context/rationale/x86_and_floating_point_env.html
> 
> where legacy x87 and MMX registers are not used or preserved on x86_64/amd64, as 
> SSE... instructions and XMM registers are used.

Thanks for the advice. I read throuh the web pages and related documents
and made a patch which uses fxsave/fxrstor and xsave/xrstror to
cygwin-patches AT cygwin DOT com mailing list.
https://cygwin.com/pipermail/cygwin-patches/2024q4/012804.html

Is this as you intended?

-- 
Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>

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