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Date: Tue, 8 Oct 2024 10:37:14 -0600
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Subject: Re: cygwin 3.5.4-1: signal handling destroys 'long double' values
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From: Brian Inglis via Cygwin <cygwin@cygwin.com>
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Cc: Brian Inglis <Brian.Inglis@SystematicSW.ab.ca>
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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 AIUI x87 FP handling is outdated and mainly unused on current systems, as 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.

-- 
Take care. Thanks, Brian Inglis              Calgary, Alberta, Canada

La perfection est atteinte                   Perfection is achieved
non pas lorsqu'il n'y a plus rien à ajouter  not when there is no more to add
mais lorsqu'il n'y a plus rien à retirer     but when there is no more to cut
                                 -- Antoine de Saint-Exupéry

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