cygwin 3.5.4-1: signal handling destroys 'long double' values

[Christian Franke]

Takashi Yano via Cygwin wrote:
> 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 AIUI x87 FP handling is outdated and mainly unused on current systems, as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@cygwin.com mailing list.
> https://cygwin.com/pipermail/cygwin-patches/2024q4/012804.html

The patch fixes the testcases from my original post on a CPU with XSAFE 
support and also if usage of FXSAVE if forced by this hack:

         movl    \$1,%eax
         cpuid
-       andl    \$0x04000000,%ecx # xsave available?
+       andl    \$0x00000000,%ecx # xsave available?
         jnz     1f

-- 
Regards,
Christian