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

[Takashi Yano]

On Sun, 13 Oct 2024 11:55:46 +0200
Christian Franke wrote:
> 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 AIUI x87 FP handling is outdated and mainly unused on current systems, as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

Thanks for testing!

-- 
Takashi Yano <takashi.yano@nifty.ne.jp>