Re: AVX on Cygwin

[Corinna Vinschen <corinna-cygwin@cygwin.com>]

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On Nov 27 10:32, Corinna Vinschen wrote:
> Hi Achim,
> 
> On Nov 26 19:17, Achim Gratz wrote:
> > 
> > You may have noted that the recent gmp update makes problems on some
> > machines and two of the three reports come from Broadwell CPU.  There is
> > one thing that did indeed change with the update and that is use of the
> > AVX ADC instruction on Broadwell/Skylake.  Is it possible that somehow
> > the stack model or some register save/restore is different on Cygwin
> > that would produce that problem?  I can only test on SandyBridge and
> > IvyBridge for Intel and these have no problem.
> 
> Prior to the invention of the AVX stuff, a thread context structure had
> a fixed size.  To hold AVX registers, the context structure became a
> variable sized struct which has to be initialized with the function
> InitializeContext().  It also requires an additional call to
> SetXStateFeaturesMask() before being able to call GetThreadContext().
> None of that is implemanted in Cygwin.
> 
> The affected scenarios are:
> 
> - sigdelayed, the function handling delayed signals, does not handle
>   AVX registers,  This potentially affects almost all system calls.
> 
> - setjmp/longjmp
> 
> - Saving and restoring thread contexts during signal handling or
>   in the setcontext/getcontext/swapcontext/makecontext calls
>   use the CONTEXT_FULL flag which omits AVX registers.
> 
> - The mcontext_t structure has no provisions to hold AVX registers
>   so they are not available in SA_SIGINFO signal handlers.
> 
> The problem with all of that is that the exposed structures are fixed
> sized and have no room for extension without breaking the ABI.
> 
> So adding AVX register support is quite some work, especially in the
> assembler code in the gendef script.  For that, I'd need help.
> Fortunately there are not so many calls exposing contexts, only the
> aforemetioned ones.
> 
> I hope this affects 64 bit only?

Btw., for the time being it might be prudent to disable AVX in gmp...


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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