Vc::currentImplementationSupported() fails

[Giordano Khouri]

Use the -mavx2 only for code that gets called after you know that avx2 exists on the machine. It must be compiled separately. Your main() and other boring code should be compiled for your base expected machine, e.g. sse2.


> -----Original Message-----
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> frankfurt.de] On Behalf Of Kay F. Jahnke
> Sent: Wednesday, May 03, 2017 2:18 AM
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> Subject: Re: Vc::currentImplementationSupported() fails
>
> Am 02.05.2017 um 21:54 schrieb Matthias Kretz:
> > On Dienstag, 2. Mai 2017 19:19:48 CEST Kay F. Jahnke wrote:
> >> Am 02.05.2017 um 13:30 schrieb Matthias Kretz:
> >>> I think you're seeing this because AVX512 is not implemented on
> >>> master. Can you try compiling for AVX2 and running on an older
> >>> system? That should give you your expected error message.
> >>
> >> I tried on my laptop. There I compiled with -mavx2, and it can only
> >> handle AVX. same effect:
> >>
> >> Ungültiger Maschinenbefehl (Speicherabzug geschrieben) (illegal
> >> instruction, core dumped)
> > [...]
> >> That must be what it's doing - I'll try and narrow it down some more,
> >> then get in touch again. Maybe there's some code which is executed
> >> before main, could that be it?
> >
> > Yes. E.g. all dynamic initializers of global variables would run before
> main.
> > E.g. if you have global Vc::Vector objects that might be the reason.
>
> Okay, I had two things going on at the same time on my desktop system: I had
> code using Vc before main (some static object's constructor). When I removed
> that, I came past the currentImplementationSupported() call...
>
> > You can run the application in gdb and produce a backtrace on SIGILL.
> > Call `disassemble` to have gdb print the instructions it just
> > executed. Let me know where it happens, maybe there's something I can do
> in Vc.
>
> ... just to hit an illegal instruction which, as you suspected, the compiler
> had generated itself, having been called with -mavx512f, the offender being
>
>   vpxord %zmm0,%zmm0,%zmm0
>
> which was used to set some variables to zero. My first proof of functioning
> autovectorization ;)
>
> Roughly the same on the laptop, though the illegal instruction here occured
> in the initialization of another global variable.
>
> While this explains the illegal instructions, it leaves me wondering how I
> can safely run the currentImplementationSupported() test. I may be able to
> avoid the illegal ops before the test, but if the test is actually executed,
> I feel it should not maintain all is well, when the program hits an illegal
> instruction just a few ops later.
>
> With regards
> Kay
>
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Khouri Giordano
Software Technology Researcher


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