Re: How to define HAVE_SSE42 with autotools?

[Jakub Zawadzki <darkjames-ws () darkjames pl>]

Hello,

On Tue, Jun 10, 2014 at 05:06:24PM +0000, Anders Broman wrote:
> > From: wireshark-dev-bounces () wireshark org [mailto:wireshark-dev-bounces () wireshark org] On Behalf Of Guy Harris
> >
> > HAVE_SSE42 is used in two places:
> >
> >     1) wsutil/ws_mempbrk_sse42.c, where it controls whether to build that file at all;
> >
> >     2) wsutil/ws_mempbrk.c, where it controls whether to check whether the CPU has SSE 4.2 and, if it does, call 
> > the SSE 4.2 version of the code.
> >
> > So what HAVE_SSE42 really seems to mean is "does the compiler support the intrinsics used in 
> > wsutil/ws_mempbrk_sse42.c?"
> >
> > Should we, instead, check for compiler support for those intrinsics?
>
> Yes I guess...
>
> > And will the compiler generate code for those intrinsics, using the SSE 4.2 instructions, even if you haven't told 
> > it, with a command-line option, to generate SSE 4.2 code in general?  Or does the >compiler not do a good job of 
> > supporting choosing whether to use SSE 4.2 instructions at run time rather than at compile time?
>
> I have recently added some macros I found that define HAVE_SSE4_2 so at least the stuff compiles now on all the 
> buildbots(I think).
>
> I'm not sure if there should be a command line option or not. But ideally we should compile ws_mempbrk_sse42.c if the 
> compiler supports it and check cpu_info() at runtime to use it or not.


What about using simple standard AC_TRY_COMPILE( )?

something like:

#v+

AC_MSG_CHECKING([whether the compiler support -msse4.2 and if there is nmmintrin.h header])

save_CFLAGS="$CFLAGS"
CFLAGS="$CFLAGS -msse4.2"

AC_TRY_COMPILE(
[#include <nmmintrin.h>],
[return 0;],
[
        ac_cv_wireshark_have_sse42=yes
        AC_DEFINE(HAVE_SSE42,,[Support SSSE4.2 (Streaming SIMD Extensions 4.2) instructions])
        AC_MSG_RESULT([yes])
],
[
        ac_cv_wireshark_have_sse42=no
        AC_MSG_RESULT([no])
])

dnl build libwsutil_sse42 only if there is SSE4.2
AM_CONDITIONAL(SSE42_SUPPORTED, test "x$ac_cv_wireshark_have_sse42" = "xno")

CFLAGS="$save_CFLAGS"

#v-

+ uncommented if SSE42_SUPPORTED in wsutil/Makefile.am

Anders it's ok for you?


I like idea to force (don't do runtime detection) SSE4.2 if -msse4.2 is supported & host CPU support can do sse4.2, but 
I think it's not worth troubles :)


With Regards,
Jakub.
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