Re: How to define HAVE_SSE42 with autotools?

[Anders Broman <anders.broman () ericsson com>]

-----Original Message-----
From: wireshark-dev-bounces () wireshark org [mailto:wireshark-dev-bounces () wireshark org] On Behalf Of Jakub Zawadzki
Sent: den 10 juni 2014 23:55
To: Developer support list for Wireshark
Subject: Re: [Wireshark-dev] How to define HAVE_SSE42 with autotools?

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?

Sure, perhaps the macro (ax_check_compile_flag) I imported can be used?

> 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.

Regards
Anders

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