Re: GCC -msse2 portability question

[Laurent GUERBY <laurent@guerby.net>]

On Tue, 2014-03-25 at 10:56 +0100, Loic Dachary wrote:
> Hi Laurent,

Hi Loic,

> It occurs to me that all we're after is to enable SSE functions such as _mm_set_epi32. We're not trying to have the binary optimized in any implicit way, it is all explicit. The problem seems to be that -msse4.2 will do both 
> 
> * activate _mm_set_epi32 etc functions
> * optimize the binary to use sse4.2 instructions
> 
> Do you know of a compiler flag that would only 
> 
> * activate _mm_set_epi32 etc functions 

This is a function part of an Intel defined standard to access processor
feature, this standard will have one or more implementation depending on
your compiler/libc/OS. IIRC these functions are closely aligned with
specific processor feature, if the feature isn't there in general it
makes no sense to use them.

In the particular case of  _mm_set_epi32 it seems
to be a data formating inline function:

/usr/lib/gcc/x86_64-linux-gnu/4.7.2/include/emmintrin.h
...
typedef long long __m128i __attribute__ ((__vector_size__ (16),
__may_alias__));
...
extern __inline __m128i __attribute__((__gnu_inline__,
__always_inline__, __artificial__))
_mm_set_epi32 (int __q3, int __q2, int __q1, int __q0)
{
  return __extension__ (__m128i)(__v4si){ __q0, __q1, __q2, __q3 };
}

Functions in this include files are using GCC builtins:

http://gcc.gnu.org/onlinedocs/gcc-4.8.2/gcc/X86-Built-in-Functions.html#X86-Built-in-Functions

To avoid any issue I wouldn't use these functions at all
on a non SSE machine.

Sincerely,

Laurent

> and not
> 
> * optimize the binary to use sse4.2 instructions
> 
> ? It may be a RTFM question and I apologize for that. Reading http://gcc.gnu.org/onlinedocs/gcc-4.8.2/gcc/i386-and-x86-64-Options.html#i386-and-x86-64-Options it looks like this is more or less what --mtune=corei7-avx would do (because gf-complete uses PCLMUL when available). But it feels weird to specify a specific processor model where what we need is a set of features. 
> 
> Thanks for your help :-)
> 
> On 25/03/2014 10:43, Laurent GUERBY wrote:
> > On Mon, 2014-03-24 at 22:27 +0100, Loic Dachary wrote:
> >>
> >> On 23/03/2014 23:34, Laurent GUERBY wrote:
> >>> http://gcc.gnu.org/onlinedocs/gcc-4.8.2/gcc/i386-and-x86-64-Options.html#i386-and-x86-64-Options
> >>>
> >>> So unless you want to run your code a very very old x86 32 bit processor
> >>> "-msse" shouldn't be an issue. "-msse2" is similar.
> >>
> >> This is good to know :) Should I be worried about unintended side effects of -msse4.2 -mssse3 -msse4.1 or -mpclmul ? These are the flags that gf-complete are using, specifically.
> > 
> > Hi,
> > 
> > SSE4.2 will be available only in more recent
> > processors as documented on the page above.
> > 
> > If your library already is dynamically checking for processor
> > feature I would advise to be conservative in your
> > -m flags, ie using what debian would use for maximum
> > x86 portability.
> > 
> > Sincerely,
> > 
> > Laurent
> > 
>