[kde-freebsd] Fixing devel/qt4-corelib to build with clang

[Dimitry Andric]

On 2013-05-08 16:11, Raphael Kubo da Costa wrote:
> Dimitry Andric <dimitry at andric.com> writes:
>> And it does similar includes for other intrinsics.  Note that it tests
>> both QT_HAVE_SSE3 (which is a configure-time feature), and __SSE3__,
>> which is defined by the compiler, if it supports SSE3 in the current
>> mode.  That is, if you either pass -msse3 on the command line, or use a
>> -march= or -mcpu= setting which is high enough to support the SSE3
>> feature.
>
> Related question: do you know why versions of GCC which do support those
> CPU instructions work in i386/i486 without any additional flags?

The test stage of the configure script passes the appropriate compiler
flags (-msse, etc) when it compiles the test cases.  However, it does
not *run* the test cases, it only looks at whether they compile and
link.  If they do, it considers the feature supported by the compiler.


> This whole issue got me really curious, and after installing a Debian
> i386 virtual machine to build a file which just includes
> <mm3dnow.h>. Both GCC 4.6 and 4.7 build it fine regardless of the values
> I pass to -march (or if I don't pass any value at all), whereas clang 3.0
> works by default but fails with those errors you mentioned if I pass
> -march={i386,i486,etc}.

This is because gcc's mm3dnow.h header has:

   #ifndef _MM3DNOW_H_INCLUDED
   #define _MM3DNOW_H_INCLUDED

   #ifdef __3dNOW__

   #include <mmintrin.h>

   [...3DNow stuff...]

   #endif /* __3dNOW_A__ */
   #endif /* __3dNOW__ */

   #endif /* _MM3DNOW_H_INCLUDED */

so it will only do anything if 3DNow is enabled.  This is not consistent
with some other gcc intrinsics headers, for example xmmintrin.h, which
has:

   #ifndef _XMMINTRIN_H_INCLUDED
   #define _XMMINTRIN_H_INCLUDED

   #ifndef __SSE__
   # error "SSE instruction set not enabled"
   #else
   [...SSE stuff...]
   #endif /* __SSE__ */
   #endif /* _XMMINTRIN_H_INCLUDED */

E.g. if you compile this file without enabling SSE, it will result in an
error.

-Dimitry