RE: Misalignment of local SSE variables in thread function

[Danny Smith <dannysmith at clear dot net dot nz>]

> 
>     * From: Jonathan Kinsey <jon_kinsey at hotmail dot com>
>     * To: gcc-help at gcc dot gnu dot org
>     * Date: Wed, 17 Jan 2007 14:47:39 +0000
>     * Subject: Misalignment of local SSE variables in thread function
> 
> This is a problem that I've seen in the archives and is marked as not
> being a problem with gcc.  I'm not sure what the solution is though,
the
> simple test below runs fine using the Microsoft compiler (on windows)
> and fails using gcc (3.4.5) compiler.  There is no pthreads or glib
code
> here.
> 
> The output shows that the second call to f(), in the thread, has a
> misaligned s variable (which will crash in a SSE call).
> 
> Jon
> 
> #include <stdio.h>
> #include <process.h>
> #include <windows.h>
> #include <xmmintrin.h>
> void f(void *p)
> {
>   __m128 s;
>   printf("%d\n", ((int)&s) % 16);
> }
> 
> int main(int argc, char ** argv)
> {
>   f(0);
>   _beginthread(f, 0, 0);
> 
>   Sleep(2000);
>   return 0;
> }


With gcc 4.1, add this attribute to definition to force 16-byte
alignmnent:

void __attribute__ ((force_align_arg_pointer)) f (void * p) {...}

Search GCC's bugzilla.  This bug has been reported before. 

Finally, with 3.4.5, look up the mingw bug report archives were this
problem this discussed. and a workaround (using a wrapper function to
force align) suggested.

Danny