pthreads-win32 2.8.0, stack alignment, and SSE code

[Sébastien Kunz-Jacques]

Ramiro Polla a écrit :
> Sébastien Kunz-Jacques wrote:
>> Hi,
>>
>> I  encountered problems with SSE code compiled with recent mingw GCC 
>> (4.3.2, TDM release, http://www.tdragon.net/recentgcc/) and using 
>> pthreads 2.8.0. After inverstigation, crashes occured because the 
>> code was trying to read operands on the stack, assuming the stack was 
>> 16-byte aligned as is the case in the main thread (the main function 
>> aligns the stack and alignment is maintained during each function 
>> call).  I solved the issue with a very simple patch that uses some 
>> GCC wizardry to force stack realignment upon entry in a new thread:
>>
>> --- ptw32_threadStart.c    Sun May 15 17:28:27 2005
>> +++ ptw32_threadStart.c    Mon Sep 29 21:28:16 2008
>> @@ -116,6 +116,9 @@
>>
>> #endif
>>
>> +#if defined(__GNUC__) && (__GNUC__ > 4 || __GNUC__ == 4 && 
>> __GNUC_MINOR__>1)
>> +__attribute__((force_align_arg_pointer))
>> +#endif
>> #if ! defined (__MINGW32__) || (defined (__MSVCRT__) && ! defined 
>> (__DMC__))
>> unsigned
>>   __stdcall
>>
>>
>> The attribute force_align_arg_pointer should be added to every 
>> function that is called with a stack with insufficient alignment; as 
>> far as I am concerned doing this for threadStart only solved my 
>> problems. Maybe this small patch could be added to the pthread code?
>
> I think that's related to:
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37216
>
> So I think this patch shouldn't be applied to pthreads-win32, and 
> people should rather use another version of MinGW (or unset automatic 
> SSE code, if that makes any sense).
>
> Ramiro Polla
>
Not at all ; I use a modified binutils that enforces 16-byte alignment 
alignment of .bss sections (basically I reverted the part of the 
binutils patch that is linked to in the gcc bug 37216 thread). The bug I 
experienced shows up only in threaded code and comes from the fact that 
a stack of a win32 thread in only 4-byte aligned. The crash occurs when 
a data is read on the stack and not in a .bss segment.

To give some contextual information, I tried to build a math library, 
ATLAS, with mingw. First, for the non-threaded version, I encountered 
bug 37216 that you mention ; to get rid of this I patched binutils 
(adding -fno-common to gcc works also). But the threaded version was 
still crashing, and indeed the symptoms looked much similiar to what 
occurred in the non-threaded case. Then I found the solution evoked in 
my first post.

Please note that adding the attribute force_align_arg_pointer to 
threadStart has a negligible performance penalty (a few machine 
instructions each time this function is entered/exited)