Re: pthreads-win32 2.8.0, stack alignment, and SSE code

[Sébastien Kunz-Jacques <kunzjacq at yahoo dot fr>]

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)