> > From gcc 3.4:
> >
> >  /* Validate -mpreferred-stack-boundary= value, or provide default.
> >     The default of 128 bits is for Pentium III's SSE __m128, but we
> >     don't want additional code to keep the stack aligned when
> >     optimizing for code size.  */
> >  ix86_preferred_stack_boundary = (optimize_size
> >                                   ? TARGET_64BIT ? 128 : 32
> >                                   : 128);
> >
> > If you compile code with -Os, you will get 4 byte stack alignment.
> > Just step back, we changed stack alignment from 4 byte to 16byte
> > for SSE since we couldn't realign stack at the time. Now we can
> > realign the stack very efficiently. I think we should do it for SSE
> > to support the existing Linux binaries which have 4 byte stack
> > alignment. If it helps, I can compare -m32 -O3 -msse2 -mfp-math=sse
> > results with SPEC CPU 2006, before and after my patch.
> >
> 
> Here are the differences of -m32 -O3 -msse2 -mfpmath=sse -ffast-math
> -funroll-loops
> before and after my patch:
> 
> 400.perlbench                    -0.384615%
> 401.bzip2                        0%
> 403.gcc                          -0.362319%
> 429.mcf                          -0.813008%
> 445.gobmk                        0.921659%
> 456.hmmer                        0.549451%
> 458.sjeng                        -0.438596%
> 462.libquantum                   0%
> 464.h264ref                      0%
> 471.omnetpp                      -0.478469%
> 473.astar                        -0.645161%
> 483.xalancbmk                    -0.727273%
> SPECint(R)_base2006                      -0.411523%
> 410.bwaves                       -0.406504%
> 416.gamess                       0%
> 433.milc                         -1.36986%
> 434.zeusmp                       -0.44843%
> 435.gromacs                      0%
> 436.cactusADM                    0%
> 437.leslie3d                     -0.888889%
> 444.namd                         1.20482%
> 447.dealII                       -0.350877%
> 450.soplex                       -0.31746%
> 453.povray                       0.458716%
> 454.calculix                     0%
> 459.GemsFDTD                     0%
> 465.tonto                        0%
> 470.lbm                          0%
> 481.wrf                          0.480769%
> 482.sphinx3                      0.940439%
> SPECfp(R)_base2006                       0%
> 
> I think we should align stack if SSE variables are put on stack.
> 
> -- 
> H.J.

Your patch is buggy, it aligns the stack in functions with vector types 
but doesn't align if the autovectorizer creates 16-bit SSE instructions 
(that is even more dangerous than vector types because it may make crashes 
at random places). You must put that test somewhere else.

Mikulas