Re: PATCH: PR target/40838: gcc shouldn't assume that the stack is aligned

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Fri, Aug 7, 2009 at 5:53 AM, H.J. Lu<hjl.tools@gmail.com> wrote:
> On Fri, Aug 7, 2009 at 12:13 AM, Jakub Jelinek<jakub@redhat.com> wrote:
>> On Fri, Aug 07, 2009 at 02:54:46AM +0200, Mikulas Patocka wrote:
>>> > > In 32bit, the incoming stack may not be 16 byte aligned. ?This patch
>>> > > assumes the incoming stack is 4 byte aligned and realigns stack if any
>>> > > SSE variable is put on stack. Any comments?
>>> >
>>> > IMHO this is wrong, I could live with a non-default option for those who
>>> > don't care about performance and think a SCO document from 1996 has any
>>> > relevance to Linux these days. ?In reality a Linux ABI for years assumes
>>> > 16 byte stack alignment for 32-bit code.
>>>
>>> Tell me which Linux distribution did you run with 16-byte stack alignment
>>> checking (as proposed in bug 40838) and what was the result?
>>>
>>> For me, the result was that 75% of binaries in /bin in Debian Lenny do not
>>> align the stack on 16-byte boundary.
>>
>> Besides the obstack glibc bug which has been fixed since then you haven't
>> reported anything particular. ?It is true that parts of i?86 glibc is
>> compiled with -mpreferered-stack-boundary=2, but only parts that don't call
>> callbacks. ?Async signals AFAIK will align the stack properly.
>>
>> I simply don't trust your 75% claim, lots of stuff would break if things
>> weren't aligned properly.
>>
>
> 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.