On Thu, 19 Nov 2009, Linus Torvalds wrote:
> On Fri, 20 Nov 2009, Thomas Gleixner wrote:
> >
> > While testing various kernel configs we found out that the problem
> > comes and goes. Finally I started to compare the gcc command line
> > options and after some fiddling it turned out that the following
> > minimal deltas change the code generator behaviour:
> >
> > Bad: -march=pentium-mmx -Wa,-mtune=generic32
> > Good: -march=i686 -mtune=generic -Wa,-mtune=generic32
> > Good: -march=pentium-mmx -mtune-generic -Wa,-mtune=generic32
> >
> > I'm not supposed to understand the logic behind that, right ?
>
> Are you sure it's just the compiler flags?
I first captured the command line with V=1 and created a script of
it. Then I changed the -march -mtune options in that script and
compiled just that single file manually w/o changing .config or
invoking the kernel make magic.
The good ones produce:
650: 55 push %ebp
651: 89 e5 mov %esp,%ebp
653: 83 e4 f0 and $0xfffffff0,%esp
The bad one:
000005f0 <timer_stats_update_stats>:
5f0: 57 push %edi
5f1: 8d 7c 24 08 lea 0x8(%esp),%edi
5f5: 83 e4 f0 and $0xfffffff0,%esp
5f8: ff 77 fc pushl -0x4(%edi)
5fb: 55 push %ebp
5fc: 89 e5 mov %esp,%ebp
> There's another configuration portion: the size of the alignment itself.
> That's dependent on L1_CACHE_SHIFT, which in turn is taken from the kernel
> config CONFIG_X86_L1_CACHE_SHIFT.
>
> Maybe that value matters too - for example maybe gcc will not try to align
> the stack if it's big?
That does not change any of the compiler options, but yes it could
have some effect via the various include magics, but all I have seen
so far is linkage.h which should not affect the compiler. And the
manual compile did not change any of this.
> [ Btw, looking at that, why are X86_L1_CACHE_BYTES and X86_L1_CACHE_SHIFT
> totally unrelated numbers? Very confusing. ]
Agreed.
> The compiler flags we use are tied to some of the same choices that choose
> the cache shift, so the correlation you found while debugging this would
> still hold.
Digging further tomorrow when my brain is more awake.
Thanks,
tglx
