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?
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?
[ Btw, looking at that, why are X86_L1_CACHE_BYTES and X86_L1_CACHE_SHIFT
totally unrelated numbers? Very confusing. ]
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.
Linus
