Re: BUG: GCC-4.4.x changes the function frame on some functions

[Thomas Gleixner <tglx@linutronix.de>]

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