Re: [PATCH v9 1/5] x86/boot: Remove "cc" clobber from memcmp()

[Borislav Petkov]

On Wed, Sep 02, 2026 at 03:29:44PM +0200, Michael Matz wrote:
> Strictly speaking, on x86, the '=@ccXY' constraints are register outputs 
> into normal random integer registers (though they are of course 
> initialized in a funny way), while the 'cc' clobber is not a register at 
> all, but rather a fuzzy idea of "state" in old cc0-based compilers (which 
> the x86 backend isn't anymore since, ... well, about forever, 1999).  As 
> such they both really don't conflict and ...
> 
> > But then I'd expect that gcc would enforce that. I know it can't have it 
> > when the clobbers contain input or output regs:
> > 
> > In function ‘__memcmp’,
> >     inlined from ‘main’ at memcmp.c:25:6:
> > memcmp.c:11:9: error: ‘asm’ operand has impossible constraints or there are not enough registers
> >    11 |         asm volatile("test %3, %3\n\t"
> >       |         ^~~
> > 
> > but with "cc" clobbers it works.
> 
> ... hence there's nothing to report. 

Aaaha, so the enforcement is solely documentation-based. :-)

> In fact what an explicit 'cc' clobber once meant in cc0 backends (that
> indiscriminated flag "state") is manufactured by the non-cc0 backends (all
> of them now) automatically whenever an asm has no flag output constraints at
> all.  (On x86 that means it adds the "flags" register (internal name for the
> collection of flag status bits) to the clobber set automatically when there
> are no =@ccXY constraints).
> 
> You can regard all 'cc' clobbers as pure source compatibility, they have 
> no meaning anymore.  But as they are so ubiquitous (even in our own docu), 
> they remain recognized.

Ah ok, I see. so we'll simply forget them.

Thanks Micha!

-- 
Regards/Gruss,
    Boris.

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