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

[Michael Matz <matz@xxxxxxx>]

Hello,

On Tue, 1 Sep 2026, Borislav Petkov wrote:

> On Sat, Aug 22, 2026 at 03:33:17PM -0300, Mauricio Faria de Oliveira wrote:
> > According to the GCC documentation, conditions in the flags register
> > (e.g., "=@ccnz") are output operands [1] and the compiler is aware [2].
> > 
> > Also, clobbers (e.g., "cc") may not overlap with an output operand [2].
> > 
> > Thus, remove the "cc" clobber as it is redudant, and overlaps with, the
> > "=@ccnz" output operand.

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.  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.


Ciao,
Michael.