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

[Borislav Petkov]

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.
> 
>     """
>     6.11.2.4 Flag Output Operands
> 
>         On some targets, a special form of output operand exists by which
>         conditions in the flags register may be outputs of the asm. [...]
> 
>     6.11.2.6 Clobbers and Scratch Registers
> 
>         While the compiler is aware of changes to entries listed in the
>         output operands, [...]
> 
>         Clobber descriptions may not in any way overlap with an input or
>         output operand. [...]
>     """
> 
> Reported-by: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Link: https://lore.kernel.org/all/5e19b195-0ca2-4510-81cb-497b40e4aaf5@xxxxxxxxx/
> Fixes: a8c171c107c0 ("x86/boot: Add volatile, clobbers and zero-length test in memcmp()")
> Signed-off-by: Mauricio Faria de Oliveira <mfo@xxxxxxxxxx>
> Link: https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Flag-Output-Operands [1]
> Link: https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Clobbers-and-Scratch-Registers-1 [2]
> ---
>  arch/x86/boot/string.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/boot/string.c b/arch/x86/boot/string.c
> index 1632d40e1f545ae0665597069b568ea6b6c263e5..03278b4393887cb71cb063818d3378c4f52b06f8 100644
> --- a/arch/x86/boot/string.c
> +++ b/arch/x86/boot/string.c
> @@ -40,7 +40,7 @@ int memcmp(const void *s1, const void *s2, size_t len)
>  	asm volatile("test %3, %3\n\t"
>  		     "repe cmpsb"
>  		     : "=@ccnz" (diff), "+D" (s1), "+S" (s2), "+c" (len)
> -		     : : "cc", "memory");
> +		     : : "memory");
>  	return diff;
>  }

So far, so good.

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.

That's gcc-16 btw.

Micha, Richi?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette