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

["H. Peter Anvin" <hpa@zytor.com>]

On September 2, 2026 1:33:54 AM PDT, David Laight <david.laight.linux@gmail.com> wrote:
>On Tue, 1 Sep 2026 15:16:58 +0200
>Jan Beulich <jbeulich@suse.com> wrote:
>
>> On 22.08.2026 20:33, 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@zytor.com>
>> > Link: https://lore.kernel.org/all/5e19b195-0ca2-4510-81cb-497b40e4aaf5@zytor.com/
>> > Fixes: a8c171c107c0 ("x86/boot: Add volatile, clobbers and zero-length test in memcmp()")
>> > Signed-off-by: Mauricio Faria de Oliveira <mfo@igalia.com>
>> > 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]  
>> 
>> Reviewed-by: Jan Beulich <jbeulich@suse.com>
>> 
>> > --- 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");  
>> 
>> In fact I'm using a modified gcc which properly rejects such conflicting
>> uses of output and clobber. ("cc" clobbers are redundant on x86 anyway.)
>
>And, if "cc" clobber wasn't redundant, you'd need clobbers for the cc flags
>that weren't being used as output values.
>
>David
>
>> 
>> Jan
>> 
>

No. The condition codes aren't orthogonal like that. There is only one flags register. 

(On x86 I believe asm statements are assumed to clobber the flags unconditionally, because it is so common.)