Re: [PATCH 8/8] x86/alternative: Convert alternatives to assembler macros

[Peter Zijlstra]

On Sat, Dec 06, 2025 at 01:41:15PM -0800, Josh Poimboeuf wrote:
> Improve code generation readability by converting the alternatives into
> assembler macros which are created when alternative.h is included.
> 
> Before:
> 
>   # ./arch/x86/include/asm/smap.h:47: 	alternative("", "stac", X86_FEATURE_SMAP);
>   # 47 "./arch/x86/include/asm/smap.h" 1
>   	# ALT: oldinstr
>   771:
> 
>   772:
>   # ALT: padding
>   .skip -(((775f-774f)-(772b-771b)) > 0) * ((775f-774f)-(772b-771b)),0x90
>   773:
>   .pushsection .altinstructions, "aM", @progbits, 14
>    .long 771b - .
>    .long 774f - .
>    .4byte ( 9*32+20)
>    .byte 773b-771b
>    .byte 775f-774f
>   .popsection
>   .pushsection .altinstr_replacement, "ax"
>   ANNOTATE_DATA_SPECIAL
>   # ALT: replacement
>   774:
>   	stac
>   775:
>   .popsection
> 
> After:
> 
>   # ./arch/x86/include/asm/smap.h:47: 	alternative("", "stac", X86_FEATURE_SMAP);
>   # 47 "./arch/x86/include/asm/smap.h" 1
> 	ALTERNATIVE "", "stac", "( 9*32+20)"
> 

So the problem with the gas macro thing is that it doesn't allow for
that nesting. I don't think we currently use it other than to define the
ALTERNATIVE_2 and ALTERNATIVE_3 macros, but IIRC the reason I started
all that was because it was fairly trivial to use things like
CALL_NOSPEC in an alternative (where CALL_NOSPEC is already an
alternative).

Anyway, yes, the macro variant is easier to read in the .s output.