linux-kernel - Re: [PATCH RFC 4/5] x86/alternative: Improve code generation readability

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Message-ID: <20250409143821.GE9833@noisy.programming.kicks-ass.net>
Date: Wed, 9 Apr 2025 16:38:21 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH RFC 4/5] x86/alternative: Improve code generation
 readability

On Tue, Apr 08, 2025 at 01:21:17AM -0700, Josh Poimboeuf wrote:
> Improve the readability and compactness of alternatives code.
> 
> ---------------------
> ALTERNATIVE() before:
> ---------------------
> 
>   	# ALT: oldinstr
>   771:
>   	rep movsb
>   772:
>   # ALT: padding
>   .skip -(((775f-774f)-(772b-771b)) > 0) * ((775f-774f)-(772b-771b)),0x90
>   773:
>   .pushsection .altinstructions,"a"
>    .long 771b - .
>    .long 774f - .
>    .4byte (((1 << 0) << 16) | ((18*32+ 4)))
>    .byte 773b-771b
>    .byte 775f-774f
>   .popsection
>   .pushsection .altinstr_replacement, "ax"
>   # ALT: replacement
>   774:
>   	call rep_movs_alternative
>   775:
>   .popsection
> 
> --------------------
> ALTERNATIVE() after:
> --------------------
> 
>  # <ALTERNATIVE>
>  771:	rep movsb
>  772:	.skip -(((775f-774f)-(772b-771b)) > 0) * ((775f-774f)-(772b-771b)),0x90
>  773:
>  # ALT ENTRY:
>  .pushsection .altinstructions,"a";  .long 771b - .;  .long 774f - .;  .4byte (((1 << 0) << 16) | ((18*32+ 4)));  .byte 773b-771b;  .byte 775f-774f; .popsection

I find this very hard to read. I prefer the multi line form it had
before.

Other than that, I like the new layout.

>  # ALT REPLACEMENT:
>  .pushsection .altinstr_replacement, "ax"
>  774:	call rep_movs_alternative
>  775:
>  .popsection
>  # </ALTERNATIVE>