linux-kernel - Re: [PATCH] x86/locking: Remove semicolon from "lock" prefix

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Message-ID: <20250228091532.GD5880@noisy.programming.kicks-ass.net>
Date: Fri, 28 Feb 2025 10:15:32 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Uros Bizjak <ubizjak@...il.com>
Cc: x86@...nel.org, llvm@...ts.linux.dev, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>
Subject: Re: [PATCH] x86/locking: Remove semicolon from "lock" prefix

On Fri, Feb 28, 2025 at 09:51:15AM +0100, Uros Bizjak wrote:
> Minimum version of binutils required to compile the kernel is 2.25.
> This version correctly handles the "lock" prefix, so it is possible
> to remove the semicolon, which was used to support ancient versions
> of GNU as.
> 
> Due to the semicolon, the compiler considers "lock; insn" as two
> separate instructions. Removing the semicolon makes asm length
> calculations more accurate, consequently making scheduling and
> inlining decisions of the compiler more accurate.
> 
> Removing the semicolon also enables assembler checks involving lock
> prefix. Trying to assemble e.g. "lock andl %eax, %ebx" results in:
> 
>   Error: expecting lockable instruction after `lock'
> 
> Signed-off-by: Uros Bizjak <ubizjak@...il.com>

Ah, I always wondered why that ; was there.

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>