linux-kernel - Re: [PATCH 2/2] x86/irqflags: Use ASM_OUTPUT_RM in native_save

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Message-ID: <AB52301D-25E4-429B-9AD1-3EBD348478E1@zytor.com>
Date: Sun, 21 Dec 2025 15:44:13 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>,
        Eric Dumazet <edumazet@...gle.com>
CC: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Uros Bizjak <ubizjak@...il.com>, x86@...nel.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH 2/2] x86/irqflags: Use ASM_OUTPUT_RM in native_save_fl()

On December 21, 2025 3:20:43 PM PST, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>On Sun, 21 Dec 2025 at 13:47, Eric Dumazet <edumazet@...gle.com> wrote:
>>
>> Unfortunately I was told it was not better, some clang versions had
>> bugs with it.
>
>The whole "builtin is worse than inline asm" is actually a thing even
>when not actively buggy. We've had things like that a few times
>before.
>
>It's why we have out own variable_ffs() implementation, for example,
>and do things like
>
>   #define ffs(x) (__builtin_constant_p(x) ? __builtin_ffs(x) : variable_ffs(x))
>
>because __builtin_ffs() generates pointlessly silly code on at least
>some compiler versions:
>
>    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106471
>
>So then just using inline asm gets us at least reliable results, and
>we can - and do - tweak it for specific known configs.
>
>             Linus

Sigh.

It's just disappointing to see this kind of stuff.