From: "H. Peter Anvin" <hpa@zytor.com>
To: Linus Torvalds <torvalds@linux-foundation.org>,
Eric Dumazet <edumazet@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
Uros Bizjak <ubizjak@gmail.com>,
x86@kernel.org, linux-kernel <linux-kernel@vger.kernel.org>,
Eric Dumazet <eric.dumazet@gmail.com>
Subject: Re: [PATCH 2/2] x86/irqflags: Use ASM_OUTPUT_RM in native_save_fl()
Date: Sun, 21 Dec 2025 15:44:13 -0800 [thread overview]
Message-ID: <AB52301D-25E4-429B-9AD1-3EBD348478E1@zytor.com> (raw)
In-Reply-To: <CAHk-=wh3qPK8u8WCgw_ueCkQDP1AtE7AgQxafGR4DKKamuaBkA@mail.gmail.com>
On December 21, 2025 3:20:43 PM PST, Linus Torvalds <torvalds@linux-foundation.org> wrote:
>On Sun, 21 Dec 2025 at 13:47, Eric Dumazet <edumazet@google.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.
next prev parent reply other threads:[~2025-12-21 23:44 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-12-19 11:20 [PATCH 0/2] x86: speedup local_irq_save() for clang Eric Dumazet
2025-12-19 11:20 ` [PATCH 1/2] clang: work around asm output constraint problems Eric Dumazet
2025-12-20 22:32 ` Linus Torvalds
2025-12-21 3:46 ` Eric Dumazet
2025-12-19 11:20 ` [PATCH 2/2] x86/irqflags: Use ASM_OUTPUT_RM in native_save_fl() Eric Dumazet
2025-12-21 21:39 ` H. Peter Anvin
2025-12-21 21:47 ` Eric Dumazet
2025-12-21 23:20 ` Linus Torvalds
2025-12-21 23:44 ` H. Peter Anvin [this message]
2025-12-22 1:38 ` Linus Torvalds
2025-12-22 4:25 ` H. Peter Anvin
2025-12-22 16:51 ` Andrew Cooper
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