From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Linus Torvalds <torvalds@linux-foundation.org>,
"H. Peter Anvin" <hpa@zytor.com>
Cc: "Ingo Molnar" <mingo@kernel.org>, "Arnd Bergmann" <arnd@arndb.de>,
"Arnd Bergmann" <arnd@kernel.org>,
"Thomas Gleixner" <tglx@linutronix.de>,
"Ingo Molnar" <mingo@redhat.com>,
"Borislav Petkov" <bp@alien8.de>,
"Dave Hansen" <dave.hansen@linux.intel.com>,
x86@kernel.org, "Juergen Gross" <jgross@suse.com>,
"Boris Ostrovsky" <boris.ostrovsky@oracle.com>,
"Alexander Usyskin" <alexander.usyskin@intel.com>,
"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
"Mateusz Jończyk" <mat.jonczyk@o2.pl>,
"Mike Rapoport" <rppt@kernel.org>,
"Ard Biesheuvel" <ardb@kernel.org>,
"Peter Zijlstra" <peterz@infradead.org>,
linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org
Subject: Re: [PATCH] bitops/32: Convert variable_ffs() and fls() zero-case handling to C
Date: Tue, 29 Apr 2025 22:59:52 +0100 [thread overview]
Message-ID: <565ac06a-005f-4b77-930e-d63285c025b9@citrix.com> (raw)
In-Reply-To: <CAHk-=wi=nuDW6yCXSA-dEztZhXNuzLOaH--s_V7GOAE7n6RsRw@mail.gmail.com>
On 29/04/2025 10:53 pm, Linus Torvalds wrote:
> I just did
>
> gcc -m32 -O2 -S -fomit-frame-pointer t.c
>
> (with, and without that -m32) and looked at the result to see that it
> looks sane. No *actual* testing.
do_variable_ffs() doesn't quite work.
REP BSF is LZCNT, and unconditionally writes it's output operand, and
defeats the attempt to preload with -1.
Drop the REP prefix, and it should work as intended.
~Andrew
next prev parent reply other threads:[~2025-04-29 21:59 UTC|newest]
Thread overview: 48+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-04-25 14:15 [PATCH] [RFC] x86/cpu: rework instruction set selection Arnd Bergmann
2025-04-25 15:34 ` H. Peter Anvin
2025-04-25 16:13 ` Arnd Bergmann
2025-04-25 20:15 ` H. Peter Anvin
2025-04-26 9:08 ` Ingo Molnar
2025-04-26 13:17 ` H. Peter Anvin
2025-04-26 18:55 ` Ingo Molnar
2025-04-27 0:35 ` H. Peter Anvin
2025-04-26 18:58 ` Arnd Bergmann
2025-04-26 19:09 ` Ingo Molnar
2025-04-27 13:24 ` Arnd Bergmann
2025-04-27 21:17 ` H. Peter Anvin
2025-04-26 19:24 ` Linus Torvalds
2025-04-26 19:55 ` Linus Torvalds
2025-04-26 23:47 ` H. Peter Anvin
2025-04-27 10:18 ` Ingo Molnar
2025-04-27 0:02 ` H. Peter Anvin
2025-04-27 19:17 ` Andrew Cooper
2025-04-27 19:34 ` Linus Torvalds
2025-04-27 21:14 ` H. Peter Anvin
2025-04-28 6:58 ` [PATCH] bitops/32: Convert variable_ffs() and fls() zero-case handling to C Ingo Molnar
2025-04-28 7:05 ` Ingo Molnar
2025-04-28 7:14 ` Ingo Molnar
2025-04-28 12:30 ` Arnd Bergmann
2025-04-28 13:41 ` H. Peter Anvin
2025-04-28 16:23 ` Linus Torvalds
2025-04-29 10:08 ` Ingo Molnar
2025-04-29 14:32 ` H. Peter Anvin
2025-04-28 16:14 ` Linus Torvalds
2025-04-28 21:38 ` H. Peter Anvin
2025-04-29 0:12 ` Andrew Cooper
2025-04-29 2:00 ` H. Peter Anvin
2025-04-29 2:22 ` Linus Torvalds
2025-04-29 2:25 ` Andrew Cooper
2025-04-29 3:13 ` H. Peter Anvin
2025-04-29 14:38 ` Andrew Cooper
2025-04-29 18:05 ` Linus Torvalds
2025-04-29 19:13 ` Andrew Cooper
2025-04-29 20:12 ` Linus Torvalds
2025-04-29 21:23 ` H. Peter Anvin
2025-04-29 21:53 ` Linus Torvalds
2025-04-29 21:59 ` Andrew Cooper [this message]
2025-04-29 22:04 ` Linus Torvalds
2025-04-29 22:10 ` H. Peter Anvin
2025-04-29 22:22 ` Andrew Cooper
2025-04-29 22:34 ` Linus Torvalds
2025-04-27 9:50 ` [PATCH] [RFC] x86/cpu: rework instruction set selection Ingo Molnar
2025-04-30 21:54 ` David Laight
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