From: Xie Yuanbin <qq570070308@gmail.com>
To: willy@infradead.org
Cc: Liam.Howlett@oracle.com, akpm@linux-foundation.org,
david.laight.linux@gmail.com, david@kernel.org,
justinstitt@google.com, linux-kernel@vger.kernel.org,
linux-mm@kvack.org, ljs@kernel.org, llvm@lists.linux.dev,
mhocko@suse.com, morbo@google.com, nathan@kernel.org,
nick.desaulniers+lkml@gmail.com, qq570070308@gmail.com,
rppt@kernel.org, surenb@google.com, vbabka@kernel.org
Subject: Re: [PATCH] mm: optimize the implementation of WARN_ON_ONCE_GFP()
Date: Mon, 9 Mar 2026 23:59:33 +0800 [thread overview]
Message-ID: <20260309155933.41179-1-qq570070308@gmail.com> (raw)
In-Reply-To: <aa7p3eQfEuh2wObQ@casper.infradead.org>
On Mon, 9 Mar 2026 15:40:13 +0000, Matthew Wilcox wrote:
>On Mon, Mar 09, 2026 at 11:38:11PM +0800, Xie Yuanbin wrote:
>> As shown in the commit message of commit 242b872239f6a7deacbc
>> ("include/linux/once_lite.h: fix judgment in WARN_ONCE with clang"),
>> the code "unlikely(a && b)" may generate poor assembly code if it is
>> actually "unlikely(a) && unlikely(b)" or "unlikely(a) && b".
>
> Why fix this in multiple places in the kernel instead of once in clang?
If a and b is both unlikely, then "unlikely(a) && unlikely(b)" will
generate better code than "unlikely(a && b)". This is also true for gcc.
As for the issue of clang judging twice, I have already submitted it to
clang:
Link: https://github.com/llvm/llvm-project/issues/167117
However, even if clang fixes it, this optimization will not be merged
back to the old version of clang.
next prev parent reply other threads:[~2026-03-09 15:59 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-03-09 15:38 Xie Yuanbin
2026-03-09 15:40 ` Matthew Wilcox
2026-03-09 15:59 ` Xie Yuanbin [this message]
2026-03-10 10:55 ` Vlastimil Babka (SUSE)
2026-03-10 14:52 ` David Laight
2026-03-27 5:34 ` Andrew Morton
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