Re: [PATCH] mm: optimize the implementation of WARN_ON_ONCE_GFP()

[Xie Yuanbin]

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.