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Mon, 09 Mar 2026 08:59:47 -0700 (PDT) From: Xie Yuanbin 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 Message-ID: <20260309155933.41179-1-qq570070308@gmail.com> X-Mailer: git-send-email 2.53.0 In-Reply-To: References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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.