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Mon, 10 Nov 2025 06:48:54 -0800 (PST) Received: from DESKTOP-8TIG9K0.localdomain ([119.28.20.50]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-3437221b329sm7579364a91.16.2025.11.10.06.48.50 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 10 Nov 2025 06:48:54 -0800 (PST) From: Xie Yuanbin To: nathan@kernel.org, nick.desaulniers+lkml@gmail.com, morbo@google.com, justinstitt@google.com, qq570070308@gmail.com, masahiroy@kernel.org, jack@suse.cz, akpm@linux-foundation.org, maninder1.s@samsung.com Cc: linux-kernel@vger.kernel.org, llvm@lists.linux.dev, will@kernel.org Subject: [PATCH v2] Fix redundant judgment in WARN_ONCE with clang Date: Mon, 10 Nov 2025 22:48:44 +0800 Message-ID: <20251110144844.1376-1-qq570070308@gmail.com> X-Mailer: git-send-email 2.51.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" For c code: ```c extern int xx; void test(void) { if (WARN_ONCE(xx, "x")) __asm__ volatile ("nop":::); } ``` Clang will generate the following assembly code: ```assemble test: movl xx(%rip), %eax // Assume xx =3D=3D 0 (likely case) testl %eax, %eax // judge once je .LBB0_3 // jump to .LBB0_3 testb $1, test.__already_done(%rip) je .LBB0_2 .LBB0_3: testl %eax, %eax // judge again je .LBB0_5 // jump to .LBB0_5 .LBB0_4: nop .LBB0_5: retq // omit ``` In the above code, `xx =3D=3D 0` should be a likely case, but in this case, xx has been judged twice. Test info: 1. kernel source: linux-next commit 9c0826a5d9aa4d52206d ("Add linux-next specific files for 20251107") 2. compiler: clang: Debian clang version 21.1.4 (8) with Debian LLD 21.1.4 (compatible with GNU linkers) 3. config: base on default x86_64_defconfig, and setting: CONFIG_MITIGATION_RETHUNK=3Dn CONFIG_STACKPROTECTOR=3Dn v1->v2: https://lore.kernel.org/20251109083715.24495-1-qq570070308@gmail.com - Code format adjustment Signed-off-by: Xie Yuanbin Cc: Andrew Morton --- include/linux/once_lite.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/include/linux/once_lite.h b/include/linux/once_lite.h index 27de7bc32a06..236592c4eeb1 100644 --- a/include/linux/once_lite.h +++ b/include/linux/once_lite.h @@ -16,7 +16,7 @@ bool __ret_cond =3D !!(condition); \ bool __ret_once =3D false; \ \ - if (unlikely(__ret_cond && !__already_done)) { \ + if (unlikely(__ret_cond) && unlikely(!__already_done)) {\ __already_done =3D true; \ __ret_once =3D true; \ } \ --=20 2.51.0