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[124.33.176.97]) by smtp.gmail.com with ESMTPSA id w11-20020a63c10b000000b003c14af50624sm42825pgf.60.2022.05.11.09.04.05 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 11 May 2022 09:04:08 -0700 (PDT) Sender: Vincent Mailhol From: Vincent Mailhol To: Nick Desaulniers , Thomas Gleixner , Ingo Molnar , Borislav Petkov Cc: Dave Hansen , x86@kernel.org, "H . Peter Anvin" , Nathan Chancellor , Tom Rix , linux-kernel@vger.kernel.org, llvm@lists.linux.dev, David Howells , Jan Beulich , Vincent Mailhol Subject: [PATCH v2 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl to evaluate constant expressions Date: Thu, 12 May 2022 01:03:19 +0900 Message-Id: <20220511160319.1045812-3-mailhol.vincent@wanadoo.fr> X-Mailer: git-send-email 2.35.1 In-Reply-To: <20220511160319.1045812-1-mailhol.vincent@wanadoo.fr> References: <20220511160319.1045812-1-mailhol.vincent@wanadoo.fr> Precedence: bulk X-Mailing-List: llvm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit __ffs(x) is equivalent to (unsigned long)__builtin_ctzl(x) and ffz(x) is equivalent to (unsigned long)__builtin_ctzl(~x). Because __builting_ctzl() returns an int, a cast to (unsigned long) is necessary to avoid potential warnings on implicit casts. For x86_64, the current __ffs() and ffz() implementations do not produce optimized code when called with a constant expression. On the contrary, the __builtin_ctzl() gets simplified into a single instruction. However, for non constant expressions, the __ffs() and ffz() asm versions of the kernel remains slightly better than the code produced by GCC (it produces a useless instruction to clear eax). This patch uses the __builtin_constant_p() to select between the kernel's __ffs()/ffz() and the __builtin_ctzl() depending on whether the argument is constant or not. ** Statistics ** On a allyesconfig, before applying this patch...: | $ objdump -d vmlinux.o | grep tzcnt | wc -l | 3607 ...and after: | $ objdump -d vmlinux.o | grep tzcnt | wc -l | 2600 So, roughly 27.9% of the call to either __ffs() or ffz() were using constant expression and were optimized out. (tests done on linux v5.18-rc5 x86_64 using GCC 11.2.1) Note: on x86_64, the asm bsf instruction produces tzcnt when used with the ret prefix (which is why we grep tzcnt instead of bsf in above benchmark). c.f. [1] [1] commit e26a44a2d618 ("x86: Use REP BSF unconditionally") http://lkml.kernel.org/r/5058741E020000780009C014@nat28.tlf.novell.com CC: Nick Desaulniers Signed-off-by: Vincent Mailhol --- arch/x86/include/asm/bitops.h | 38 ++++++++++++++++++++++------------- 1 file changed, 24 insertions(+), 14 deletions(-) diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h index 6ed979547086..7cf5374ce403 100644 --- a/arch/x86/include/asm/bitops.h +++ b/arch/x86/include/asm/bitops.h @@ -224,13 +224,7 @@ static __always_inline bool variable_test_bit(long nr, volatile const unsigned l ? constant_test_bit((nr), (addr)) \ : variable_test_bit((nr), (addr))) -/** - * __ffs - find first set bit in word - * @word: The word to search - * - * Undefined if no bit exists, so code should check against 0 first. - */ -static __always_inline unsigned long __ffs(unsigned long word) +static __always_inline unsigned long __variable_ffs(unsigned long word) { asm("rep; bsf %1,%0" : "=r" (word) @@ -238,13 +232,18 @@ static __always_inline unsigned long __ffs(unsigned long word) return word; } -/** - * ffz - find first zero bit in word - * @word: The word to search - * - * Undefined if no zero exists, so code should check against ~0UL first. - */ -static __always_inline unsigned long ffz(unsigned long word) +/** + * __ffs - find first set bit in word + * @word: The word to search + * + * Undefined if no bit exists, so code should check against 0 first. + */ +#define __ffs(word) \ + (__builtin_constant_p(word) ? \ + (unsigned long)__builtin_ctzl(word) : \ + __variable_ffs(word)) + +static __always_inline unsigned long __variable_ffz(unsigned long word) { asm("rep; bsf %1,%0" : "=r" (word) @@ -252,6 +251,17 @@ static __always_inline unsigned long ffz(unsigned long word) return word; } +/** + * ffz - find first zero bit in word + * @word: The word to search + * + * Undefined if no zero exists, so code should check against ~0UL first. + */ +#define ffz(word) \ + (__builtin_constant_p(word) ? \ + (unsigned long)__builtin_ctzl(~word) : \ + __variable_ffz(word)) + /* * __fls: find last set bit in word * @word: The word to search -- 2.35.1