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[124.33.176.97]) by smtp.gmail.com with ESMTPSA id w11-20020a63c10b000000b003c14af50624sm42825pgf.60.2022.05.11.09.03.52 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 11 May 2022 09:03:54 -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 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate constant expressions Date: Thu, 12 May 2022 01:03:18 +0900 Message-Id: <20220511160319.1045812-2-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> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org For x86_64, the current ffs() implementation does not produce optimized code when called with a constant expression. On the contrary, the __builtin_ffs() function of both GCC and clang is able to simplify the expression into a single instruction. * Example * Let's consider two dummy functions foo() and bar() as below: | #include | #define CONST 0x01000000 | | unsigned int foo(void) | { | return ffs(CONST); | } | | unsigned int bar(void) | { | return __builtin_ffs(CONST); | } GCC would produce below assembly code: | 0000000000000000 : | 0: ba 00 00 00 01 mov $0x1000000,%edx | 5: b8 ff ff ff ff mov $0xffffffff,%eax | a: 0f bc c2 bsf %edx,%eax | d: 83 c0 01 add $0x1,%eax | 10: c3 ret | 11: 66 66 2e 0f 1f 84 00 data16 cs nopw 0x0(%rax,%rax,1) | 18: 00 00 00 00 | 1c: 0f 1f 40 00 nopl 0x0(%rax) | | 0000000000000020 : | 20: b8 19 00 00 00 mov $0x19,%eax | 25: c3 ret And clang would produce: | 0000000000000000 : | 0: b8 ff ff ff ff mov $0xffffffff,%eax | 5: 0f bc 05 00 00 00 00 bsf 0x0(%rip),%eax # c | c: 83 c0 01 add $0x1,%eax | f: c3 ret | | 0000000000000010 : | 10: b8 19 00 00 00 mov $0x19,%eax | 15: c3 ret For both example, we clearly see the benefit of using __builtin_ffs() instead of the kernel's asm implementation for constant expressions. However, for non constant expressions, the ffs() asm version of the kernel remains better for x86_64 because, contrary to GCC, it doesn't emit the CMOV assembly instruction, c.f. [1] (noticeably, clang is able optimize out the CMOV call). This patch uses the __builtin_constant_p() to select between the kernel's ffs() and the __builtin_ffs() depending on whether the argument is constant or not. As a side benefit, this patch also removes below -Wshadow warning: | ./arch/x86/include/asm/bitops.h:283:28: warning: declaration of 'ffs' shadows a built-in function [-Wshadow] | 283 | static __always_inline int ffs(int x) ** Statistics ** On a allyesconfig, before applying this patch...: | $ objdump -d vmlinux.o | grep bsf | wc -l | 3607 ...and after: | $ objdump -d vmlinux.o | grep bsf | wc -l | 792 So, roughly 26.7% of the call to ffs() were using constant expression and were optimized out. (tests done on linux v5.18-rc5 x86_64 using GCC 11.2.1) [1] commit ca3d30cc02f7 ("x86_64, asm: Optimise fls(), ffs() and fls64()") http://lkml.kernel.org/r/20111213145654.14362.39868.stgit@warthog.procyon.org.uk CC: Nick Desaulniers Signed-off-by: Vincent Mailhol --- arch/x86/include/asm/bitops.h | 26 ++++++++++++++------------ 1 file changed, 14 insertions(+), 12 deletions(-) diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h index a288ecd230ab..6ed979547086 100644 --- a/arch/x86/include/asm/bitops.h +++ b/arch/x86/include/asm/bitops.h @@ -269,18 +269,7 @@ static __always_inline unsigned long __fls(unsigned long word) #undef ADDR #ifdef __KERNEL__ -/** - * ffs - find first set bit in word - * @x: the word to search - * - * This is defined the same way as the libc and compiler builtin ffs - * routines, therefore differs in spirit from the other bitops. - * - * ffs(value) returns 0 if value is 0 or the position of the first - * set bit if value is nonzero. The first (least significant) bit - * is at position 1. - */ -static __always_inline int ffs(int x) +static __always_inline int variable_ffs(int x) { int r; @@ -310,6 +299,19 @@ static __always_inline int ffs(int x) return r + 1; } +/** + * ffs - find first set bit in word + * @x: the word to search + * + * This is defined the same way as the libc and compiler builtin ffs + * routines, therefore differs in spirit from the other bitops. + * + * ffs(value) returns 0 if value is 0 or the position of the first + * set bit if value is nonzero. The first (least significant) bit + * is at position 1. + */ +#define ffs(x) (__builtin_constant_p(x) ? __builtin_ffs(x) : variable_ffs(x)) + /** * fls - find last set bit in word * @x: the word to search -- 2.35.1