Message ID | 20221125023330.195581-2-mailhol.vincent@wanadoo.fr |
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State | New |
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[124.33.176.97]) by smtp.gmail.com with ESMTPSA id m127-20020a625885000000b0056bc5ad4862sm1893822pfb.28.2022.11.24.18.33.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 24 Nov 2022 18:33:55 -0800 (PST) Sender: Vincent Mailhol <vincent.mailhol@gmail.com> From: Vincent Mailhol <mailhol.vincent@wanadoo.fr> To: x86@kernel.org, Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de> Cc: Nick Desaulniers <ndesaulniers@google.com>, Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org, Yury Norov <yury.norov@gmail.com>, llvm@lists.linux.dev, Peter Zijlstra <peterz@infradead.org>, Vincent Mailhol <mailhol.vincent@wanadoo.fr>, Borislav Petkov <bp@suse.de> Subject: [PATCH v2 1/2] x86/asm/bitops: Replace __fls() by its generic builtin implementation Date: Fri, 25 Nov 2022 11:33:29 +0900 Message-Id: <20221125023330.195581-2-mailhol.vincent@wanadoo.fr> X-Mailer: git-send-email 2.37.4 In-Reply-To: <20221125023330.195581-1-mailhol.vincent@wanadoo.fr> References: <20221106095106.849154-1-mailhol.vincent@wanadoo.fr> <20221125023330.195581-1-mailhol.vincent@wanadoo.fr> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Spam-Status: No, score=-1.5 required=5.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_EF,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,RCVD_IN_DNSWL_NONE,SPF_HELO_NONE, SPF_PASS autolearn=no autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: <linux-kernel.vger.kernel.org> X-Mailing-List: linux-kernel@vger.kernel.org X-getmail-retrieved-from-mailbox: =?utf-8?q?INBOX?= X-GMAIL-THRID: =?utf-8?q?1748740039693708765?= X-GMAIL-MSGID: =?utf-8?q?1750434200255788742?= |
Series |
x86/asm/bitops: optimize fls functions for constant expressions
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Commit Message
Vincent Mailhol
Nov. 25, 2022, 2:33 a.m. UTC
Below snippet: #include <linux/bitops.h> unsigned int foo(unsigned long word) { return __fls(word); } produces this on GCC 12.1.0: 0000000000000000 <foo>: 0: f3 0f 1e fa endbr64 4: e8 00 00 00 00 call 9 <foo+0x9> 9: 53 push %rbx a: 48 89 fb mov %rdi,%rbx d: e8 00 00 00 00 call 12 <foo+0x12> 12: 48 0f bd c3 bsr %rbx,%rax 16: 5b pop %rbx 17: 31 ff xor %edi,%edi 19: e9 00 00 00 00 jmp 1e <foo+0x1e> and that on clang 14.0.6: 0000000000000000 <foo>: 0: f3 0f 1e fa endbr64 4: e8 00 00 00 00 call 9 <foo+0x9> 9: 53 push %rbx a: 50 push %rax b: 48 89 fb mov %rdi,%rbx e: e8 00 00 00 00 call 13 <foo+0x13> 13: 48 89 1c 24 mov %rbx,(%rsp) 17: 48 0f bd 04 24 bsr (%rsp),%rax 1c: 48 83 c4 08 add $0x8,%rsp 20: 5b pop %rbx 21: c3 ret The implementation from <asm-generic/bitops/builtin-__fls.h> [1] produces the exact same code on GCC and below code on clang: 0000000000000000 <foo>: 0: f3 0f 1e fa endbr64 4: e8 00 00 00 00 call 9 <foo+0x9> 9: 53 push %rbx a: 48 89 fb mov %rdi,%rbx d: e8 00 00 00 00 call 12 <foo+0x12> 12: 48 0f bd c3 bsr %rbx,%rax 16: 5b pop %rbx 17: c3 ret The builtin implementation is better for two reasons: 1/ it saves two instructions on clang (a push and a stack pointer decrement). 2/ when used on constant expressions, the compiler is only able to fold the builtin version (c.f. [2]). For those two reasons, replace the assembly implementation by its builtin counterpart. [1] https://elixir.bootlin.com/linux/v6.0/source/include/asm-generic/bitops/builtin-__fls.h [2] commit 146034fed6ee ("x86/asm/bitops: Use __builtin_ffs() to evaluate constant expressions") CC: Borislav Petkov <bp@suse.de> CC: Yury Norov <yury.norov@gmail.com> Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com> --- arch/x86/include/asm/bitops.h | 14 +------------- 1 file changed, 1 insertion(+), 13 deletions(-)
diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h index 2edf68475fec..a31453d7686d 100644 --- a/arch/x86/include/asm/bitops.h +++ b/arch/x86/include/asm/bitops.h @@ -285,19 +285,7 @@ static __always_inline unsigned long variable_ffz(unsigned long word) (unsigned long)__builtin_ctzl(~word) : \ variable_ffz(word)) -/* - * __fls: find last set bit in word - * @word: The word to search - * - * Undefined if no set bit exists, so code should check against 0 first. - */ -static __always_inline unsigned long __fls(unsigned long word) -{ - asm("bsr %1,%0" - : "=r" (word) - : "rm" (word)); - return word; -} +#include <asm-generic/bitops/builtin-__fls.h> #undef ADDR