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([2001:288:7001:2703:86b4:46:3f1e:43bf]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-737113d71fbsm8407612b3a.0.2025.03.17.20.43.12 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 17 Mar 2025 20:43:13 -0700 (PDT) From: I Hsin Cheng To: hirofumi@mail.parknet.co.jp Cc: linux-kernel@vger.kernel.org, skhan@linuxfoundation.org, linux-kernel-mentees@lists.linux.dev, jserv@ccns.ncku.edu.tw, I Hsin Cheng Subject: [PATCH] fat: Refactor fat_tolower with branchless implementation Date: Tue, 18 Mar 2025 11:43:09 +0800 Message-ID: <20250318034309.920866-1-richard120310@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-kernel-mentees@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Elimate the need of if-else branch within fat_tolower, replace it with a branchless bitwise operation. This can reduce the number of branch ~130 regarding to the test script[1]. Test size can also be reduced: $ ./scripts/bloat-o-meter vmlinux_old vmlinux_new add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-68 (-68) Function old new delta fat_parse_short 1901 1833 -68 Total: Before=22471023, After=22470955, chg -0.00% Signed-off-by: I Hsin Cheng --- [1]: static inline unsigned char old_tolower(unsigned char c) { return ((c >= 'A') && (c <= 'Z')) ? c+32 : c; } static inline unsigned char new_tolower(unsigned char c) { return c | 0x20; } int main(void) { for (unsigned char i = 0; i < 26; i++) { if (old_tolower('a' + i) != old_tolower('A' + i)) return 1; } return 0; } Utilize perf to profile the difference when using old_tolower() and new_tolower(). $ perf stat -e branches,branch-misses --repeat 100 ./old Performance counter stats for './old': 2,6302 branches:u 2334 branch-misses:u 0.000754710 seconds time elapsed 0.000000000 seconds user 0.000804000 seconds sys $ perf stat -e branches,branch-misses --repeat 100 ./new Performance counter stats for './main': 2,6172 branches:u 2338 branch-misses:u 0.000782670 seconds time elapsed 0.000853000 seconds user 0.000000000 seconds sys Best regards, I Hsin Cheng --- fs/fat/dir.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/fs/fat/dir.c b/fs/fat/dir.c index acbec5bdd521..77d212b4d4db 100644 --- a/fs/fat/dir.c +++ b/fs/fat/dir.c @@ -35,7 +35,7 @@ static inline unsigned char fat_tolower(unsigned char c) { - return ((c >= 'A') && (c <= 'Z')) ? c+32 : c; + return c | 0x20; } static inline loff_t fat_make_i_pos(struct super_block *sb, -- 2.43.0