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Message-ID: <20250318034309.920866-1-richard120310@gmail.com>
Date: Tue, 18 Mar 2025 11:43:09 +0800
From: I Hsin Cheng <richard120310@...il.com>
To: hirofumi@...l.parknet.co.jp
Cc: linux-kernel@...r.kernel.org,
skhan@...uxfoundation.org,
linux-kernel-mentees@...ts.linux.dev,
jserv@...s.ncku.edu.tw,
I Hsin Cheng <richard120310@...il.com>
Subject: [PATCH] fat: Refactor fat_tolower with branchless implementation
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 <richard120310@...il.com>
---
[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
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