[PATCH] fat: Refactor fat_tolower with branchless implementation

I Hsin Cheng
2025-03-18T03:43:15Z

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@gmail.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