[PATCH] fat: Refactor fat_tolower with branchless implementation

[I Hsin Cheng <richard120310@gmail.com>]

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