linux-kernel - Re: [PATCH] fat: Refactor fat_tolower with branchless implementation

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Message-ID: <87tt7rkn90.fsf@mail.parknet.co.jp>
Date: Tue, 18 Mar 2025 12:53:47 +0900
From: OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To: I Hsin Cheng <richard120310@...il.com>
Cc: linux-kernel@...r.kernel.org, skhan@...uxfoundation.org,
        linux-kernel-mentees@...ts.linux.dev, jserv@...s.ncku.edu.tw
Subject: Re: [PATCH] fat: Refactor fat_tolower with branchless implementation

I Hsin Cheng <richard120310@...il.com> writes:

> 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;
> }

Looks like doesn't work correctly. For example, new one changes TAB to ')'.

Thanks.

> 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,

-- 
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>