[v18,11/15] block: Fix bio_flagged() so that gcc can better optimise it

Commit Message

David Howells March 14, 2023, 10:07 p.m. UTC

Fix bio_flagged() so that multiple instances of it, such as:

	if (bio_flagged(bio, BIO_PAGE_REFFED) ||
	    bio_flagged(bio, BIO_PAGE_PINNED))

can be combined by the gcc optimiser into a single test in assembly
(arguably, this is a compiler optimisation issue[1]).

The missed optimisation stems from bio_flagged() comparing the result of
the bitwise-AND to zero.  This results in an out-of-line bio_release_page()
being compiled to something like:

   <+0>:     mov    0x14(%rdi),%eax
   <+3>:     test   $0x1,%al
   <+5>:     jne    0xffffffff816dac53 <bio_release_pages+11>
   <+7>:     test   $0x2,%al
   <+9>:     je     0xffffffff816dac5c <bio_release_pages+20>
   <+11>:    movzbl %sil,%esi
   <+15>:    jmp    0xffffffff816daba1 <__bio_release_pages>
   <+20>:    jmp    0xffffffff81d0b800 <__x86_return_thunk>

However, the test is superfluous as the return type is bool.  Removing it
results in:

   <+0>:     testb  $0x3,0x14(%rdi)
   <+4>:     je     0xffffffff816e4af4 <bio_release_pages+15>
   <+6>:     movzbl %sil,%esi
   <+10>:    jmp    0xffffffff816dab7c <__bio_release_pages>
   <+15>:    jmp    0xffffffff81d0b7c0 <__x86_return_thunk>

instead.

Also, the MOVZBL instruction looks unnecessary[2] - I think it's just
're-booling' the mark_dirty parameter.

Signed-off-by: David Howells <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Reviewed-by: John Hubbard <[e-mail]>
cc: Jens Axboe <[e-mail]>
cc: [e-mail]
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108370 [1]
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108371 [2]
Link: https://lore.kernel.org/r/[e-mail]/ # v6
---
 include/linux/bio.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Patch

diff --git a/include/linux/bio.h b/include/linux/bio.h
index d766be7152e1..d9d6df62ea57 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -229,7 +229,7 @@  static inline void bio_cnt_set(struct bio *bio, unsigned int count)
 
 static inline bool bio_flagged(struct bio *bio, unsigned int bit)
 {
-	return (bio->bi_flags & (1U << bit)) != 0;
+	return bio->bi_flags & (1U << bit);
 }
 
 static inline void bio_set_flag(struct bio *bio, unsigned int bit)