Re: [PATCH v21 2/6] block: Fix bio_flagged() so that gcc can better optimise it

[Christian Brauner]

On Mon, May 22, 2023 at 09:57:40PM +0100, David Howells wrote:
> 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 <dhowells@xxxxxxxxxx>
> Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>
> cc: Jens Axboe <axboe@xxxxxxxxx>
> cc: linux-block@xxxxxxxxxxxxxxx
> 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/167391056756.2311931.356007731815807265.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v6
> ---

Reviewed-by: Christian Brauner <brauner@xxxxxxxxxx>