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>
