On Mon, 22 Mar 2021, Matthew Wilcox wrote:
> On Mon, Mar 22, 2021 at 10:05:05AM -0400, Mikulas Patocka wrote:
> > This patch replaces a loop with a "tzcnt" instruction.
>
> Are you sure that's an optimisation? The loop would execute very few
> times under normal circumstances (a maximum of three times on x86).
> Some numbers would be nice.
According to Agner's instruction tables, tzcnt has latency 3 on Skylake
and 2 on Zen. (386, 486, Pentium and K6 didn't have hardware for the bsf
instruction, they executed it in microcode bit-by-bit, but newer
processors execute it quickly).
The patch reduces code size by 16 bytes.
Mikulas
> > Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> >
> > Index: linux-2.6/fs/buffer.c
> > ===================================================================
> > --- linux-2.6.orig/fs/buffer.c
> > +++ linux-2.6/fs/buffer.c
>
> Are ... are you still using CVS?!
>
> > @@ -1020,11 +1020,7 @@ grow_buffers(struct block_device *bdev,
> > pgoff_t index;
> > int sizebits;
> >
> > - sizebits = -1;
> > - do {
> > - sizebits++;
> > - } while ((size << sizebits) < PAGE_SIZE);
> > -
> > + sizebits = PAGE_SHIFT - __ffs(size);
> > index = block >> sizebits;
> >
> > /*
> >
>
