Messages in this thread Patch in this message | | | Date | Sat, 24 Dec 2016 12:13:35 +0530 | From | Yury Norov <> | Subject | Re: [PATCH] find_bit: Micro-optimise find_next_*_bit |
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Hi Mattew,
On Fri, Dec 23, 2016 at 09:20:03AM -0800, Matthew Wilcox wrote: > From: Matthew Wilcox <mawilcox@microsoft.com> > > This saves 20 bytes on my x86-64 build, mostly due to alignment > considerations ... I think it actually saves about five bytes of > instructions. There's really two parts to this commit. First, the > first half of the test: (!nbits || start >= nbits) is trivially a subset > of the second half, since nbits and start are both unsigned.
Yes... It's obvious... when you point it out.
ARM64 GCC compiler didn't notice it as well as me:
37 0000000000000070 <find_next_bit>: 38 70: eb1f003f cmp x1, xzr 39 74: fa421020 ccmp x1, x2, #0x0, ne 40 78: 54000088 b.hi 88 <find_next_bit+0x18> 41 7c: aa0103e0 mov x0, x1 42 80: d65f03c0 ret 43 84: d503201f nop 44 88: a9bf7bfd stp x29, x30, [sp,#-16]! 45 8c: 910003fd mov x29, sp 46 90: d2800003 mov x3, #0x0 // #0 47 94: 97ffffdb bl 0 <_find_next_bit.part.0> 48 98: a8c17bfd ldp x29, x30, [sp],#16 49 9c: d65f03c0 ret
> Second, > while looking at the disassembly, I noticed that GCC was predicting the > branch taken. Since this is a failure case, it's clearly the less likely > of the two branches, so add an unlikely() to override GCC's heuristics. > > Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com> > --- > lib/find_bit.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/lib/find_bit.c b/lib/find_bit.c > index 18072ea9c20e..7d4a681d625f 100644 > --- a/lib/find_bit.c > +++ b/lib/find_bit.c > @@ -33,7 +33,7 @@ static unsigned long _find_next_bit(const unsigned long *addr, > { > unsigned long tmp; > > - if (!nbits || start >= nbits) > + if (unlikely(start >= nbits)) > return nbits; > > tmp = addr[start / BITS_PER_LONG] ^ invert; > -- > 2.11.0
There's also _find_next_bit_le() with same code. I think it should be also patched.
Acked-by: Yury Norov <ynorov@caviumnetworks.com>
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