From: Vlastimil Babka
> Sent: 06 November 2018 12:51
>
> On 11/6/18 12:07 PM, David Laight wrote:
> > From: Vlastimil Babka [mailto:vba...@suse.cz]
> >> Sent: 06 November 2018 10:22
> > ...
> >>>> - return type_dma + (is_reclaimable & !is_dma) * KMALLOC_RECLAIM;
> >>>> + return type_dma + is_reclaimable * !is_dma * KMALLOC_RECLAIM;
...
> > I've done some experiments, compiled with gcc 4.7.3 and -O2
> > The constants match those from the kernel headers.
> >
> > It is noticable that there isn't a cmov in sight.
>
> There is with newer gcc: https://godbolt.org/z/qFdByQ
>
> But even that didn't remove the imul in f3() so that's indeed a bust.
>
> > The code would also be better if the KMALLOC constants matched the GFP ones.
>
> That would be hard, as __GFP flags have also other constraints
> (especially __GFP_DMA relative to other zone restricting __GFP flags)
> and KMALLOC_* are used as array index.
Hmmm...
With only 2 or three values conditionals are probably better than
table lookups - especially if they are function pointers.
>
> > unsigned int f1(unsigned int flags)
> > {
> > return !__builtin_expect(flags & (__GFP_DMA | __GFP_RECLAIM), 0) ?
> > 0 : flags & __GFP_DMA ?
> KMALLOC_DMA : KMALLOC_RECLAIM;
> > }
> >
>
> ...
>
> > 0000000000000020 <f1>:
> > 20: 40 f6 c7 11 test $0x11,%dil
> > 24: 75 03 jne 29 <f1+0x9>
> > 26: 31 c0 xor %eax,%eax
> > 28: c3 retq
> > 29: 83 e7 01 and $0x1,%edi
> > 2c: 83 ff 01 cmp $0x1,%edi
> > 2f: 19 c0 sbb %eax,%eax
> > 31: 83 c0 02 add $0x2,%eax
> > 34: c3 retq
> >
> > The jne will be predicted not taken and the retq predicted.
> > So this might only be 1 clock in the normal case.
>
> I think this is the winner. It's also a single branch and not two,
> because the compiler could figure out some of the "clever arithmetics"
> itself. Care to send a full patch?
I've not got a suitable source tree lurking.
So someone else would need to do it.
I'll waive any copyright that could plausibly be assigned to the above!
David
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