RE: [PATCH] slab.h: Avoid using & for logical and of booleans

[David Laight]

From: Vlastimil Babka
> Sent: 06 November 2018 12:51
> 
> On 11/6/18 12:07 PM, David Laight wrote:
> > From: Vlastimil Babka [mailto:vbabka@xxxxxxx]
> >> 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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