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    SubjectRe: [PATCH] slab.h: Avoid using & for logical and of booleans
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    On 11/6/18 12:07 PM, David Laight wrote:
    > From: Vlastimil Babka [mailto:vbabka@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;
    >>>
    >>> ISTM that changing is_dma and is_reclaimable from int to bool will stop the bleating.
    >>>
    >>> It is also strange that this code is trying so hard here to avoid conditional instructions
    >
    > 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.

    > 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?

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