linux-kernel - Re: [PATCH] slab.h: Avoid using & for logical and of booleans

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Date:   Tue, 6 Nov 2018 13:51:12 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     David Laight <David.Laight@...LAB.COM>,
        'Bart Van Assche' <bvanassche@....org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Christoph Lameter <cl@...ux.com>, Roman Gushchin <guro@...com>
Subject: Re: [PATCH] slab.h: Avoid using & for logical and of booleans

On 11/6/18 12:07 PM, David Laight wrote:
> From: Vlastimil Babka [mailto:vbabka@...e.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?