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

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Date:   Mon, 05 Nov 2018 16:01:06 -0800
From:   Bart Van Assche <bvanassche@....org>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     linux@...musvillemoes.dk,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Christoph Lameter <cl@...ux.com>, guro@...com,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH] slab.h: Avoid using & for logical and of booleans

On Mon, 2018-11-05 at 14:48 -0800, Alexander Duyck wrote:
> On Mon, Nov 5, 2018 at 2:41 PM Bart Van Assche <bvanassche@....org> wrote:
> > How about this version, still untested? My compiler is able to evaluate
> > the switch expression if the argument is constant.
> > 
> >  static __always_inline enum kmalloc_cache_type kmalloc_type(gfp_t flags)
> >  {
> > -       int is_dma = 0;
> > -       int type_dma = 0;
> > -       int is_reclaimable;
> > +       unsigned int dr = !!(flags & __GFP_RECLAIMABLE);
> > 
> >  #ifdef CONFIG_ZONE_DMA
> > -       is_dma = !!(flags & __GFP_DMA);
> > -       type_dma = is_dma * KMALLOC_DMA;
> > +       dr |= !!(flags & __GFP_DMA) << 1;
> >  #endif
> > 
> > -       is_reclaimable = !!(flags & __GFP_RECLAIMABLE);
> > -
> >         /*
> >          * If an allocation is both __GFP_DMA and __GFP_RECLAIMABLE, return
> >          * KMALLOC_DMA and effectively ignore __GFP_RECLAIMABLE
> >          */
> > -       return type_dma + (is_reclaimable & !is_dma) * KMALLOC_RECLAIM;
> > +       switch (dr) {
> > +       default:
> > +       case 0:
> > +               return 0;
> > +       case 1:
> > +               return KMALLOC_RECLAIM;
> > +       case 2:
> > +       case 3:
> > +               return KMALLOC_DMA;
> > +       }
> >  }
>
> Doesn't this defeat the whole point of the code which I thought was to
> avoid conditional jumps and branches? Also why would you bother with
> the "dr" value when you could just mask the flags value and switch on
> that directly?

Storing the relevant bits of 'flags' in the 'dr' variable avoids that the
bit selection expressions have to be repeated and allows to use a switch
statement instead of multiple if / else statements.

Most kmalloc() calls pass a constant to the gfp argument. That allows the
compiler to evaluate kmalloc_type() at compile time. So the conditional jumps
and branches only appear when the gfp argument is not a constant. What makes
you think it is important to optimize for that case?

Bart.