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

[Bart Van Assche <bvanassche@acm.org>]

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@acm.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.