On Mon, Nov 5, 2018 at 2:41 PM Bart Van Assche <bvanassche@xxxxxxx> wrote:
>
> On Mon, 2018-11-05 at 23:14 +0100, Rasmus Villemoes wrote:
> > Won't that pessimize the cases where gfp is a constant to actually do
> > the table lookup, and add 16 bytes to every translation unit?
> >
> > Another option is to add a fake KMALLOC_DMA_RECLAIM so the
> > kmalloc_caches[] array has size 4, then assign the same dma
> > kmalloc_cache pointer to [2][i] and [3][i] (so that costs perhaps a
> > dozen pointers in .data), and then just compute kmalloc_type() as
> >
> > ((flags & __GFP_RECLAIMABLE) >> someshift) | ((flags & __GFP_DMA) >>
> > someothershift).
> >
> > Perhaps one could even shuffle the GFP flags so the two shifts are the same.
>
> 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;
> + }
> }
>
> Bart.
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?
