Messages in this thread | | | Subject | Re: [PATCH] slab.h: Avoid using & for logical and of booleans | From | Bart Van Assche <> | Date | Mon, 05 Nov 2018 13:48:09 -0800 |
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On Mon, 2018-11-05 at 13:13 -0800, Andrew Morton wrote: > On Mon, 5 Nov 2018 12:40:00 -0800 Bart Van Assche <bvanassche@acm.org> wrote: > > > This patch suppresses the following sparse warning: > > > > ./include/linux/slab.h:332:43: warning: dubious: x & !y > > > > ... > > > > --- a/include/linux/slab.h > > +++ b/include/linux/slab.h > > @@ -329,7 +329,7 @@ static __always_inline enum kmalloc_cache_type kmalloc_type(gfp_t flags) > > * 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; > > + return type_dma + is_reclaimable * !is_dma * KMALLOC_RECLAIM; > > } > > > > /* > > I suppose so. > > That function seems too clever for its own good :(. I wonder if these > branch-avoiding tricks are really worthwhile.
From what I have seen in gcc disassembly it seems to me like gcc uses the cmov instruction to implement e.g. the ternary operator (?:). So I think none of the cleverness in kmalloc_type() is really necessary to avoid conditional branches. I think this function would become much more readable when using a switch statement or when rewriting it as follows (untested):
static __always_inline enum kmalloc_cache_type kmalloc_type(gfp_t flags) { - int is_dma = 0; - int type_dma = 0; - int is_reclaimable; - -#ifdef CONFIG_ZONE_DMA - is_dma = !!(flags & __GFP_DMA); - type_dma = is_dma * KMALLOC_DMA; -#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; + static const enum kmalloc_cache_type flags_to_type[2][2] = { + { 0, KMALLOC_RECLAIM }, + { KMALLOC_DMA, KMALLOC_DMA }, + }; +#ifdef CONFIG_ZONE_DMA + bool is_dma = !!(flags & __GFP_DMA); +#endif + bool is_reclaimable = !!(flags & __GFP_RECLAIMABLE); + + return flags_to_type[is_dma][is_reclaimable]; }
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