On 11/19/18 12:04 PM, Pavel Machek wrote: > On Mon 2018-11-05 12:40:00, Bart Van Assche wrote: >> This patch suppresses the following sparse warning: >> >> ./include/linux/slab.h:332:43: warning: dubious: x & !y >> >> Fixes: 1291523f2c1d ("mm, slab/slub: introduce kmalloc-reclaimable caches") >> Cc: Vlastimil Babka <vba...@suse.cz> >> Cc: Mel Gorman <mgor...@techsingularity.net> >> Cc: Christoph Lameter <c...@linux.com> >> Cc: Roman Gushchin <g...@fb.com> >> Signed-off-by: Bart Van Assche <bvanass...@acm.org> >> --- >> include/linux/slab.h | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/include/linux/slab.h b/include/linux/slab.h >> index 918f374e7156..97d0599ddb7b 100644 >> --- 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; >> } >> > > What is wrong with && ?
Nothing, it would work and generate the same assembly as '&'. But Andrew noted that this code is probably too clever for its own good, and he has a point. The single predictable branch is also likely faster than the chain of arithmetic calculations anyway. Nobody has actually measured it, so I'd go with the easier-to-read variant. > If logical and is better done by multiply, > that's compiler job, and compiler should be fixed to do it... Multiply was just another way (equivalent to '&&' semantically) to shut up sparse warning. But gcc actually emits IMUL in that case, which is wasteful, so yeah there's a bug report now: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87954 > > Pavel >