Messages in this thread | | | Date | Fri, 9 Nov 2018 13:47:38 -0600 | From | "Darryl T. Agostinelli" <> | Subject | Re: [PATCH] slab.h: Avoid using & for logical and of booleans |
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On Fri, Nov 09, 2018 at 08:16:07PM +0100, Vlastimil Babka wrote: > On 11/9/18 8:00 PM, Andrew Morton wrote: > > On Fri, 9 Nov 2018 09:12:09 +0100 Vlastimil Babka <vbabka@suse.cz> wrote: > > > >> Multiple people have reported the following sparse warning: > >> > >> ./include/linux/slab.h:332:43: warning: dubious: x & !y > >> > >> The minimal fix would be to change the logical & to boolean &&, which emits the > >> same code, but Andrew has suggested that the branch-avoiding tricks are maybe > >> not worthwile. David Laight provided a nice comparison of disassembly of > >> multiple variants, which shows that the current version produces a 4 deep > >> dependency chain, and fixing the sparse warning by changing logical and to > >> multiplication emits an IMUL, making it even more expensive. > >> > >> The code as rewritten by this patch yielded the best disassembly, with a single > >> predictable branch for the most common case, and a ternary operator for the > >> rest, which gcc seems to compile without a branch or cmov by itself. > >> > >> The result should be more readable, without a sparse warning and probably also > >> faster for the common case. > >> > >> Reported-by: Bart Van Assche <bvanassche@acm.org> > >> Reported-by: Darryl T. Agostinelli <dagostinelli@gmail.com> > >> Suggested-by: Andrew Morton <akpm@linux-foundation.org> > >> Suggested-by: David Laight <David.Laight@ACULAB.COM> > >> Fixes: 1291523f2c1d ("mm, slab/slub: introduce kmalloc-reclaimable caches") > >> Signed-off-by: Vlastimil Babka <vbabka@suse.cz> > >> --- > >> include/linux/slab.h | 24 ++++++++++++------------ > >> 1 file changed, 12 insertions(+), 12 deletions(-) > >> > >> diff --git a/include/linux/slab.h b/include/linux/slab.h > >> index 918f374e7156..18c6920c2803 100644 > >> --- a/include/linux/slab.h > >> +++ b/include/linux/slab.h > >> @@ -304,6 +304,8 @@ enum kmalloc_cache_type { > >> KMALLOC_RECLAIM, > >> #ifdef CONFIG_ZONE_DMA > >> KMALLOC_DMA, > >> +#else > >> + KMALLOC_DMA = KMALLOC_NORMAL, > >> #endif > >> NR_KMALLOC_TYPES > >> }; > > > > I don't think this works correctly. Resetting KMALLOC_DMA to 0 will > > cause NR_KMALLOC_TYPES to have value 1. > > Doh, right! Thanks for catching this. > > This? Not terribly elegant, but I don't see a nicer way right now... >
How about the solution I proposed yesterday?
https://lkml.org/lkml/2018/11/9/750
It doesn't involve any tricks.
As it is, this sparse warning is begging for a trick. Let's not oblidge it to much.
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