On 11/9/18 8:47 PM, Darryl T. Agostinelli wrote: > 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 <vba...@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 <bvanass...@acm.org> >>>> Reported-by: Darryl T. Agostinelli <dagostine...@gmail.com> >>>> Suggested-by: Andrew Morton <a...@linux-foundation.org> >>>> Suggested-by: David Laight <david.lai...@aculab.com> >>>> Fixes: 1291523f2c1d ("mm, slab/slub: introduce kmalloc-reclaimable caches") >>>> Signed-off-by: Vlastimil Babka <vba...@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.
It doesn't remove the "trick" that calculates return value as a sum of booleans multiplying constants. The patch converts one part of the expression of those booleans to a ternary operator. I think the result is even harder to follow and meanwhile Andrew's suggestion was to remove all the tricks. > As it is, this sparse warning is begging for a trick. Let's not > oblidge it to much. The sparse warning could be silenced just by changing '&' to '&&' which would emit the same code. But we decided to untrick the code.