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SubjectRe: [PATCH] slab.h: Avoid using & for logical and of booleans
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.


enum foo {
a = 0,
b,
c = 0,
d
}

main()
{
printf("%d %d %d %d\n", a, b, c, d);
}

akpm3> ./a.out
0 1 0 1

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