linux-kernel - Re: [PATCH] slab.h: Avoid using & for logical and of booleans

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Date:   Fri, 9 Nov 2018 11:00:19 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     David Laight <David.Laight@...LAB.COM>,
        "'Bart Van Assche'" <bvanassche@....org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Christoph Lameter <cl@...ux.com>, Roman Gushchin <guro@...com>,
        "Darryl T. Agostinelli" <dagostinelli@...il.com>
Subject: Re: [PATCH] slab.h: Avoid using & for logical and of booleans

On Fri, 9 Nov 2018 09:12:09 +0100 Vlastimil Babka <vbabka@...e.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@....org>
> Reported-by: Darryl T. Agostinelli <dagostinelli@...il.com>
> Suggested-by: Andrew Morton <akpm@...ux-foundation.org>
> Suggested-by: David Laight <David.Laight@...LAB.COM>
> Fixes: 1291523f2c1d ("mm, slab/slub: introduce kmalloc-reclaimable caches")
> Signed-off-by: Vlastimil Babka <vbabka@...e.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