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

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Date:   Fri, 9 Nov 2018 13:47:38 -0600
From:   "Darryl T. Agostinelli" <dagostinelli@...il.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        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>
Subject: Re: [PATCH] slab.h: Avoid using & for logical and of booleans

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@...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.
> 
> 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.