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

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Date:   Mon, 05 Nov 2018 13:48:09 -0800
From:   Bart Van Assche <bvanassche@....org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Christoph Lameter <cl@...ux.com>, Roman Gushchin <guro@...com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, linux-mm@...ck.org
Subject: Re: [PATCH] slab.h: Avoid using & for logical and of booleans

On Mon, 2018-11-05 at 13:13 -0800, Andrew Morton wrote:
> On Mon,  5 Nov 2018 12:40:00 -0800 Bart Van Assche <bvanassche@....org> wrote:
> 
> > This patch suppresses the following sparse warning:
> > 
> > ./include/linux/slab.h:332:43: warning: dubious: x & !y
> > 
> > ...
> > 
> > --- a/include/linux/slab.h
> > +++ b/include/linux/slab.h
> > @@ -329,7 +329,7 @@ static __always_inline enum kmalloc_cache_type kmalloc_type(gfp_t flags)
> >  	 * If an allocation is both __GFP_DMA and __GFP_RECLAIMABLE, return
> >  	 * KMALLOC_DMA and effectively ignore __GFP_RECLAIMABLE
> >  	 */
> > -	return type_dma + (is_reclaimable & !is_dma) * KMALLOC_RECLAIM;
> > +	return type_dma + is_reclaimable * !is_dma * KMALLOC_RECLAIM;
> >  }
> >  
> >  /*
> 
> I suppose so.
> 
> That function seems too clever for its own good :(.  I wonder if these
> branch-avoiding tricks are really worthwhile.

>From what I have seen in gcc disassembly it seems to me like gcc uses the
cmov instruction to implement e.g. the ternary operator (?:). So I think none
of the cleverness in kmalloc_type() is really necessary to avoid conditional
branches. I think this function would become much more readable when using a
switch statement or when rewriting it as follows (untested):

 static __always_inline enum kmalloc_cache_type kmalloc_type(gfp_t flags)
 {
-	int is_dma = 0;
-	int type_dma = 0;
-	int is_reclaimable;
-
-#ifdef CONFIG_ZONE_DMA
-	is_dma = !!(flags & __GFP_DMA);
-	type_dma = is_dma * KMALLOC_DMA;
-#endif
-
-	is_reclaimable = !!(flags & __GFP_RECLAIMABLE);
-
 	/*
 	 * If an allocation is both __GFP_DMA and __GFP_RECLAIMABLE, return
 	 * KMALLOC_DMA and effectively ignore __GFP_RECLAIMABLE
 	 */
-	return type_dma + (is_reclaimable & !is_dma) * KMALLOC_RECLAIM;
+	static const enum kmalloc_cache_type flags_to_type[2][2] = {
+		{ 0,		KMALLOC_RECLAIM },
+		{ KMALLOC_DMA,	KMALLOC_DMA },
+	};
+#ifdef CONFIG_ZONE_DMA
+	bool is_dma = !!(flags & __GFP_DMA);
+#endif
+	bool is_reclaimable = !!(flags & __GFP_RECLAIMABLE);
+
+	return flags_to_type[is_dma][is_reclaimable];
 }