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

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Message-ID: <1541457654.196084.159.camel@acm.org>
Date:   Mon, 05 Nov 2018 14:40:54 -0800
From:   Bart Van Assche <bvanassche@....org>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        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 23:14 +0100, Rasmus Villemoes wrote:
> Won't that pessimize the cases where gfp is a constant to actually do
> the table lookup, and add 16 bytes to every translation unit?
> 
> Another option is to add a fake KMALLOC_DMA_RECLAIM so the
> kmalloc_caches[] array has size 4, then assign the same dma
> kmalloc_cache pointer to [2][i] and [3][i] (so that costs perhaps a
> dozen pointers in .data), and then just compute kmalloc_type() as
> 
> ((flags & __GFP_RECLAIMABLE) >> someshift) | ((flags & __GFP_DMA) >>
> someothershift).
> 
> Perhaps one could even shuffle the GFP flags so the two shifts are the same.

How about this version, still untested? My compiler is able to evaluate
the switch expression if the argument is constant.

 static __always_inline enum kmalloc_cache_type kmalloc_type(gfp_t flags)
 {
-	int is_dma = 0;
-	int type_dma = 0;
-	int is_reclaimable;
+	unsigned int dr = !!(flags & __GFP_RECLAIMABLE);
 
 #ifdef CONFIG_ZONE_DMA
-	is_dma = !!(flags & __GFP_DMA);
-	type_dma = is_dma * KMALLOC_DMA;
+	dr |= !!(flags & __GFP_DMA) << 1;
 #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;
+	switch (dr) {
+	default:
+	case 0:
+		return 0;
+	case 1:
+		return KMALLOC_RECLAIM;
+	case 2:
+	case 3:
+		return KMALLOC_DMA;
+	}
 }
 
Bart.