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

[Vlastimil Babka <vbabka@suse.cz>]

On 11/9/18 8:00 PM, Andrew Morton wrote:
> 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.

Doh, right! Thanks for catching this.

This? Not terribly elegant, but I don't see a nicer way right now...

----8<----
From 40b84707e1b5aeccff11bd5f0563bb938e2c22d6 Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <vbabka@suse.cz>
Date: Fri, 9 Nov 2018 08:47:12 +0100
Subject: [PATCH] mm, slab: fix sparse warning in kmalloc_type()

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 | 26 ++++++++++++++------------
 1 file changed, 14 insertions(+), 12 deletions(-)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 918f374e7156..ca113d4ecc6f 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -314,22 +314,24 @@ kmalloc_caches[NR_KMALLOC_TYPES][KMALLOC_SHIFT_HIGH + 1];
 
 static __always_inline enum kmalloc_cache_type kmalloc_type(gfp_t flags)
 {
-	int is_dma = 0;
-	int type_dma = 0;
-	int is_reclaimable;
+	int gfp_dma = IS_ENABLED(CONFIG_ZONE_DMA) ? __GFP_DMA : 0;
 
-#ifdef CONFIG_ZONE_DMA
-	is_dma = !!(flags & __GFP_DMA);
-	type_dma = is_dma * KMALLOC_DMA;
-#endif
-
-	is_reclaimable = !!(flags & __GFP_RECLAIMABLE);
+	/*
+	 * The most common case is KMALLOC_NORMAL, so test for it
+	 * with a single branch for both flags.
+	 */
+	if (likely((flags & (gfp_dma | __GFP_RECLAIMABLE)) == 0))
+		return KMALLOC_NORMAL;
 
 	/*
-	 * If an allocation is both __GFP_DMA and __GFP_RECLAIMABLE, return
-	 * KMALLOC_DMA and effectively ignore __GFP_RECLAIMABLE
+	 * At least one of the flags has to be set. If both are, __GFP_DMA
+	 * is more important.
 	 */
-	return type_dma + (is_reclaimable & !is_dma) * KMALLOC_RECLAIM;
+#ifdef CONFIG_ZONE_DMA
+	return flags & gfp_dma ? KMALLOC_DMA : KMALLOC_RECLAIM;
+#else
+	return KMALLOC_RECLAIM;
+#endif
 }
 
 /*
-- 
2.19.1