include/linux/slab.h: 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.

Link: http://lkml.kernel.org/r/80340595-d7c5-97b9-4f6c-23fa893a91e9@suse.cz
Fixes: 36e78392f462 ("mm, slab/slub: introduce kmalloc-reclaimable caches")
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reported-by: Bart Van Assche <bvanassche@acm.org>
Reported-by: Darryl T. Agostinelli <dagostinelli@gmail.com>
Reported-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Suggested-by: David Laight <David.Laight@ACULAB.COM>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 351ac48dabc4d43670fc9ea5b108497c91798875..6d9bd6fc0c571775b4b7d49cba613f7d6940dd97 100644 (file)
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -314,22 +314,22 @@ 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;
-
 #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;
+       return flags & __GFP_DMA ? KMALLOC_DMA : KMALLOC_RECLAIM;
+#else
+       return flags & __GFP_RECLAIMABLE ? KMALLOC_RECLAIM : KMALLOC_NORMAL;
+#endif
 }
 
 /*