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Subject[PATCH] remove sparse warning for mmzone.h
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include/linux/mmzone.h:640:22: warning: potentially expensive pointer subtraction

Calculate the offset into the node_zones array rather than the index
using casts to (char *) and comparing against the index * sizeof(struct zone).

On X86_32 this saves a sar, but code size increases by one byte per
is_highmem() use due to 32-bit cmps rather than 16 bit cmps.

Before:
207: 2b 80 8c 07 00 00 sub 0x78c(%eax),%eax
20d: c1 f8 0b sar $0xb,%eax
210: 83 f8 02 cmp $0x2,%eax
213: 74 16 je 22b <kmap_atomic_prot+0x144>
215: 83 f8 03 cmp $0x3,%eax
218: 0f 85 8f 00 00 00 jne 2ad <kmap_atomic_prot+0x1c6>
21e: 83 3d 00 00 00 00 02 cmpl $0x2,0x0
225: 0f 85 82 00 00 00 jne 2ad <kmap_atomic_prot+0x1c6>
22b: 64 a1 00 00 00 00 mov %fs:0x0,%eax

After:
207: 2b 80 8c 07 00 00 sub 0x78c(%eax),%eax
20d: 3d 00 10 00 00 cmp $0x1000,%eax
212: 74 18 je 22c <kmap_atomic_prot+0x145>
214: 3d 00 18 00 00 cmp $0x1800,%eax
219: 0f 85 8f 00 00 00 jne 2ae <kmap_atomic_prot+0x1c7>
21f: 83 3d 00 00 00 00 02 cmpl $0x2,0x0
226: 0f 85 82 00 00 00 jne 2ae <kmap_atomic_prot+0x1c7>
22c: 64 a1 00 00 00 00 mov %fs:0x0,%eax

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
---
include/linux/mmzone.h | 7 ++++---
1 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 8d8d197..3109a65 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -637,9 +637,10 @@ static inline int is_normal_idx(enum zone_type idx)
static inline int is_highmem(struct zone *zone)
{
#ifdef CONFIG_HIGHMEM
- int zone_idx = zone - zone->zone_pgdat->node_zones;
- return zone_idx == ZONE_HIGHMEM ||
- (zone_idx == ZONE_MOVABLE && zone_movable_is_highmem());
+ int zone_off = (char *)zone - (char *)zone->zone_pgdat->node_zones;
+ return zone_off == ZONE_HIGHMEM * sizeof(*zone) ||
+ (zone_off == ZONE_MOVABLE * sizeof(*zone) &&
+ zone_movable_is_highmem());
#else
return 0;
#endif
--
1.5.4.1.1278.gc75be




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