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    Subject[PATCH] fix sparse warning from include/linux/mmzone.h
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    Date
    include/linux/mmzone.h:640:22: warning: potentially expensive pointer subtraction

    The code in question was doing a pointer subtraction to find the index of
    the zone argument in the node_zones array. This essentially boils down to:

    ((unsigned long)zone - (unsigned long)zone->zone_pgdat->node_zones)/sizeof(struct zone)

    Which can be expensive if struct zone is not a power of two. Instead, just
    calculate the offsets rather than the index in the array.

    Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
    ---
    This shows up > 30 times in an x86 allyesconfig, would be nice to get
    rid of it and makes the code a little more efficient. I believe this
    is correct, unless there is a subtlety I have missed.

    include/linux/mmzone.h | 9 ++++++---
    1 files changed, 6 insertions(+), 3 deletions(-)

    diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
    index 8d8d197..cb07758 100644
    --- a/include/linux/mmzone.h
    +++ b/include/linux/mmzone.h
    @@ -637,9 +637,12 @@ 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());
    + const int highmem_off = ZONE_HIGHMEM * sizeof(*zone);
    + const int movable_off = ZONE_MOVABLE * sizeof(*zone);
    +
    + int zone_off = (unsigned long)zone - (unsigned long)zone->zone_pgdat->node_zones;
    + return zone_off == highmem_off ||
    + (zone_off == movable_off && zone_movable_is_highmem());
    #else
    return 0;
    #endif
    --
    1.5.4.1219.g65b9




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