On Thu, 07 Feb 2008 12:52:23 -0800
Harvey Harrison <[EMAIL PROTECTED]> wrote:
> 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 <[EMAIL PROTECTED]>
> ---
> 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
> --
hrm. For my i386 build (and that's where CONFIG_HIGHMEM really matters)
this patch makes mm/page_alloc.o six bytes larger, and I don't think the
change did much for readability.
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