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Date: Wed, 13 Feb 2008 11:56:46 -0800
From: Harvey Harrison <harvey.harrison@...il.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, viro@...IV.linux.org.uk,
linux-kernel@...r.kernel.org, mingo@...e.hu
Subject: Re: [PATCH] fix sparse warning from include/linux/mmzone.h
Maybe I'm beating a dead horse, but here's what I came up with:
include/linux/mmzone.h:640:22: warning: potentially expensive pointer subtraction
Using arch/x86/mm/highmem_32.o as an example, line 82, PageHighMem
expands to is_highmem.
if (!PageHighMem(page))
return page_address(page);
I've run through the current code, one of Linus' suggestions and Linus'
alternate version of my original patch without the stupid unsigned
long casts, using char *. The best version appears to be the char *
version. Code size will increase by one byte for each use of
is_highmem, but there is one fewer instruction (saves a sar over the
base code). The reason for the code size increase is the two cmps
use 32-bit compares rather than 16 bit compares in the original due
to the sar beforehand. (-3 bytes for no sar +4 bytes for longer cmp)
It's a small thing, if you agree that the new code is better, I'll
send a patch.
Original:
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());
#else
return 0;
#endif
}
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
Linus' suggestion:
static inline int is_highmem(struct zone *zone)
{
#ifdef CONFIG_HIGHMEM
struct zone *base = zone->zone_pgdat->node_zones;
return zone == base + ZONE_HIGHMEM ||
(zone == base + ZONE_MOVABLE && zone_movable_is_highmem());
#else
return 0;
#endif
}
202: 8d 90 00 00 00 00 lea 0x0(%eax),%edx
208: 8b 8a 8c 07 00 00 mov 0x78c(%edx),%ecx
20e: 8d 81 00 10 00 00 lea 0x1000(%ecx),%eax
214: 39 c2 cmp %eax,%edx
216: 74 1b je 233 <kmap_atomic_prot+0x14c>
218: 8d 81 00 18 00 00 lea 0x1800(%ecx),%eax
21e: 39 c2 cmp %eax,%edx
220: 0f 85 8f 00 00 00 jne 2b5 <kmap_atomic_prot+0x1ce>
226: 83 3d 00 00 00 00 02 cmpl $0x2,0x0
22d: 0f 85 82 00 00 00 jne 2b5 <kmap_atomic_prot+0x1ce>
233: 64 a1 00 00 00 00 mov %fs:0x0,%eax
An alternate suggestion (excuse the long line):
static inline int is_highmem(struct zone *zone)
{
#ifdef CONFIG_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
}
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
Cheers,
Harvey
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