Messages in this thread Patch in this message | | | Date | Tue, 18 Dec 2007 22:53:47 +0100 | From | Eric Dumazet <> | Subject | [PATCH] lib/extable.c : Removes an expensive integer divide in search_extable() |
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Actual code let compiler generates idiv instruction on x86.
Using a right shift is OK here and readable as well.
Before patch 10: 57 push %edi 11: 56 push %esi 12: 89 d6 mov %edx,%esi 14: 53 push %ebx 15: 89 c3 mov %eax,%ebx 17: eb 22 jmp 3b <search_extable+0x2b> 19: 89 f0 mov %esi,%eax 1b: ba 02 00 00 00 mov $0x2,%edx 20: 29 d8 sub %ebx,%eax 22: 89 d7 mov %edx,%edi 24: c1 f8 03 sar $0x3,%eax 27: 99 cltd 28: f7 ff idiv %edi 2a: 8d 04 c3 lea (%ebx,%eax,8),%eax 2d: 39 08 cmp %ecx,(%eax) ...
After patch
00000010 <search_extable>: 10: 53 push %ebx 11: 89 c3 mov %eax,%ebx 13: eb 18 jmp 2d <search_extable+0x1d> 15: 89 d0 mov %edx,%eax 17: 29 d8 sub %ebx,%eax 19: c1 f8 04 sar $0x4,%eax 1c: 8d 04 c3 lea (%ebx,%eax,8),%eax 1f: 39 08 cmp %ecx,(%eax) ...
Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
diff --git a/lib/extable.c b/lib/extable.c index 463f456..179c087 100644 --- a/lib/extable.c +++ b/lib/extable.c @@ -57,10 +57,10 @@ search_extable(const struct exception_table_entry *first, while (first <= last) { const struct exception_table_entry *mid; - mid = (last - first) / 2 + first; + mid = ((last - first) >> 1) + first; /* - * careful, the distance between entries can be - * larger than 2GB: + * careful, the distance between value and insn + * can be larger than MAX_LONG: */ if (mid->insn < value) first = mid + 1; | |