lib/extable.c: remove an expensive integer divide in search_extable()
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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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 @@
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;