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Date: Tue, 18 Dec 2007 22:53:47 +0100
From: Eric Dumazet <dada1@cosmosbay.com>
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To: Andrew Morton <akpm@linux-foundation.org>
CC: Linux kernel <linux-kernel@vger.kernel.org>
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>


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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;

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