Subject: [PATCH 3/3] xdiff: import new 32-bit version of count_masked_bytes()

From: René Scharfe <[email-masked]>
Date: Tue, 22 May 2012 22:36:57 +0200

Import the latest 32-bit implementation of count_masked_bytes() from
Linux (arch/x86/include/asm/word-at-a-time.h).  It's shorter and avoids
overflows and negative numbers.

This fixes test failures on 32-bit, where negative partial results had
been shifted right using the "wrong" method (logical shift right instead
of arithmetic short right).  The compiler is free to chose the method,
so it was only wrong in the sense that it didn't work as intended by us.

Reported-by: Øyvind A. Holm <[email-masked]>
Signed-off-by: Rene Scharfe <[email-masked]>
---
Does this fix all the warnings, and do the tests pass?  I can only
reproduce the "shifting to far to the right" warning..

 xdiff/xutils.c |   18 +++++-------------
 1 file changed, 5 insertions(+), 13 deletions(-)

diff --git a/xdiff/xutils.c b/xdiff/xutils.c
index 78549e3..9504eae 100644
--- a/xdiff/xutils.c
+++ b/xdiff/xutils.c
@@ -280,19 +280,11 @@ static inline long count_masked_bytes(unsigned long mask)
 		long a = (REPEAT_BYTE(0x01) / 0xff + 1);
 		return mask * a >> (sizeof(long) * 7);
 	} else {
-		/*
-		 * Modified Carl Chatfield G+ version for 32-bit *
-		 *
-		 * (a) gives us
-		 *   -1 (0, ff), 0 (ffff) or 1 (ffffff)
-		 * (b) gives us
-		 *   0 for 0, 1 for (ff ffff ffffff)
-		 * (a+b+1) gives us
-		 *   correct 0-3 bytemask count result
-		 */
-		long a = (mask - 256) >> 23;
-		long b = mask & 1;
-		return a + b + 1;
+		/* Carl Chatfield / Jan Achrenius G+ version for 32-bit */
+		/* (000000 0000ff 00ffff ffffff) -> ( 1 1 2 3 ) */
+		long a = (0x0ff0001 + mask) >> 23;
+		/* Fix the 1 for 00 case */
+		return a & mask;
 	}
 }
 
-- 
1.7.10.2