makedumpfile: optimize is_zero_page

[Marc Milgram]

There are local complaints that  filtering out only zero pages is slow.  I
found that is_zero_page was inefficient.  It checks if the page contains any
non-zero bytes - one byte at a time.

Improve performance by checking for non-zero data 64 bits at a time.  Also,
unroll the loop for additional performance.

Did testing in x86_64 mode on an Intel Xeon x5560 system with 18GB RAM.
Executed:
  time makedumpfile -d 1 /proc/vmcore <destination>

The amount of time taken in User space was reduced by 75%.  The total time to
dump memory was reduced by 28%.

is_zero_page 
Signed-off-by: Marc Milgram <mmilgram at redhat.com>
---
diff --git a/makedumpfile.h b/makedumpfile.h
index 3d270c6..0f211c4 100644
--- a/makedumpfile.h
+++ b/makedumpfile.h
@@ -1634,10 +1634,27 @@ static inline int
 is_zero_page(unsigned char *buf, long page_size)
 {
 	size_t i;
+	unsigned long long *vect = (unsigned long long *) buf;
+	long page_len = page_size / (sizeof(unsigned long long));
 
-	for (i = 0; i < page_size; i++)
-		if (buf[i])
+	for (i = 0; i < page_len; i+=8) {
+		if (vect[i])
 			return FALSE;
+		if (vect[i+1])
+			return FALSE;
+		if (vect[i+2])
+			return FALSE;
+		if (vect[i+3])
+			return FALSE;
+		if (vect[i+4])
+			return FALSE;
+		if (vect[i+5])
+			return FALSE;
+		if (vect[i+6])
+			return FALSE;
+		if (vect[i+7])
+			return FALSE;
+	}
 	return TRUE;
 }