improve base85 generated assembly code

This code is arguably pretty hot, if you use binary patches of course.
This patch helps gcc generate both smaller and faster code especially in
the error free path.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

base85.c

@@ -44,14 +44,19 @@ int decode_85(char *dst, char *buffer, int len)
 	say2("decode 85 <%.*s>", len/4*5, buffer);
 	while (len) {
 		unsigned acc = 0;
-		int cnt;
-		for (cnt = 0; cnt < 5; cnt++, buffer++) {
-			int ch = *((unsigned char *)buffer);
-			int de = de85[ch];
-			if (!de)
+		int de, cnt = 4;
+		unsigned char ch;
+		do {
+			ch = *buffer++;
+			de = de85[ch];
+			if (--de < 0)
+				return error("invalid base85 alphabet %c", ch);
+			acc = acc * 85 + de;
+		} while (--cnt);
+		ch = *buffer++;
+		de = de85[ch];
+		if (--de < 0)
 			return error("invalid base85 alphabet %c", ch);
-			de--;
-			if (cnt == 4) {
 		/*
 		 * Detect overflow.  The largest
 		 * 5-letter possible is "|NsC0" to
@@ -60,18 +65,17 @@ int decode_85(char *dst, char *buffer, int len)
 		 * 0xffffffff = 0x03030303 * 85).
 		 */
 		if (0x03030303 < acc ||
-				    (0x03030303 == acc && de))
-					error("invalid base85 sequence %.5s",
-					      buffer-3);
-			}
-			acc = acc * 85 + de;
-			say1(" <%08x>", acc);
-		}
+		    0xffffffff - de < (acc *= 85))
+			error("invalid base85 sequence %.5s", buffer-5);
+		acc += de;
 		say1(" %08x", acc);
-		for (cnt = 0; cnt < 4 && len; cnt++, len--) {
-			*dst++ = (acc >> 24) & 0xff;
-			acc = acc << 8;
-		}
+
+		cnt = (len < 4) ? len : 4;
+		len -= cnt;
+		do {
+			acc = (acc << 8) | (acc >> 24);
+			*dst++ = acc;
+		} while (--cnt);
 	}
 	say("\n");
 
@@ -86,15 +90,17 @@ void encode_85(char *buf, unsigned char *data, int bytes)
 	while (bytes) {
 		unsigned acc = 0;
 		int cnt;
-		for (cnt = 0; cnt < 4 && bytes; cnt++, bytes--) {
+		for (cnt = 24; cnt >= 0; cnt -= 8) {
 			int ch = *data++;
-			acc |= ch << ((3-cnt)*8);
+			acc |= ch << cnt;
+			if (--bytes == 0)
+				break;
 		}
 		say1(" %08x", acc);
-		for (cnt = 0; cnt < 5; cnt++) {
+		for (cnt = 4; cnt >= 0; cnt--) {
 			int val = acc % 85;
 			acc /= 85;
-			buf[4-cnt] = en85[val];
+			buf[cnt] = en85[val];
 		}
 		buf += 5;
 	}