gzip: speed up send_bits()

Replace one RMW op with store. This speeds up gzip of a png file by ~2%.

function                                             old     new   delta
send_bits                                             62      66      +4

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

diff --git a/archival/gzip.c b/archival/gzip.c
index f253a217a4b4db489e08779f9f6e447dc7213f61..08633d667c1b8122e1ba20ef9bcc32e0e8b11856 100644 (file)
--- a/archival/gzip.c
+++ b/archival/gzip.c
@@ -523,7 +523,6 @@ static unsigned file_read(void *buf, unsigned size)
 static void send_bits(unsigned value, unsigned length)
 {
        unsigned new_buf;
-       unsigned remain;
 
 #ifdef DEBUG
        Tracev((stderr, " l %2d v %4x ", length, value));
@@ -534,25 +533,26 @@ static void send_bits(unsigned value, unsigned length)
 
        new_buf = G1.bi_buf | (value << G1.bi_valid);
        /* NB: the above may sometimes do "<< 32" shift (undefined)
-        * if check below is changed to "length > remain" instead of >= */
-       remain = BUF_SIZE - G1.bi_valid;
+        * if check below is changed to "length > BUF_SIZE" instead of >= */
+       length += G1.bi_valid;
 
        /* If bi_buf is full */
-       if (length >= remain) {
+       if (length >= BUF_SIZE) {
                /* ...use (valid) bits from bi_buf and
                 * (BUF_SIZE - bi_valid) bits from value,
                 *  leaving (width - (BUF_SIZE-bi_valid)) unused bits in value.
                 */
+               value >>= (BUF_SIZE - G1.bi_valid);
                if (BUF_SIZE == 32) {
                        put_32bit(new_buf); /* maybe unroll to 2*put_16bit()? */
                } else { /* 16 */
                        put_16bit(new_buf);
                }
-               new_buf = value >> remain;
+               new_buf = value;
                length -= BUF_SIZE;
        }
        G1.bi_buf = new_buf;
-       G1.bi_valid += length;
+       G1.bi_valid = length;
 }