decompress_bunzip2: keep bd->writeCRC in CPU reg in the hot loop

-5 bytes on 64-bit, +7 bytes on 32-bit.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>

diff --git a/archival/libunarchive/decompress_bunzip2.c b/archival/libunarchive/decompress_bunzip2.c
index 3a5d233456227985fefd63cc089d94624e62500d..8d7746a79788916b87db74aa6f26d221d413ef78 100644 (file)
--- a/archival/libunarchive/decompress_bunzip2.c
+++ b/archival/libunarchive/decompress_bunzip2.c
@@ -492,15 +492,20 @@ static int get_next_block(bunzip_data *bd)
 int FAST_FUNC read_bunzip(bunzip_data *bd, char *outbuf, int len)
 {
        const uint32_t *dbuf;
-       int pos, current, previous, gotcount;
+       int pos, current, previous, out_count;
+       uint32_t CRC;
 
-       /* If last read was short due to end of file, return last block now */
-       if (bd->writeCount < 0) return bd->writeCount;
+       /* If we already have error/end indicator, return it */
+       if (bd->writeCount < 0)
+               return bd->writeCount;
 
-       gotcount = 0;
+       out_count = 0;
        dbuf = bd->dbuf;
+
+       /* Register-cached state (hopefully): */
        pos = bd->writePos;
        current = bd->writeCurrent;
+       CRC = bd->writeCRC; /* small loss on x86-32 (not enough regs), win on x86-64 */
 
        /* We will always have pending decoded data to write into the output
           buffer unless this is the very first call (in which case we haven't
@@ -514,8 +519,8 @@ int FAST_FUNC read_bunzip(bunzip_data *bd, char *outbuf, int len)
                /* Loop outputting bytes */
                for (;;) {
 
-                       /* If the output buffer is full, snapshot state and return */
-                       if (gotcount >= len) {
+                       /* If the output buffer is full, save cached state and return */
+                       if (out_count >= len) {
                                /* Unlikely branch.
                                 * Use of "goto" instead of keeping code here
                                 * helps compiler to realize this. */
@@ -523,17 +528,16 @@ int FAST_FUNC read_bunzip(bunzip_data *bd, char *outbuf, int len)
                        }
 
                        /* Write next byte into output buffer, updating CRC */
-                       outbuf[gotcount++] = current;
-                       bd->writeCRC = (bd->writeCRC << 8)
-                               ^ bd->crc32Table[(bd->writeCRC >> 24) ^ current];
+                       outbuf[out_count++] = current;
+                       CRC = (CRC << 8) ^ bd->crc32Table[(CRC >> 24) ^ current];
 
                        /* Loop now if we're outputting multiple copies of this byte */
                        if (bd->writeCopies) {
                                /* Unlikely branch */
                                /*--bd->writeCopies;*/
                                /*continue;*/
-                               /* Same, but (ab)using other existing --writeCopies operation.
-                                * Luckily, this also compiles into just one branch insn: */
+                               /* Same, but (ab)using other existing --writeCopies operation
+                                * (and this if() compiles into just test+branch pair): */
                                goto dec_writeCopies;
                        }
  decode_next_byte:
@@ -549,7 +553,7 @@ int FAST_FUNC read_bunzip(bunzip_data *bd, char *outbuf, int len)
                        /* After 3 consecutive copies of the same byte, the 4th
                         * is a repeat count.  We count down from 4 instead
                         * of counting up because testing for non-zero is faster */
-                       if (--bd->writeRunCountdown) {
+                       if (--bd->writeRunCountdown != 0) {
                                if (current != previous)
                                        bd->writeRunCountdown = 4;
                        } else {
@@ -568,11 +572,11 @@ int FAST_FUNC read_bunzip(bunzip_data *bd, char *outbuf, int len)
                } /* for(;;) */
 
                /* Decompression of this input block completed successfully */
-               bd->writeCRC = ~bd->writeCRC;
-               bd->totalCRC = ((bd->totalCRC << 1) | (bd->totalCRC >> 31)) ^ bd->writeCRC;
+               bd->writeCRC = CRC = ~CRC;
+               bd->totalCRC = ((bd->totalCRC << 1) | (bd->totalCRC >> 31)) ^ CRC;
 
-               /* If this block had a CRC error, force file level CRC error. */
-               if (bd->writeCRC != bd->headerCRC) {
+               /* If this block had a CRC error, force file level CRC error */
+               if (CRC != bd->headerCRC) {
                        bd->totalCRC = bd->headerCRC + 1;
                        return RETVAL_LAST_BLOCK;
                }
@@ -581,23 +585,26 @@ int FAST_FUNC read_bunzip(bunzip_data *bd, char *outbuf, int len)
        /* Refill the intermediate buffer by Huffman-decoding next block of input */
        {
                int r = get_next_block(bd);
-               if (r) {
+               if (r) { /* error/end */
                        bd->writeCount = r;
-                       return (r != RETVAL_LAST_BLOCK) ? r : gotcount;
+                       return (r != RETVAL_LAST_BLOCK) ? r : out_count;
                }
        }
 
-       bd->writeCRC = ~0;
+       CRC = ~0;
        pos = bd->writePos;
        current = bd->writeCurrent;
        goto decode_next_byte;
 
  outbuf_full:
-       /* Output buffer is full, snapshot state and return */
+       /* Output buffer is full, save cached state and return */
        bd->writePos = pos;
        bd->writeCurrent = current;
+       bd->writeCRC = CRC;
+
        bd->writeCopies++;
-       return gotcount;
+
+       return out_count;
 }
 
 /* Allocate the structure, read file header.  If in_fd==-1, inbuf must contain