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>

1 files changed, 28 insertions, 21 deletions

diff --git a/archival/libunarchive/decompress_bunzip2.c b/archival/libunarchive/decompress_bunzip2.c
index 3a5d23345..8d7746a79 100644
--- 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