decompress_bunzip2: relieve register pressure in hot function read_bunzip

function                                             old     new   delta
unpack_bz2_stream                                    318     329     +11
read_bunzip                                          268     262      -6

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

2 files changed, 15 insertions, 7 deletions

diff --git a/archival/libunarchive/decompress_bunzip2.c b/archival/libunarchive/decompress_bunzip2.c
index 8d7746a79..00ec47c75 100644
--- a/archival/libunarchive/decompress_bunzip2.c
+++ b/archival/libunarchive/decompress_bunzip2.c
@@ -488,18 +488,21 @@ static int get_next_block(bunzip_data *bd)
    data are written to outbuf.  Return value is number of bytes written or
    error (all errors are negative numbers).  If out_fd!=-1, outbuf and len
    are ignored, data is written to out_fd and return is RETVAL_OK or error.
+
+   NB: read_bunzip returns < 0 on error, or the number of *unfilled* bytes
+   in outbuf. IOW: on EOF returns len ("all bytes are not filled"), not 0.
+   (Why? This allows to get rid of one local variable)
 */
 int FAST_FUNC read_bunzip(bunzip_data *bd, char *outbuf, int len)
 {
         const uint32_t *dbuf;
-        int pos, current, previous, out_count;
+        int pos, current, previous;
         uint32_t CRC;
 
         /* If we already have error/end indicator, return it */
         if (bd->writeCount < 0)
                 return bd->writeCount;
 
-        out_count = 0;
         dbuf = bd->dbuf;
 
         /* Register-cached state (hopefully): */
@@ -520,7 +523,7 @@ int FAST_FUNC read_bunzip(bunzip_data *bd, char *outbuf, int len)
                 for (;;) {
 
                         /* If the output buffer is full, save cached state and return */
-                        if (out_count >= len) {
+                        if (--len < 0) {
                                 /* Unlikely branch.
                                  * Use of "goto" instead of keeping code here
                                  * helps compiler to realize this. */
@@ -528,7 +531,7 @@ int FAST_FUNC read_bunzip(bunzip_data *bd, char *outbuf, int len)
                         }
 
                         /* Write next byte into output buffer, updating CRC */
-                        outbuf[out_count++] = current;
+                        *outbuf++ = current;
                         CRC = (CRC << 8) ^ bd->crc32Table[(CRC >> 24) ^ current];
 
                         /* Loop now if we're outputting multiple copies of this byte */
@@ -587,7 +590,7 @@ int FAST_FUNC read_bunzip(bunzip_data *bd, char *outbuf, int len)
                 int r = get_next_block(bd);
                 if (r) { /* error/end */
                         bd->writeCount = r;
-                        return (r != RETVAL_LAST_BLOCK) ? r : out_count;
+                        return (r != RETVAL_LAST_BLOCK) ? r : len;
                 }
         }
 
@@ -604,7 +607,7 @@ int FAST_FUNC read_bunzip(bunzip_data *bd, char *outbuf, int len)
 
         bd->writeCopies++;
 
-        return out_count;
+        return 0;
 }
 
 /* Allocate the structure, read file header.  If in_fd==-1, inbuf must contain
@@ -698,7 +701,10 @@ unpack_bz2_stream(int src_fd, int dst_fd)
                 if (i == 0) {
                         while (1) { /* "Produce some output bytes" loop */
                                 i = read_bunzip(bd, outbuf, IOBUF_SIZE);
-                                if (i <= 0)
+                                if (i < 0) /* error? */
+                                        break;
+                                i = IOBUF_SIZE - i; /* number of bytes produced */
+                                if (i == 0) /* EOF? */
                                         break;
                                 if (i != full_write(dst_fd, outbuf, i)) {
                                         bb_error_msg("short write");
diff --git a/include/unarchive.h b/include/unarchive.h
index 11d8c77a0..ba6d323e0 100644
--- a/include/unarchive.h
+++ b/include/unarchive.h
@@ -194,6 +194,8 @@ extern const llist_t *find_list_entry2(const llist_t *list, const char *filename
 /* A bit of bunzip2 internals are exposed for compressed help support: */
 typedef struct bunzip_data bunzip_data;
 int start_bunzip(bunzip_data **bdp, int in_fd, const void *inbuf, int len) FAST_FUNC;
+/* NB: read_bunzip returns < 0 on error, or the number of *unfilled* bytes
+ * in outbuf. IOW: on EOF returns len ("all bytes are not filled"), not 0: */
 int read_bunzip(bunzip_data *bd, char *outbuf, int len) FAST_FUNC;
 void dealloc_bunzip(bunzip_data *bd) FAST_FUNC;