xpress-decompress.c: Performance tweaks
authorEric Biggers <ebiggers3@gmail.com>
Sat, 6 Sep 2014 18:05:43 +0000 (13:05 -0500)
committerEric Biggers <ebiggers3@gmail.com>
Sat, 6 Sep 2014 20:45:15 +0000 (15:45 -0500)
include/wimlib/decompress_common.h
src/xpress-decompress.c

index 3f3742a..99930b9 100644 (file)
@@ -131,11 +131,24 @@ bitstream_read_bits(struct input_bitstream *is, unsigned num_bits)
 static inline u8
 bitstream_read_byte(struct input_bitstream *is)
 {
-       if (unlikely(is->end - is->next < 1))
+       if (unlikely(is->end == is->next))
                return 0;
        return *is->next++;
 }
 
+/* Read and return the next 16-bit integer embedded in the bitstream.  */
+static inline u16
+bitstream_read_u16(struct input_bitstream *is)
+{
+       u16 v;
+
+       if (unlikely(is->end - is->next < 2))
+               return 0;
+       v = le16_to_cpu(*(const le16 *)is->next);
+       is->next += 2;
+       return v;
+}
+
 /* Read and return the next 32-bit integer embedded in the bitstream.  */
 static inline u32
 bitstream_read_u32(struct input_bitstream *is)
index 05d0c49..bf42c1f 100644 (file)
@@ -1,7 +1,7 @@
 /*
  * xpress-decompress.c
  *
- * XPRESS decompression routines.
+ * A very fast decompressor for XPRESS (Huffman version).
  */
 
 /*
 /* This value is chosen for fast decompression.  */
 #define XPRESS_TABLEBITS 12
 
-/*
- * Decodes a symbol @sym that begins an XPRESS match.
- *
- * The low 8 bits of the symbol are divided into:
- *
- * bits 0-3:  length header
- * bits 4-7:  index of high-order bit of match offset
- *
- * Returns the match length, or -1 if the data is invalid.
- */
+/* Decode the matches and literal bytes in a region of XPRESS-encoded data.  */
 static int
-xpress_decode_match(unsigned sym, u32 window_pos,
-                   u32 window_len, u8 window[restrict],
-                   struct input_bitstream * restrict istream)
+xpress_decode_window(struct input_bitstream *istream, const u16 *decode_table,
+                    u8 *window, unsigned window_size)
 {
-       unsigned len_hdr;
-       unsigned offset_bsr;
+       u8 *window_ptr = window;
+       u8 *window_end = &window[window_size];
+       unsigned sym;
        unsigned match_len;
+       unsigned offset_bsr;
        unsigned match_offset;
 
-       sym -= XPRESS_NUM_CHARS;
-       len_hdr = sym & 0xf;
-       offset_bsr = sym >> 4;
+       while (window_ptr != window_end) {
 
-       bitstream_ensure_bits(istream, 16);
-
-       match_offset = (1U << offset_bsr) | bitstream_pop_bits(istream, offset_bsr);
-
-       if (len_hdr == 0xf) {
-               match_len = bitstream_read_byte(istream);
-               if (unlikely(match_len == 0xff)) {
-                       match_len = bitstream_read_byte(istream);
-                       match_len |= (unsigned)bitstream_read_byte(istream) << 8;
-               } else {
-                       match_len += 0xf;
+               sym = read_huffsym(istream, decode_table,
+                                  XPRESS_TABLEBITS, XPRESS_MAX_CODEWORD_LEN);
+               if (sym < XPRESS_NUM_CHARS) {
+                       /* Literal  */
+                       *window_ptr++ = sym;
+                       continue;
                }
-       } else {
-               match_len = len_hdr;
-       }
-       match_len += XPRESS_MIN_MATCH_LEN;
 
-       if (unlikely(match_len > window_len - window_pos))
-               return -1;
+               /* Match  */
+               match_len = sym & 0xf;
+               offset_bsr = (sym >> 4) & 0xf;
 
-       if (unlikely(match_offset > window_pos))
-               return -1;
+               bitstream_ensure_bits(istream, 16);
 
-       lz_copy(&window[window_pos], match_len, match_offset,
-               &window[window_len]);
+               match_offset = (1 << offset_bsr) |
+                               bitstream_pop_bits(istream, offset_bsr);
 
-       return match_len;
-}
+               if (match_len == 0xf) {
+                       match_len += bitstream_read_byte(istream);
+                       if (match_len == 0xf + 0xff)
+                               match_len = bitstream_read_u16(istream);
+               }
+               match_len += XPRESS_MIN_MATCH_LEN;
 
-/* Decodes the Huffman-encoded matches and literal bytes in a region of
- * XPRESS-encoded data.  */
-static int
-xpress_lz_decode(struct input_bitstream * restrict istream,
-                u8 uncompressed_data[restrict],
-                unsigned uncompressed_len,
-                const u16 decode_table[restrict])
-{
-       u32 curpos;
-       unsigned match_len;
+               if (unlikely(match_offset > window_ptr - window))
+                       return -1;
 
-       for (curpos = 0; curpos < uncompressed_len; curpos += match_len) {
-               unsigned sym;
-               int ret;
+               if (unlikely(match_len > window_end - window_ptr))
+                       return -1;
 
-               sym = read_huffsym(istream, decode_table,
-                                  XPRESS_TABLEBITS, XPRESS_MAX_CODEWORD_LEN);
-               if (sym < XPRESS_NUM_CHARS) {
-                       /* Literal  */
-                       uncompressed_data[curpos] = sym;
-                       match_len = 1;
-               } else {
-                       /* Match  */
-                       ret = xpress_decode_match(sym,
-                                                 curpos,
-                                                 uncompressed_len,
-                                                 uncompressed_data,
-                                                 istream);
-                       if (unlikely(ret < 0))
-                               return -1;
-                       match_len = ret;
-               }
+               lz_copy(window_ptr, match_len, match_offset, window_end);
+
+               window_ptr += match_len;
        }
        return 0;
 }
 
-
 static int
 xpress_decompress(const void *compressed_data, size_t compressed_size,
                  void *uncompressed_data, size_t uncompressed_size, void *_ctx)
@@ -198,8 +159,8 @@ xpress_decompress(const void *compressed_data, size_t compressed_size,
        init_input_bitstream(&istream, cdata + XPRESS_NUM_SYMBOLS / 2,
                             compressed_size - XPRESS_NUM_SYMBOLS / 2);
 
-       return xpress_lz_decode(&istream, uncompressed_data,
-                               uncompressed_size, decode_table);
+       return xpress_decode_window(&istream, decode_table,
+                                   uncompressed_data, uncompressed_size);
 }
 
 static int