* decompress_common.h
*
* Header for decompression code shared by multiple compression formats.
+ *
+ * The following copying information applies to this specific source code file:
+ *
+ * Written in 2012-2016 by Eric Biggers <ebiggers3@gmail.com>
+ *
+ * To the extent possible under law, the author(s) have dedicated all copyright
+ * and related and neighboring rights to this software to the public domain
+ * worldwide via the Creative Commons Zero 1.0 Universal Public Domain
+ * Dedication (the "CC0").
+ *
+ * This software is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+ * FOR A PARTICULAR PURPOSE. See the CC0 for more details.
+ *
+ * You should have received a copy of the CC0 along with this software; if not
+ * see <http://creativecommons.org/publicdomain/zero/1.0/>.
*/
#ifndef _WIMLIB_DECOMPRESS_COMMON_H
#define _WIMLIB_DECOMPRESS_COMMON_H
-#include "wimlib/assert.h"
+#include <string.h>
+
#include "wimlib/compiler.h"
-#include "wimlib/error.h"
-#include "wimlib/endianness.h"
#include "wimlib/types.h"
+#include "wimlib/unaligned.h"
-#ifndef INPUT_IDX_T_DEFINED
-#define INPUT_IDX_T_DEFINED
-typedef u32 input_idx_t;
-#endif
+/******************************************************************************/
+/* Input bitstream for XPRESS and LZX */
+/*----------------------------------------------------------------------------*/
-/* Structure to encapsulate a block of in-memory data that is being interpreted
- * as a stream of bits.
- *
- * This is geared specifically towards the XPRESS and LZX compression formats
- * with regards to the actual ordering the bits within the byte sequence. */
+/* Structure that encapsulates a block of in-memory data being interpreted as a
+ * stream of bits, optionally with interwoven literal bytes. Bits are assumed
+ * to be stored in little endian 16-bit coding units, with the bits ordered high
+ * to low. */
struct input_bitstream {
- /* A variable of length at least 32 bits that is used to hold bits that
- * have been read from the stream. The bits are ordered from high-order
- * to low-order, and the next bit is always the high-order bit. */
+ /* Bits that have been read from the input buffer. The bits are
+ * left-justified; the next bit is always bit 31. */
u32 bitbuf;
- /* Number of bits in @bitbuf that are valid. */
- unsigned bitsleft;
+ /* Number of bits currently held in @bitbuf. */
+ u32 bitsleft;
- /* Pointer to the next byte to be retrieved from the input. */
- const u8 *data;
+ /* Pointer to the next byte to be retrieved from the input buffer. */
+ const u8 *next;
- /* Number of bytes of data that are left. */
- input_idx_t data_bytes_left;
+ /* Pointer past the end of the input buffer. */
+ const u8 *end;
};
-/* Initializes a bitstream to receive its input from @data. */
+/* Initialize a bitstream to read from the specified input buffer. */
static inline void
-init_input_bitstream(struct input_bitstream *istream,
- const void *data, input_idx_t num_data_bytes)
+init_input_bitstream(struct input_bitstream *is, const void *buffer, u32 size)
{
- istream->bitbuf = 0;
- istream->bitsleft = 0;
- istream->data = data;
- istream->data_bytes_left = num_data_bytes;
+ is->bitbuf = 0;
+ is->bitsleft = 0;
+ is->next = buffer;
+ is->end = is->next + size;
}
-/* Ensures the bit buffer variable for the bitstream contains at least @num_bits
+/* Note: for performance reasons, the following methods don't return error codes
+ * to the caller if the input buffer is overrun. Instead, they just assume that
+ * all overrun data is zeroes. This has no effect on well-formed compressed
+ * data. The only disadvantage is that bad compressed data may go undetected,
+ * but even this is irrelevant if higher level code checksums the uncompressed
+ * data anyway. */
+
+/* Ensure the bit buffer variable for the bitstream contains at least @num_bits
* bits. Following this, bitstream_peek_bits() and/or bitstream_remove_bits()
- * may be called on the bitstream to peek or remove up to @num_bits bits.
- *
- * If the input data is exhausted, any further bits are assumed to be 0. */
+ * may be called on the bitstream to peek or remove up to @num_bits bits. */
static inline void
-bitstream_ensure_bits(struct input_bitstream *istream, unsigned num_bits)
+bitstream_ensure_bits(struct input_bitstream *is, const unsigned num_bits)
{
- for (int nbits = num_bits; (int)istream->bitsleft < nbits; nbits -= 16) {
- u16 nextword;
- unsigned shift;
+ /* This currently works for at most 17 bits. */
- if (unlikely(istream->data_bytes_left < 2)) {
- istream->bitsleft = num_bits;
- return;
- }
+ if (is->bitsleft >= num_bits)
+ return;
+
+ if (unlikely(is->end - is->next < 2))
+ goto overflow;
+
+ is->bitbuf |= (u32)get_unaligned_le16(is->next) << (16 - is->bitsleft);
+ is->next += 2;
+ is->bitsleft += 16;
+
+ if (unlikely(num_bits == 17 && is->bitsleft == 16)) {
+ if (unlikely(is->end - is->next < 2))
+ goto overflow;
- nextword = le16_to_cpu(*(const le16*)istream->data);
- shift = sizeof(istream->bitbuf) * 8 - 16 - istream->bitsleft;
- istream->bitbuf |= (u32)nextword << shift;
- istream->data += 2;
- istream->bitsleft += 16;
- istream->data_bytes_left -= 2;
+ is->bitbuf |= (u32)get_unaligned_le16(is->next);
+ is->next += 2;
+ is->bitsleft = 32;
}
+
+ return;
+
+overflow:
+ is->bitsleft = 32;
}
-/* Returns the next @num_bits bits from the bitstream, without removing them.
+/* Return the next @num_bits bits from the bitstream, without removing them.
* There must be at least @num_bits remaining in the buffer variable, from a
* previous call to bitstream_ensure_bits(). */
static inline u32
-bitstream_peek_bits(const struct input_bitstream *istream, unsigned num_bits)
+bitstream_peek_bits(const struct input_bitstream *is, const unsigned num_bits)
{
- if (unlikely(num_bits == 0))
- return 0;
- return istream->bitbuf >> (sizeof(istream->bitbuf) * 8 - num_bits);
+ return (is->bitbuf >> 1) >> (sizeof(is->bitbuf) * 8 - num_bits - 1);
}
-/* Removes @num_bits from the bitstream. There must be at least @num_bits
+/* Remove @num_bits from the bitstream. There must be at least @num_bits
* remaining in the buffer variable, from a previous call to
* bitstream_ensure_bits(). */
static inline void
-bitstream_remove_bits(struct input_bitstream *istream, unsigned num_bits)
+bitstream_remove_bits(struct input_bitstream *is, unsigned num_bits)
{
- istream->bitbuf <<= num_bits;
- istream->bitsleft -= num_bits;
+ is->bitbuf <<= num_bits;
+ is->bitsleft -= num_bits;
}
-/* Removes and returns @num_bits bits from the bitstream. There must be at
- * least @num_bits remaining in the buffer variable, from a previous call to
+/* Remove and return @num_bits bits from the bitstream. There must be at least
+ * @num_bits remaining in the buffer variable, from a previous call to
* bitstream_ensure_bits(). */
static inline u32
-bitstream_pop_bits(struct input_bitstream *istream, unsigned num_bits)
+bitstream_pop_bits(struct input_bitstream *is, unsigned num_bits)
{
- u32 n = bitstream_peek_bits(istream, num_bits);
- bitstream_remove_bits(istream, num_bits);
- return n;
+ u32 bits = bitstream_peek_bits(is, num_bits);
+ bitstream_remove_bits(is, num_bits);
+ return bits;
}
-/* Reads and returns the next @num_bits bits from the bitstream.
- * If the input data is exhausted, the bits are assumed to be 0. */
+/* Read and return the next @num_bits bits from the bitstream. */
static inline u32
-bitstream_read_bits(struct input_bitstream *istream, unsigned num_bits)
+bitstream_read_bits(struct input_bitstream *is, unsigned num_bits)
{
- bitstream_ensure_bits(istream, num_bits);
- return bitstream_pop_bits(istream, num_bits);
+ bitstream_ensure_bits(is, num_bits);
+ return bitstream_pop_bits(is, num_bits);
}
-/* Reads and returns the next literal byte embedded in the bitstream.
- * If the input data is exhausted, the byte is assumed to be 0. */
+/* Read and return the next literal byte embedded in the bitstream. */
static inline u8
-bitstream_read_byte(struct input_bitstream *istream)
+bitstream_read_byte(struct input_bitstream *is)
{
- if (unlikely(istream->data_bytes_left == 0))
+ if (unlikely(is->end == is->next))
return 0;
- istream->data_bytes_left--;
- return *istream->data++;
+ return *is->next++;
}
-/* Reads and returns the next Huffman-encoded symbol from a bitstream. If the
- * input data is exhausted, the Huffman symbol is decoded as if the missing bits
- * are all zeroes. */
+/* Read and return the next 16-bit integer embedded in the bitstream. */
static inline u16
-read_huffsym(struct input_bitstream * restrict istream,
- const u16 decode_table[restrict],
- const u8 lens[restrict],
- unsigned num_syms,
- unsigned table_bits,
- unsigned max_codeword_len)
-{
-
- bitstream_ensure_bits(istream, max_codeword_len);
-
- /* Use the next table_bits of the input as an index into the
- * decode_table. */
- u16 key_bits = bitstream_peek_bits(istream, table_bits);
-
- u16 sym = decode_table[key_bits];
-
- if (likely(sym < num_syms)) {
- /* Fast case: The decode table directly provided the symbol. */
- bitstream_remove_bits(istream, lens[sym]);
- } else {
- /* Slow case: The symbol took too many bits to include directly
- * in the decode table, so search for it in a binary tree at the
- * end of the decode table. */
- bitstream_remove_bits(istream, table_bits);
- do {
- key_bits = sym + bitstream_pop_bits(istream, 1);
- } while ((sym = decode_table[key_bits]) >= num_syms);
+bitstream_read_u16(struct input_bitstream *is)
+{
+ u16 v;
+
+ if (unlikely(is->end - is->next < 2))
+ return 0;
+ v = get_unaligned_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)
+{
+ u32 v;
+
+ if (unlikely(is->end - is->next < 4))
+ return 0;
+ v = get_unaligned_le32(is->next);
+ is->next += 4;
+ return v;
+}
+
+/* Read into @dst_buffer an array of literal bytes embedded in the bitstream.
+ * Return 0 if there were enough bytes remaining in the input, otherwise -1. */
+static inline int
+bitstream_read_bytes(struct input_bitstream *is, void *dst_buffer, size_t count)
+{
+ if (unlikely(is->end - is->next < count))
+ return -1;
+ memcpy(dst_buffer, is->next, count);
+ is->next += count;
+ return 0;
+}
+
+/* Align the input bitstream on a coding-unit boundary. */
+static inline void
+bitstream_align(struct input_bitstream *is)
+{
+ is->bitsleft = 0;
+ is->bitbuf = 0;
+}
+
+/******************************************************************************/
+/* Huffman decoding */
+/*----------------------------------------------------------------------------*/
+
+/*
+ * Required alignment for the Huffman decode tables. We require this alignment
+ * so that we can fill the entries with vector or word instructions and not have
+ * to deal with misaligned buffers.
+ */
+#define DECODE_TABLE_ALIGNMENT 16
+
+/*
+ * Each decode table entry is 16 bits divided into two fields: 'symbol' (high 12
+ * bits) and 'length' (low 4 bits). The precise meaning of these fields depends
+ * on the type of entry:
+ *
+ * Root table entries which are *not* subtable pointers:
+ * symbol: symbol to decode
+ * length: codeword length in bits
+ *
+ * Root table entries which are subtable pointers:
+ * symbol: index of start of subtable
+ * length: number of bits with which the subtable is indexed
+ *
+ * Subtable entries:
+ * symbol: symbol to decode
+ * length: codeword length in bits, minus the number of bits with which the
+ * root table is indexed
+ */
+#define DECODE_TABLE_SYMBOL_SHIFT 4
+#define DECODE_TABLE_MAX_SYMBOL ((1 << (16 - DECODE_TABLE_SYMBOL_SHIFT)) - 1)
+#define DECODE_TABLE_MAX_LENGTH ((1 << DECODE_TABLE_SYMBOL_SHIFT) - 1)
+#define DECODE_TABLE_LENGTH_MASK DECODE_TABLE_MAX_LENGTH
+#define MAKE_DECODE_TABLE_ENTRY(symbol, length) \
+ (((symbol) << DECODE_TABLE_SYMBOL_SHIFT) | (length))
+
+/*
+ * Read and return the next Huffman-encoded symbol from the given bitstream
+ * using the given decode table.
+ *
+ * If the input data is exhausted, then the Huffman symbol will be decoded as if
+ * the missing bits were all zeroes.
+ *
+ * XXX: This is mostly duplicated in lzms_decode_huffman_symbol() in
+ * lzms_decompress.c; keep them in sync!
+ */
+static inline unsigned
+read_huffsym(struct input_bitstream *is, const u16 decode_table[],
+ unsigned table_bits, unsigned max_codeword_len)
+{
+ unsigned entry;
+ unsigned symbol;
+ unsigned length;
+
+ /* Preload the bitbuffer with 'max_codeword_len' bits so that we're
+ * guaranteed to be able to fully decode a codeword. */
+ bitstream_ensure_bits(is, max_codeword_len);
+
+ /* Index the root table by the next 'table_bits' bits of input. */
+ entry = decode_table[bitstream_peek_bits(is, table_bits)];
+
+ /* Extract the "symbol" and "length" from the entry. */
+ symbol = entry >> DECODE_TABLE_SYMBOL_SHIFT;
+ length = entry & DECODE_TABLE_LENGTH_MASK;
+
+ /* If the root table is indexed by the full 'max_codeword_len' bits,
+ * then there cannot be any subtables, and this will be known at compile
+ * time. Otherwise, we must check whether the decoded symbol is really
+ * a subtable pointer. If so, we must discard the bits with which the
+ * root table was indexed, then index the subtable by the next 'length'
+ * bits of input to get the real entry. */
+ if (max_codeword_len > table_bits &&
+ entry >= (1U << (table_bits + DECODE_TABLE_SYMBOL_SHIFT)))
+ {
+ /* Subtable required */
+ bitstream_remove_bits(is, table_bits);
+ entry = decode_table[symbol + bitstream_peek_bits(is, length)];
+ symbol = entry >> DECODE_TABLE_SYMBOL_SHIFT;
+ length = entry & DECODE_TABLE_LENGTH_MASK;
}
- return sym;
+
+ /* Discard the bits (or the remaining bits, if a subtable was required)
+ * of the codeword. */
+ bitstream_remove_bits(is, length);
+
+ /* Return the decoded symbol. */
+ return symbol;
}
+/*
+ * The DECODE_TABLE_ENOUGH() macro evaluates to the maximum number of decode
+ * table entries, including all subtable entries, that may be required for
+ * decoding a given Huffman code. This depends on three parameters:
+ *
+ * num_syms: the maximum number of symbols in the code
+ * table_bits: the number of bits with which the root table will be indexed
+ * max_codeword_len: the maximum allowed codeword length in the code
+ *
+ * Given these parameters, the utility program 'enough' from zlib, when passed
+ * the three arguments 'num_syms', 'table_bits', and 'max_codeword_len', will
+ * compute the maximum number of entries required. This has already been done
+ * for the combinations we need and incorporated into the macro below so that
+ * the mapping can be done at compilation time. If an unknown combination is
+ * used, then a compilation error will result. To fix this, use 'enough' to
+ * find the missing value and add it below. If that still doesn't fix the
+ * compilation error, then most likely a constraint would be violated by the
+ * requested parameters, so they cannot be used, at least without other changes
+ * to the decode table --- see DECODE_TABLE_SIZE().
+ */
+#define DECODE_TABLE_ENOUGH(num_syms, table_bits, max_codeword_len) ( \
+ ((num_syms) == 8 && (table_bits) == 7 && (max_codeword_len) == 15) ? 128 : \
+ ((num_syms) == 8 && (table_bits) == 5 && (max_codeword_len) == 7) ? 36 : \
+ ((num_syms) == 8 && (table_bits) == 6 && (max_codeword_len) == 7) ? 66 : \
+ ((num_syms) == 8 && (table_bits) == 7 && (max_codeword_len) == 7) ? 128 : \
+ ((num_syms) == 20 && (table_bits) == 5 && (max_codeword_len) == 15) ? 1062 : \
+ ((num_syms) == 20 && (table_bits) == 6 && (max_codeword_len) == 15) ? 582 : \
+ ((num_syms) == 20 && (table_bits) == 7 && (max_codeword_len) == 15) ? 390 : \
+ ((num_syms) == 54 && (table_bits) == 9 && (max_codeword_len) == 15) ? 618 : \
+ ((num_syms) == 54 && (table_bits) == 10 && (max_codeword_len) == 15) ? 1098 : \
+ ((num_syms) == 249 && (table_bits) == 9 && (max_codeword_len) == 16) ? 878 : \
+ ((num_syms) == 249 && (table_bits) == 10 && (max_codeword_len) == 16) ? 1326 : \
+ ((num_syms) == 249 && (table_bits) == 11 && (max_codeword_len) == 16) ? 2318 : \
+ ((num_syms) == 256 && (table_bits) == 9 && (max_codeword_len) == 15) ? 822 : \
+ ((num_syms) == 256 && (table_bits) == 10 && (max_codeword_len) == 15) ? 1302 : \
+ ((num_syms) == 256 && (table_bits) == 11 && (max_codeword_len) == 15) ? 2310 : \
+ ((num_syms) == 512 && (table_bits) == 10 && (max_codeword_len) == 15) ? 1558 : \
+ ((num_syms) == 512 && (table_bits) == 11 && (max_codeword_len) == 15) ? 2566 : \
+ ((num_syms) == 512 && (table_bits) == 12 && (max_codeword_len) == 15) ? 4606 : \
+ ((num_syms) == 656 && (table_bits) == 10 && (max_codeword_len) == 16) ? 1734 : \
+ ((num_syms) == 656 && (table_bits) == 11 && (max_codeword_len) == 16) ? 2726 : \
+ ((num_syms) == 656 && (table_bits) == 12 && (max_codeword_len) == 16) ? 4758 : \
+ ((num_syms) == 799 && (table_bits) == 9 && (max_codeword_len) == 15) ? 1366 : \
+ ((num_syms) == 799 && (table_bits) == 10 && (max_codeword_len) == 15) ? 1846 : \
+ ((num_syms) == 799 && (table_bits) == 11 && (max_codeword_len) == 15) ? 2854 : \
+ -1)
+
+/* Wrapper around DECODE_TABLE_ENOUGH() that does additional compile-time
+ * validation. */
+#define DECODE_TABLE_SIZE(num_syms, table_bits, max_codeword_len) ( \
+ \
+ /* All values must be positive. */ \
+ STATIC_ASSERT_ZERO((num_syms) > 0) + \
+ STATIC_ASSERT_ZERO((table_bits) > 0) + \
+ STATIC_ASSERT_ZERO((max_codeword_len) > 0) + \
+ \
+ /* There cannot be more symbols than possible codewords. */ \
+ STATIC_ASSERT_ZERO((num_syms) <= 1U << (max_codeword_len)) + \
+ \
+ /* There is no reason for the root table to be indexed with
+ * more bits than the maximum codeword length. */ \
+ STATIC_ASSERT_ZERO((table_bits) <= (max_codeword_len)) + \
+ \
+ /* The maximum symbol value must fit in the 'symbol' field. */ \
+ STATIC_ASSERT_ZERO((num_syms) - 1 <= DECODE_TABLE_MAX_SYMBOL) + \
+ \
+ /* The maximum codeword length in the root table must fit in
+ * the 'length' field. */ \
+ STATIC_ASSERT_ZERO((table_bits) <= DECODE_TABLE_MAX_LENGTH) + \
+ \
+ /* The maximum codeword length in a subtable must fit in the
+ * 'length' field. */ \
+ STATIC_ASSERT_ZERO((max_codeword_len) - (table_bits) <= \
+ DECODE_TABLE_MAX_LENGTH) + \
+ \
+ /* The minimum subtable index must be greater than the maximum
+ * symbol value. If this were not the case, then there would
+ * be no way to tell whether a given root table entry is a
+ * "subtable pointer" or not. (An alternate solution would be
+ * to reserve a flag bit specifically for this purpose.) */ \
+ STATIC_ASSERT_ZERO((1U << table_bits) > (num_syms) - 1) + \
+ \
+ /* The needed 'enough' value must have been defined. */ \
+ STATIC_ASSERT_ZERO(DECODE_TABLE_ENOUGH( \
+ (num_syms), (table_bits), \
+ (max_codeword_len)) > 0) + \
+ \
+ /* The maximum subtable index must fit in the 'symbol' field. */\
+ STATIC_ASSERT_ZERO(DECODE_TABLE_ENOUGH( \
+ (num_syms), (table_bits), \
+ (max_codeword_len)) - 1 <= \
+ DECODE_TABLE_MAX_SYMBOL) + \
+ \
+ /* Finally, make the macro evaluate to the needed maximum
+ * number of decode table entries. */ \
+ DECODE_TABLE_ENOUGH((num_syms), (table_bits), \
+ (max_codeword_len)) \
+)
+
+/*
+ * Declare the decode table for a Huffman code, given several compile-time
+ * constants that describe the code. See DECODE_TABLE_ENOUGH() for details.
+ *
+ * Decode tables must be aligned to a DECODE_TABLE_ALIGNMENT-byte boundary.
+ * This implies that if a decode table is nested inside a dynamically allocated
+ * structure, then the outer structure must be allocated on a
+ * DECODE_TABLE_ALIGNMENT-byte aligned boundary as well.
+ */
+#define DECODE_TABLE(name, num_syms, table_bits, max_codeword_len) \
+ u16 name[DECODE_TABLE_SIZE((num_syms), (table_bits), \
+ (max_codeword_len))] \
+ _aligned_attribute(DECODE_TABLE_ALIGNMENT)
+
+/*
+ * Declare the temporary "working_space" array needed for building the decode
+ * table for a Huffman code.
+ */
+#define DECODE_TABLE_WORKING_SPACE(name, num_syms, max_codeword_len) \
+ u16 name[2 * ((max_codeword_len) + 1) + (num_syms)];
+
extern int
make_huffman_decode_table(u16 decode_table[], unsigned num_syms,
- unsigned num_bits, const u8 lengths[],
- unsigned max_codeword_len);
+ unsigned table_bits, const u8 lens[],
+ unsigned max_codeword_len, u16 working_space[]);
-/* Minimum alignment for the decode_table parameter to
- * make_huffman_decode_table(). */
-#define DECODE_TABLE_ALIGNMENT 16
+/******************************************************************************/
+/* LZ match copying */
+/*----------------------------------------------------------------------------*/
+
+static inline void
+copy_word_unaligned(const void *src, void *dst)
+{
+ store_word_unaligned(load_word_unaligned(src), dst);
+}
+
+static inline machine_word_t
+repeat_u16(u16 b)
+{
+ machine_word_t v = b;
+
+ STATIC_ASSERT(WORDBITS == 32 || WORDBITS == 64);
+ v |= v << 16;
+ v |= v << ((WORDBITS == 64) ? 32 : 0);
+ return v;
+}
+
+static inline machine_word_t
+repeat_byte(u8 b)
+{
+ return repeat_u16(((u16)b << 8) | b);
+}
+
+/*
+ * Copy an LZ77 match of 'length' bytes from the match source at 'out_next -
+ * offset' to the match destination at 'out_next'. The source and destination
+ * may overlap.
+ *
+ * This handles validating the length and offset. It is validated that the
+ * beginning of the match source is '>= out_begin' and that end of the match
+ * destination is '<= out_end'. The return value is 0 if the match was valid
+ * (and was copied), otherwise -1.
+ *
+ * 'min_length' is a hint which specifies the minimum possible match length.
+ * This should be a compile-time constant.
+ */
+static inline int
+lz_copy(u32 length, u32 offset, u8 *out_begin, u8 *out_next, u8 *out_end,
+ u32 min_length)
+{
+ const u8 *src;
+ u8 *end;
+
+ /* Validate the offset. */
+ if (unlikely(offset > out_next - out_begin))
+ return -1;
+
+ /*
+ * Fast path: copy a match which is no longer than a few words, is not
+ * overlapped such that copying a word at a time would produce incorrect
+ * results, and is not too close to the end of the buffer. Note that
+ * this might copy more than the length of the match, but that's okay in
+ * this scenario.
+ */
+ src = out_next - offset;
+ if (UNALIGNED_ACCESS_IS_FAST && length <= 3 * WORDBYTES &&
+ offset >= WORDBYTES && out_end - out_next >= 3 * WORDBYTES)
+ {
+ copy_word_unaligned(src + WORDBYTES*0, out_next + WORDBYTES*0);
+ copy_word_unaligned(src + WORDBYTES*1, out_next + WORDBYTES*1);
+ copy_word_unaligned(src + WORDBYTES*2, out_next + WORDBYTES*2);
+ return 0;
+ }
+
+ /* Validate the length. This isn't needed in the fast path above, due
+ * to the additional conditions tested, but we do need it here. */
+ if (unlikely(length > out_end - out_next))
+ return -1;
+ end = out_next + length;
+
+ /*
+ * Try to copy one word at a time. On i386 and x86_64 this is faster
+ * than copying one byte at a time, unless the data is near-random and
+ * all the matches have very short lengths. Note that since this
+ * requires unaligned memory accesses, it won't necessarily be faster on
+ * every architecture.
+ *
+ * Also note that we might copy more than the length of the match. For
+ * example, if a word is 8 bytes and the match is of length 5, then
+ * we'll simply copy 8 bytes. This is okay as long as we don't write
+ * beyond the end of the output buffer, hence the check for (out_end -
+ * end >= WORDBYTES - 1).
+ */
+ if (UNALIGNED_ACCESS_IS_FAST && likely(out_end - end >= WORDBYTES - 1))
+ {
+ if (offset >= WORDBYTES) {
+ /* The source and destination words don't overlap. */
+ do {
+ copy_word_unaligned(src, out_next);
+ src += WORDBYTES;
+ out_next += WORDBYTES;
+ } while (out_next < end);
+ return 0;
+ } else if (offset == 1) {
+ /* Offset 1 matches are equivalent to run-length
+ * encoding of the previous byte. This case is common
+ * if the data contains many repeated bytes. */
+ machine_word_t v = repeat_byte(*(out_next - 1));
+ do {
+ store_word_unaligned(v, out_next);
+ src += WORDBYTES;
+ out_next += WORDBYTES;
+ } while (out_next < end);
+ return 0;
+ }
+ /*
+ * We don't bother with special cases for other 'offset <
+ * WORDBYTES', which are usually rarer than 'offset == 1'.
+ * Extra checks will just slow things down. Actually, it's
+ * possible to handle all the 'offset < WORDBYTES' cases using
+ * the same code, but it still becomes more complicated doesn't
+ * seem any faster overall; it definitely slows down the more
+ * common 'offset == 1' case.
+ */
+ }
+
+ /* Fall back to a bytewise copy. */
+ if (min_length >= 2)
+ *out_next++ = *src++;
+ if (min_length >= 3)
+ *out_next++ = *src++;
+ if (min_length >= 4)
+ *out_next++ = *src++;
+ do {
+ *out_next++ = *src++;
+ } while (out_next != end);
+ return 0;
+}
-#endif /* _WIMLIB_DECOMPRESS_H */
+#endif /* _WIMLIB_DECOMPRESS_COMMON_H */
git://wimlib.net / wimlib / blobdiff