xpress-decompress.c: Performance tweaks

[wimlib] / include / wimlib / decompress_common.h

/*
 * decompress_common.h
 *
 * Header for decompression code shared by multiple compression formats.
 *
 * The author dedicates this file to the public domain.
 * You can do whatever you want with this file.
 */

#ifndef _WIMLIB_DECOMPRESS_COMMON_H
#define _WIMLIB_DECOMPRESS_COMMON_H

#include "wimlib/assert.h"
#include "wimlib/compiler.h"
#include "wimlib/endianness.h"
#include "wimlib/types.h"

/* 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 {

        /* 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 currently held in @bitbuf.  */
        u32 bitsleft;

        /* Pointer to the next byte to be retrieved from the input buffer.  */
        const u8 *next;

        /* Pointer past the end of the input buffer.  */
        const u8 *end;
};

/* Initialize a bitstream to read from the specified input buffer.  */
static inline void
init_input_bitstream(struct input_bitstream *is, const void *buffer, u32 size)
{
        is->bitbuf = 0;
        is->bitsleft = 0;
        is->next = buffer;
        is->end = is->next + size;
}

/* 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.  */
static inline void
bitstream_ensure_bits(struct input_bitstream *is, const unsigned num_bits)
{
        /* This currently works for at most 17 bits.  */
        wimlib_assert2(num_bits <= 17);

        if (is->bitsleft >= num_bits)
                return;

        if (unlikely(is->end - is->next < 2))
                goto overflow;

        is->bitbuf |= (u32)le16_to_cpu(*(const 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;

                is->bitbuf |= (u32)le16_to_cpu(*(const le16 *)is->next);
                is->next += 2;
                is->bitsleft = 32;
        }

        return;

overflow:
        is->bitsleft = 32;
}

/* 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 *is, const unsigned num_bits)
{
        if (unlikely(num_bits == 0))
                return 0;
        return is->bitbuf >> (32 - 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 *is, unsigned num_bits)
{
        is->bitbuf <<= num_bits;
        is->bitsleft -= num_bits;
}

/* 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 *is, unsigned num_bits)
{
        u32 bits = bitstream_peek_bits(is, num_bits);
        bitstream_remove_bits(is, num_bits);
        return bits;
}

/* Read and return the next @num_bits bits from the bitstream.  */
static inline u32
bitstream_read_bits(struct input_bitstream *is, unsigned num_bits)
{
        bitstream_ensure_bits(is, num_bits);
        return bitstream_pop_bits(is, num_bits);
}

/* Read and return the next literal byte embedded in the bitstream.  */
static inline u8
bitstream_read_byte(struct input_bitstream *is)
{
        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)
{
        u32 v;

        if (unlikely(is->end - is->next < 4))
                return 0;
        v = le32_to_cpu(*(const le32 *)is->next);
        is->next += 4;
        return v;
}

/* Read an array of literal bytes embedded in the bitstream.  Return a pointer
 * to the resulting array, or NULL if the read overflows the input buffer.  */
static inline const u8 *
bitstream_read_bytes(struct input_bitstream *is, size_t count)
{
        const u8 *p;

        if (unlikely(is->end - is->next < count))
                return NULL;
        p = is->next;
        is->next += count;
        return p;
}

/* Align the input bitstream on a coding-unit boundary.  */
static inline void
bitstream_align(struct input_bitstream *is)
{
        is->bitsleft = 0;
        is->bitbuf = 0;
}

/* Needed alignment of decode_table parameter to make_huffman_decode_table().
 *
 * Reason: We may fill the entries with SSE instructions without worrying
 * about dealing with the unaligned case.  */
#define DECODE_TABLE_ALIGNMENT 16

/* Maximum supported symbol count for make_huffman_decode_table().
 *
 * Reason: In direct mapping entries, we store the symbol in 11 bits.  */
#define DECODE_TABLE_MAX_SYMBOLS 2048

/* Maximum supported table bits for make_huffman_decode_table().
 *
 * Reason: In internal binary tree nodes, offsets are encoded in 14 bits.
 * But the real limit is 13, because we allocate entries past the end of
 * the direct lookup part of the table for binary tree nodes.  (Note: if
 * needed this limit could be removed by encoding the offsets relative to
 * &decode_table[1 << table_bits].)  */
#define DECODE_TABLE_MAX_TABLE_BITS 13

/* Maximum supported codeword length for make_huffman_decode_table().
 *
 * Reason: In direct mapping entries, we encode the codeword length in 5
 * bits, and the top 2 bits can't both be set because that has special
 * meaning.  */
#define DECODE_TABLE_MAX_CODEWORD_LEN 23

/* 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.
 *
 * XXX: This is mostly duplicated in lzms_huffman_decode_symbol() in
 * lzms-decompress.c.  */
static inline u16
read_huffsym(struct input_bitstream *istream, const u16 decode_table[],
             unsigned table_bits, unsigned max_codeword_len)
{
        u16 entry;
        u16 key_bits;

        bitstream_ensure_bits(istream, max_codeword_len);

        /* Index the decode table by the next table_bits bits of the input.  */
        key_bits = bitstream_peek_bits(istream, table_bits);
        entry = decode_table[key_bits];
        if (likely(entry < 0xC000)) {
                /* Fast case: The decode table directly provided the
                 * symbol and codeword length.  The low 11 bits are the
                 * symbol, and the high 5 bits are the codeword length.  */
                bitstream_remove_bits(istream, entry >> 11);
                return entry & 0x7FF;
        } else {
                /* Slow case: The codeword for the symbol is longer than
                 * table_bits, so the symbol does not have an entry
                 * directly in the first (1 << table_bits) entries of the
                 * decode table.  Traverse the appropriate binary tree
                 * bit-by-bit to decode the symbol.  */
                bitstream_remove_bits(istream, table_bits);
                do {
                        key_bits = (entry & 0x3FFF) + bitstream_pop_bits(istream, 1);
                } while ((entry = decode_table[key_bits]) >= 0xC000);
                return entry;
        }
}

extern int
make_huffman_decode_table(u16 decode_table[], unsigned num_syms,
                          unsigned num_bits, const u8 lens[],
                          unsigned max_codeword_len);


/*
 * Copy a LZ77 match at (dst - offset) to dst.
 *
 * The length and offset must be already validated --- that is, (dst - offset)
 * can't underrun the output buffer, and (dst + length) can't overrun the output
 * buffer.  Also, the length cannot be 0.
 *
 * @winend points to the byte past the end of the output buffer.
 * This function won't write any data beyond this position.
 */
static inline void
lz_copy(u8 *dst, u32 length, u32 offset, const u8 *winend)
{
        const u8 *src = dst - offset;
#if defined(__x86_64__) || defined(__i386__)
        /* Copy one 'unsigned long' 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 an 'unsigned long' 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
         * (winend - (dst + length) >= sizeof(unsigned long) - 1).  */
        if (offset >= sizeof(unsigned long) &&
                        winend - (dst + length) >= sizeof(unsigned long) - 1)
        {
                /* Access memory through a packed struct.  This tricks the
                 * compiler into allowing unaligned memory accesses.  */
                struct ulong_wrapper {
                        unsigned long v;
                } _packed_attribute;

                const u8 * const end = dst + length;
                unsigned long v;

                v = ((struct ulong_wrapper *)src)->v;
                ((struct ulong_wrapper *)dst)->v = v;
                dst += sizeof(unsigned long);
                src += sizeof(unsigned long);

                if (dst < end) {
                        do {
                                v = ((struct ulong_wrapper *)src)->v;
                                ((struct ulong_wrapper *)dst)->v = v;
                                dst += sizeof(unsigned long);
                                src += sizeof(unsigned long);
                        } while (dst < end);
                }

                return;
        }
#endif
        do {
                *dst++ = *src++;
        } while (--length);
}

#endif /* _WIMLIB_DECOMPRESS_COMMON_H */