c++ - fastest way to write a bitstream on modern x86 hardware

Question

What is the fastest way to write a bitstream on x86/x86-64? (codeword <= 32bit)

by writing a bitstream I refer to the process of concatenating variable bit-length symbols into a contiguous memory buffer.

currently I've got a standard container with a 32bit intermediate buffer to write to

void write_bits(SomeContainer<unsigned int>& dst,unsigned int& buffer, unsigned int& bits_left_in_buffer,int codeword, short bits_to_write){
    if(bits_to_write < bits_left_in_buffer){
        buffer|= codeword << (32-bits_left_in_buffer);
        bits_left_in_buffer -= bits_to_write;

    }else{
        unsigned int full_bits = bits_to_write - bits_left_in_buffer;
        unsigned int towrite = buffer|(codeword<<(32-bits_left_in_buffer));
        buffer= full_bits ? (codeword >> bits_left_in_buffer) : 0;
        dst.push_back(towrite);
        bits_left_in_buffer = 32-full_bits;
    }
}

Does anyone know of any nice optimizations, fast instructions or other info that may be of use?

Cheers,

Answer 1

I wrote once a quite fast implementation, but it has several limitations: It works on 32 bit x86 when you write and read the bitstream. I don't check for buffer limits here, I was allocating larger buffer and checked it from time to time from the calling code.

unsigned char* membuff; 
unsigned bit_pos; // current BIT position in the buffer, so it's max size is 512Mb

// input bit buffer: we'll decode the byte address so that it's even, and the DWORD from that address will surely have at least 17 free bits
inline unsigned int get_bits(unsigned int bit_cnt){ // bit_cnt MUST be in range 0..17
    unsigned int byte_offset = bit_pos >> 3;
    byte_offset &= ~1;  // rounding down by 2.
    unsigned int bits = *(unsigned int*)(membuff + byte_offset);
    bits >>= bit_pos & 0xF;
    bit_pos += bit_cnt;
    return bits & BIT_MASKS[bit_cnt];
};

// output buffer, the whole destination should be memset'ed to 0
inline unsigned int put_bits(unsigned int val, unsigned int bit_cnt){
    unsigned int byte_offset = bit_pos >> 3;
    byte_offset &= ~1;
    *(unsigned int*)(membuff + byte_offset) |= val << (bit_pos & 0xf);
    bit_pos += bit_cnt;
};