fastest way to write a bitstream on modern x86 hardware

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,