kc3-lang/libjpeg-turbo

2018-10-22T10:05:18

Optimize Huffman encoding This commit improves the C and SSE2 Huffman encoding implementations in the following ways: - Avoid using xmm8-xmm15 in the x86-64 SSE2 implementation. There is no actual need to use those registers, and avoiding them produces a cleaner WIN64 function entry/exit-- as well as shorter code, since REX prefixes can be avoided (this is helpful on certain CPUs, such as Intel Atom, for which instruction fetch and decoding can be a bottleneck.) - Optimize register usage so that fewer REX prefixes and register-register moves are needed. - Use the bit counter to store the number of free bits in the bit buffer rather than the number of bits in the bit buffer. This changes the method for inserting a code into the bit buffer to: (put_buffer |= code << (free_bits -= code_size)); As a result: * Only one bit counter needs to stay in a register (we just keep it in cl.) * The bit buffer contents are already properly aligned to be written out (after a byte swap.) * Adjusting the free bits counter and checking if the bit buffer is full can be combined into a single operation. * We can wait to flush the bit buffer until the buffer is actually full and not just in danger of becoming full. Thus, eight bytes can be flushed at a time. - Speed is quite sensitive to the alignment of branch target labels, so insert some padding and remove branches from the flush code. (Flushing this way isn't actually faster when compared to using branches, but the branchless code doesn't need extra alignment and is thus smaller.) - Speculatively write out the bit buffer as a single 8-byte write, falling back to a byte-by-byte write only if there are any 0xFF bytes in the bit buffer that need to be encoded as 0xFF 0x00. - Use MMX registers for the 32-bit implementation (so the bit buffer can be 64 bits wide.) - Slightly reduce overall function code size. - Eliminate or combine a few SSE instructions. - Make some minor improvements to instruction scheduling. - Adjust flush_bits() in jchuff.c to handle cases in which the bit buffer has less than 7 free bits (apparently that couldn't happen before.) Based on: https://github.com/1camper/libjpeg-turbo/commit/947a09defa2ec848322b1bae050d1b57b316a32a https://github.com/1camper/libjpeg-turbo/commit/262ebb6b816fd8a49ff4d7185f6c5153dddde02f https://github.com/1camper/libjpeg-turbo/commit/6e9a091221bb244c8ba232a942650e94254ffcf0 See change log for performance claims. Closes #292