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:
947a09defa262ebb6b816e9a091221
See change log for performance claims.
Closes#292
087c29e07f