[Python-ideas] More compact bytecode

[Antoine Pitrou]

Serhiy Storchaka <storchaka at ...> writes:
> 
> 2. Use 16-bit opcodes as in WPython.
> 
> This will decrease mean size of opcode from 2.73 to 2.044 bytes (1.33 
> times). But this might make decoding of 85% opcodes slightly faster.

It sounds like, by 16-bit opcodes, you mean combine the opcode and the
argument in
a single 16-bit word.  But that doesn't solve the issue you want to solve: you
still have to decode the argument encoded in the 16-bit word.  I don't see
where the benefit is.

The *byte* size of bytecode is IMO largely unimportant. Python bytecode is
synthetic and high-level, its expressive density is much higher than that of
native code. I don't think the cache occupancy of bytecode is a significant
contributor to Python performance (certainly much less so than the memory
consumption of *objects*).

(as for the disk occupancy of pyc files, if that is a concern we could actually
compress those files; there are very fast compressors with decent efficiency
these days, such as lzo or snappy)

It is generally estimated the overhead of bytecode dispatch and decoding is
around
10-30% for CPython. You cannot hope to eliminate that overhead entirely without
writing a (JIT or AOT) compiler, so any heroic effort to restructure the current
opcode space and structure will at best win 5 to 20% on select benchmarks.

Regards

Antoine.