[Python-ideas] More compact bytecode

[Andrew Barnert]

On Thursday, February 4, 2016 8:55 AM, Serhiy Storchaka <storchaka at gmail.com> wrote:

> > On 04.02.16 18:16, Victor Stinner wrote:


>>>  2. Use 16-bit opcodes as in WPython.
>> 
>>  IMHO this is a much more interesting option.
>> 
>>  It would be great to remove "if (HAS_ARG(opcode)) oparg = 
> NEXTARG();"
>>  and always get the argument from the 16-bit opcode. This if() adds
>>  more work to the CPU which has to flush the pipeline on branch
>>  misprediction. Usually, it doesn't matter. For this if() is really
>>  part of the hot path code Python, it's the most important loop running
>>  the bytecode. So any instruction matters here ;-)
> 
> Actually in common case this "if" is executed at compile time. But I 
> expect a benefit from using simple read of 16-bit value instead of 3 
> reads of 8-bit values.

Only if the 16-bit reads are aligned. Can that be guaranteed somehow?

If you change co_code to some type that's like bytes, but an immutable array of shorts instead of bytes, that would certainly do it. And it would be easier to inspect/hack 16-bit bytecodes from Python if you didn't have to look at them as bytes. But that implies creating the new type, exposing it to Python, and changing what co_code returns.

Or it could just be a simple switch when constructing a code object: if the code argument is aligned, use it as-is; if not, copy its contents into an aligned array, build a new bytes around that, and store that instead. (Would that significantly slow down .pyc reads and other marshaling uses, if half of all code objects have to copy their contents?)