[Python-Dev] Wordcode: new regular bytecode using 16-bit units

[Cesare Di Mauro]

2016-04-13 18:24 GMT+02:00 Victor Stinner <victor.stinner at gmail.com>:

> Demur Rumed proposes a different change to use a regular bytecode
> using 16-bit units: an instruction has always one 8-bit argument, it's
> zero if the instruction doesn't have an argument:
>
>    http://bugs.python.org/issue26647
>
> According to benchmarks, it looks faster:
>
>   http://bugs.python.org/issue26647#msg263339
>
> IMHO it's a nice enhancement: it makes the code simpler. The most
> interesting change is made in Python/ceval.c:
>
> -        if (HAS_ARG(opcode))
> -            oparg = NEXTARG();
> +        oparg = NEXTARG();
>
> This code is the very hot loop evaluating Python bytecode. I expect
> that removing a conditional branch here can reduce the CPU branch
> misprediction.
>

Correct. The old bytecode format wasn't so much predictable for the CPU.

>
> Right now, ceval.c still fetchs opcode and then oparg with two 8-bit
> instructions. Later, we can discuss if it would be possible to ensure
> that the bytecode is always aligned to 16-bit in memory to fetch the
> two bytes using a uint16_t* pointer.
>
> Maybe we can overallocate 1 byte in codeobject.c and align manually
> the memory block if needed. Or ceval.c should maybe copy the code if
> it's not aligned?
>
> Raymond Hettinger proposes something like that, but it looks like
> there are concerns about non-aligned memory accesses:
>
>    http://bugs.python.org/issue25823
>
> The cost of non-aligned memory accesses depends on the CPU
> architecture, but it can raise a SIGBUS on some arch (MIPS and
> SPARC?).
>
> Victor
>

It should not be a problem, since every PyObject is allocated with PyAlloc
(however I don't remember if it's the correct name) which AFAIK guarantees
a base 8 bytes alignment.

So, it's safe to use an unsigned int for keeping/referencing a word at the
time.

The only problem with such approach is related to the processor endianess,
but it can be solved with proper macros (like I did with WPython).

Regards,
Cesare
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