> From: Skip Montanaro [mailto:skip@pobox.com]
>
> Damien> Ive done a static analysis of the bytecodes from
> compiling the
> Damien> python standard library:
>
> ...
>
> Nice input, but I think you should deal with dynamic opcode
> frequencies. See the #define's in the ceval.c source for how
> to enable that and the recent thread in c.l.py. In short, it
> doesn't really matter how many LOAD_CONST instructions appear
> in the bytecode stream, only how many of them are executed.
> While LOAD_CONST is a relatively frequently executed opcode,
> if I recall correctly, LOAD_FAST is much more frequently
> executed, and LOAD_CONST is already pretty fast, so adding
> special opcodes probably won't buy you much and runs the risk
> of disturbing what locality of reference exists.
I will indeed take a look at dynamic opcode frequencies. This is my
first stab at it, though.
I had originaly taken a look at this from a code compression
perspective. The proposals I made should result in a non-trival
reduction in the size of the compiled bytecodes. This, in turn should
result in _some_ speedup.
As you say, LOAD_FAST is a very frequently occuring instruction, both
statically and dynamically. Reducing it from a 3 byte instruction to a 1
byte instruction in 97% of (static) cases should be an overall good.
Most of the opcodes I proposed could be added without disturbing
locality of reference.
e.g.
switch (op = *p++) {
...
case LOAD_FAST:
index = (*p++) + (*p++)<<8
goto LOAD_FAST_MAIN;
break;
case LOAD_FAST_0:
case LOAD_FAST_1:
case LOAD_FAST_15:
index = op - LOAD_FAST_0
LOAD_FAST_MAIN:
...
break;
}
> The LOAD_IF_FALSE issue was discussed recently (here I
> think). The problem is that chained operations like
>
> if a < b < c:
> ...
>
> require the value be retained. You could get rid of the
> separate POP_TOP instruction but it would require some work
> in the compiler.
Those kind of chained operations are by far the minority (20% of static
cases). They can be handled by a DUP instruction before the JUMP opcode,
or a separate non-consuming JUMP opcode can be created.
> as-always-patches-are-welcome-ly, y'rs,
>
> Skip
>
not-quite-ready-to-push-my-hands-in-up-to-the-elbows-ly, yours
Damien
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