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