[Python-Dev] new bytecode results

[damien morton]

> From: M.-A. Lemburg [mailto:mal@lemburg.com] 
> 
> Back in the 1.5.2 days I played a lot with the ceval loop and 
> the best results I got came from:
> 
> a) moving the LOAD_FAST opcode out of the switch

Are you suggesting a test for LOAD_FAST before the switch, 

e.g.
if (opcode == LOAD_FAST) {
 // load fast
}
else switch (opcode) {
 // body
}

> b) splitting the switch statement in two: the first one for
>     more commonly used opcodes, the second one for less often
>     used opcodes

Are you suggesting something like: 

switch (opcode) {
 case COMMON_OP:
 case COMMON_OP:
 ...
 default:
   switch (opcode) {
     case UNCOMMON_OP:
     case UNCOMMON_OP:
}

Or something like this:

switch (opcode) {
 case COMMON_OP:
 case COMMON_OP:
 ...
 default:
   handle_uncommon_ops();
}


> c) ordering cases in the switch statements by usage frequency
>     (using average opcode usage frequencs obtained by
>     instrumenting the interpreter)

I might try a little simulated annealing to generate layouts with high
frequency opcodes near the front and coorcurring opcodes near each
other.

> The last point is probably compiler dependent. GCC has the 
> tendency to use the same layout for the assembler code as you 
> use in the C source code, so placing often used code close to 
> the top results in better locality (at least on my machines).