What version of CPython did you try that with? The latest py3k branch?
I had a quick look at 3.2, 2.5 and 2.7 and got the impression that
the savings is more if the interpreter loop is faster: the fewer
instructions there are, the bigger a 3 instruction difference would
make.
The NEXTARG macro is the same in all three versions:
#define NEXTARG() (next_instr += 2, (next_instr[-1]<<8) +
next_instr[-2])
and the compiler compiles this to two separate fetches.
I found out my compiler (gcc) will make better code if we used a short.
It produces a "movswl" instruction to do both fetches at the same
time, if I force it to.
That saves two instructions already.
This would imply that on little-endian machines, this would already
save a few percent changing just 1 line of code in ceval.c:
#define NEXTARG() (next_instr += 2, *(short *)&next_instr[-2])
- Jurjen
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