[issue4753] Faster opcode dispatch on gcc

[Alexandre Vassalotti]

Alexandre Vassalotti <alexandre at peadrop.com> added the comment:

> Removing the switch won't be possible unless we change the semantic
> EXTENDED_ARG. In addition, I doubt the improvement, if any, would worth
> the increased complexity.

Nevermind what I have said. I managed to remove switch pretty easily by
moving opcode fetching in the FAST_DISPATCH macro and abstracting the
control flow of the switch. There is no speed difference on pybench on
x86; on x86-64, the code is slower due to the opcode fetching change.

> I patched ceval.c to minimize f->last_i manipulations in the dispatch
> code.  On x86, I got an extra 9% speed up on pybench. However, the
> patch is a bit clumsy and a few unit tests are failing. I will see
> if I can improve it and open a new issue if worthwhile.

Nevermind that too. I found out f->last_i can be accessed anytime via
frame.getlineno(). So, you cannot really change how f->last_i is used
like I did.

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