[issue4715] optimize bytecode for conditional branches

[Jeffrey Yasskin]

Jeffrey Yasskin <jyasskin at gmail.com> added the comment:

In peephole.c:

_optimize isn't a great label name, but I don't have a great
replacement. Maybe reoptimize_current_index?

Your change to the "LOAD_CONST trueconst JUMP_IF_FALSE xx POP_TOP"
optimization doesn't preserve the optimization to:
  def f():
    return 1 and a
I suspect we don't care since "0 or a" wasn't optimized.

I wonder what the "POP_TOP JUMP_FORWARD 1 POP_TOP" was ever for. Why did
compiler_comprehension_generator() emit it in the first place?

After "if (JUMP_SIGN(j) == JUMP_SIGN(opcode)) {", it might be nice to
have a comment like, "/* The second jump will always be taken if the
first was. */" and similarly for the else branch with an explanation why
the POP should become unconditional.

Otherwise looks good.

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