[issue11549] Build-out an AST optimizer, moving some functionality out of the peephole optimizer

[Eugene Toder]

Eugene Toder <eltoder at gmail.com> added the comment:

I found a problem in constant de-duplication, already performed by compiler, that needs to be fixed before this change can be merged. 

Compiler tries to eliminate duplicate constants by putting them into a dict. However, "duplicate" in this case doesn't mean just "equal", we need a stronger relationship, as there are many equal values that behave differently in some contexts, e.g. 0 and 0.0 and False or 0.0 and -0.0. To this end for each value we create a tuple of the value and it's type and have some logic for -0.0. This is handled in compiler_add_o in Python/compile.c.

This logic, however, only works for scalar values -- if we get a container with 0 and the same container with False we collapse them into one. This was not a problem before, because constant tuples were only created by peephole, which doesn't attempt de-duplication. If tuple folding is moved to AST we start hitting this problem:

>>> dis(lambda: print((0,1),(False,True)))
  1           0 LOAD_GLOBAL              0 (print)
              3 LOAD_CONST               1 ((0, 1))
              6 LOAD_CONST               1 ((0, 1))
              9 CALL_FUNCTION            2
             12 RETURN_VALUE

The cleanest solution seems to be to introduce a new rich comparison code: Py_EQUIV (equivalent) and implement it at least in types that we support in marshal. This will simplify compiler_add_o quite a bit and make it work for tuples and frozensets.
I'm open to other suggestions.

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<http://bugs.python.org/issue11549>
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