Constant fold initializers of final variables #14283
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| @@ -3273,7 +3273,7 @@ L2: | |||
| [case testFinalStaticInt] | |||
| from typing import Final | |||
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| x: Final = 1 + 1 | |||
| x: Final = 1 + int() | |||
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The intent of changes like these is to preserve the old test case behavior where constant folding wasn't performed.
| L2: | ||
| r2 = CPyTagged_Add(r0, 2) | ||
| a = r2 | ||
| a = 14 |
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This illustrates how much we can simplify the generated code. The simpler code is also significantly faster, since there is no memory read any more.
| @@ -8,8 +8,9 @@ x | |||
| [out] | |||
| MypyFile:1( | |||
| AssignmentStmt:1( | |||
| NameExpr(x* [__main__.x]) | |||
| IntExpr(1)) | |||
| NameExpr(x [__main__.x]) | |||
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Changes such as these are a side effect of making semantic analyzer tests use the constant folding logic. Previously it was disabled due to historical reasons. The actual behavior outside tests wasn't changed (for most of these test changes).
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According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉 |
| if isinstance(expr, IntExpr): | ||
| return expr.value | ||
| if isinstance(expr, StrExpr): | ||
| return expr.value | ||
| if isinstance(expr, FloatExpr): | ||
| return expr.value | ||
| elif isinstance(expr, NameExpr): | ||
| if expr.name == "True": | ||
| return True | ||
| elif expr.name == "False": | ||
| return False |
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Interesting, this is very similar logic to the code @JelleZijlstra added in evalexpr.py in a9c62c5. I wonder if that logic could be reused here?
(Apologies if this comment makes no sense; I don't know much about mypyc internals!)
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Yeah, there is definitely some overlap. Right now the benefits of sharing code don't seem big enough to me to make it worthwhile to increase coupling, but if we add support for more things, it may make sense to refactor and share parts of the implementations. Note that mypy and mypyc already share some of the constant folding code.
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I'm not using a visitor, since only relatively few AST node types are handled here, and others have shared default behavior. I generally use a visitor when I need to implement logic for many node types. However, I think that in this case both using and not using a visitor would have been reasonable.
Now mypy can figure out the values of final variables even if the initializer has some operations on constant values:
Currently we support integer arithmetic and bitwise operations, and string concatenation.
This can be useful with literal types, but my main goal was to improve constant folding in mypyc. In particular, this helps constant folding with native ints in cases like these:
We still have another constant folding pass in mypyc, since it does some things more aggressively (e.g. it constant folds some member expression references).
Work on mypyc/mypyc#772.
Also helps with mypyc/mypyc#862.