Useless is an optimization pass for the SSA IntermediateLanguage, invoked from SSASimplify.
Description
This pass:
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builds some kind of dependence graph where
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a value of ground type is useful if it is an arg to a primitive
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a tuple is useful if it contains a useful component
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a constructor is useful if it contains a useful component or is used in a
Casetransfer
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If a useful tuple is coerced to another useful tuple, then all of their components must agree (exactly). It is trivial to convert a useful value to a useless one.
Implementation
Details and Notes
It is also trivial to convert a useful tuple to one of its useful components — but this seems hard.
Suppose that you have a ref/array/vector that is useful, but the
components aren’t — then the components are converted to type unit,
and any primitive args must be as well.