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Useless

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Useless is an optimization pass for the SSA IntermediateLanguage, invoked from SSASimplify.

## Description

This pass:

- removes components of tuples that are constants (use unification)

- removes function arguments that are constants

- builds some kind of dependence graph where

- a value of ground type is useful if it is an arg to a primitive - a tuple is useful if it contains a useful component - a constructor is useful if it contains a useful component or is used in a `Case` transfer

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

useless.sig useless.fun

## 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.

Unify all handler arguments so that `raise`/`handle` has a consistent calling convention.

Last edited on 2006-11-02 17:56:21 by MatthewFluet.