MLton 20210117

# Shrink

Shrink is a rewrite pass for the SSA and SSA2 IntermediateLanguages, invoked from every optimization pass (see SSASimplify and SSA2Simplify).

## Description

This pass implements a whole family of compile-time reductions, like:

- #1(a, b) ⇒ a

- case C x of C y => e ⇒ let y = x in e

- constant folding, copy propagation

- eta blocks

- tuple reconstruction elimination

## Implementation

- shrink.sig

- shrink.fun

- shrink2.sig

- shrink2.fun

## Details and Notes

The Shrink pass is run after every SSA and SSA2 optimization pass.

The Shrink implementation also includes functions to eliminate unreachable blocks from a SSA or SSA2 program or function. The Shrink pass does not guarantee to eliminate all unreachable blocks. Doing so would unduly complicate the implementation, and it is almost always the case that all unreachable blocks are eliminated. However, a small number of optimization passes require that the input have no unreachable blocks (essentially, when the analysis works on the control flow graph and the rewrite iterates on the vector of blocks). These passes explicitly call eliminateDeadBlocks.

The Shrink pass has a special case to turn a non-tail call where the continuation and handler only do Profile statements into a tail call where the Profile statements precede the tail call.