MLton 20210117

# SSA2

SSA2 is an IntermediateLanguage, translated from SSA by ToSSA2, optimized by SSA2Simplify, and translated by ToRSSA to RSSA.

## Description

SSA2 is a FirstOrder, SimplyTyped IntermediateLanguage, a slight variant of the SSA IntermediateLanguage,

Like SSA, an SSA2 program consists of a collection of datatype declarations, a sequence of global statements, and a collection of functions, along with a distinguished "main" function. Each function consists of a collection of basic blocks, where each basic block is a sequence of statements ending with some control transfer.

Unlike SSA, SSA2 includes mutable fields in objects and makes the vector type constructor n-ary instead of unary. This allows optimizations like RefFlatten and DeepFlatten to be expressed.

## Implementation

- ssa2.sig

- ssa2.fun

- ssa-tree2.sig

- ssa-tree2.fun

## Type Checking

Type checking (type-check2.sig, type-check2.fun) of a SSA2 program verifies the following:

- no duplicate definitions (tycons, cons, vars, labels, funcs)

- no out of scope references (tycons, cons, vars, labels, funcs)

- variable definitions dominate variable uses

- case transfers are exhaustive and irredundant

- Enter/Leave profile statements match

- "traditional" well-typedness

## Details and Notes

SSA is an abbreviation for Static Single Assignment.