MLton

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

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.