SSA IR

Script compiles to a Static Single Assignment (SSA) intermediate representation for optimization and code generation.

What is SSA?

In SSA form, each variable is assigned exactly once. This enables powerful optimizations and makes the code easier to analyze.

IR Structure

Types

• Number, String, Boolean, Object, Array, Function, Any, Never, Void

Ownership

• Owned, Moved, BorrowedImm, BorrowedMut, Captured

Storage

• Stack, Heap, Register

Operations

• Constants: Const
• Arithmetic: AddNum, SubNum, MulNum and dynamic AddAny, SubAny, ...
• Control flow: Jump, Branch, Return, Phi
• Memory: LoadLocal, StoreLocal, LoadProp, StoreProp

Example Transformation

// Source: let x = 1 + 2; let y = x * 3;

fn main() -> any {
bb0:
    v0 = const 1
    v1 = const 2
    v2 = add.num v0, v1      // Specialized to numeric add
    store.local $0, v2
    v3 = load.local $0
    v4 = const 3
    v5 = mul.any v3, v4
    return
}

// After optimization:
bb0:
    v2 = const 3             // 1+2 constant-folded!
    store.local $0, v2
    ...

Type Specialization

The type inference pass specializes dynamic operations:

Before	After	Speedup
add.any v0, v1	add.num v0, v1	~10x
mul.any v0, v1	mul.num v0, v1	~10x

Optimization Passes

1. Dead Code Elimination (DCE) - Remove unused code
2. Constant Folding - Evaluate constant expressions at compile time
3. Common Subexpression Elimination (CSE) - Reuse computed values
4. Copy Propagation - Replace copies with original values
5. Branch Simplification - Simplify conditional branches
6. Unreachable Block Elimination - Remove unreachable code

Inspecting IR

You can dump the IR for any Script program:

./target/release/script ir myprogram.tscl

This prints:

• Bytecode
• SSA before optimization
• SSA after type inference
• SSA after optimizations

Phi Nodes

Phi nodes handle values that come from multiple control flow paths:

IR:
  bb2: phi v5 = [(bb0, v1), (bb1, v3)]

Cranelift:
  bb2(v5: i64):
    ...
  bb0: jump bb2(v1)
  bb1: jump bb2(v3)

Verification

The IR is validated for:

• Exactly-once definitions (SSA property)
• Valid control flow (jumps and blocks)
• Use-after-move detection
• Borrow checker rules