Static Single Assignment (SSA) form intermediate representation.

This crate provides an SSA form representation for JavaScript/ECMAScript code, which is a lower-level IR than TAC (Three-Address Code) that enforces stronger invariants for optimization.

Static Single Assignment Form

In SSA form:

• Each variable is assigned exactly once (static single assignment)
• Variables are versioned when they would be reassigned in non-SSA form
• Control flow merges use block parameters instead of φ-functions
• Enables more powerful dataflow analysis and optimizations

Block Parameters Instead of Phi Nodes

This SSA representation uses block parameters instead of traditional φ-functions. Basic blocks take parameters just like functions, and predecessors pass arguments when jumping to them. This approach:

• Makes SSA construction and transformation easier to reason about
• Avoids edge cases that occur with φ-nodes (e.g., critical edges)
• Provides a cleaner representation for optimization passes
• Naturally handles multiple values being merged at the same point

For example, instead of:

block:
  x = φ(x1 from pred1, x2 from pred2)

We use:

block(x):        // x is a block parameter
  ...
pred1:
  jump block(x1) // pass x1 as argument
pred2:
  jump block(x2) // pass x2 as argument

Key Types

• SFunc: A function in SSA form
• SCfg: The SSA control flow graph
• SBlock: A basic block with parameters (replacing φ-functions)
• SValue: An SSA value (operation, parameter, load, store, etc.)
• SValueW: A wrapper around SValue for arena storage
• STerm: Block terminator with target blocks and arguments
• STarget: A jump target (block + arguments for its parameters)

Conversion from TAC

The primary entry point is converting from TAC to SSA form. This involves:

1. Identifying variables that need versioning
2. Creating block parameters at control flow merges (instead of φ-functions)
3. Threading SSA values through the control flow graph
4. Converting assignments to SSA form
5. Passing appropriate arguments when jumping to blocks

Modules

• consts: Constant propagation in SSA form
• conv: Conversion from TAC to SSA
• impls: Trait implementations for SSA types
• opt_stub: Optimization stub/framework
• [rew]: Rewriting passes
• simplify: Simplification passes