TODO

Implement a basic block type, and functionality allowing us to lift the AST to a CFG.
Implement vars_read, vars_written, signals_read, signals_written, and signals_constrained on Statement.
Compute dominators, dominator frontiers, and immediate dominators on basic blocks. (See A Simple, Fast Dominance Algorithm.)
Implement (pruned) SSA.
Implement analyses enabled by SSA:
- Constant propagation
  - Implement constant propagation.
  - Implement/update is_constant and value on Expression.
- Dead code analysis
- Value-range analysis (simple overflow detection)
- Intraprocedural data flow
- Unconstrained signals (simple)
Implement emulation.
- Unconstrained signals (specific)
Implement symbolic execution.
- Unconstrained signals (complete)
- Overflow detection (complete)

Potential issues

Bit level arithmetic does not commute with modular reduction. This means that
- Currently, (p | 1) - 1 != 0 (see circom_algebra/src/modular_arithmetic.rs)
- !x (256-bit complement) will typically overflow which means that !x does not satisfy (!x)_i = x_i ^ 1 for all i.
Arithmetic is done in (p/2, p/2] which may produce unexpected results.
- E.g. p/2 + 1 < p/2 - 1.
Typically you want to constrain all input and output signals for each instantiated component in each circuit. There are exceptions from this rule (e.g. the circomlib AliasCheck template). We should add an analysis pass ensuring that signals belonging to instantiated subcomponents are properly constrained.
Find cases when it is possible to prove that the output from a component is not uniquely determined by the input.