The Register IR

WebAssembly specifies a stack machine: instructions consume operands from and push results to an implicit operand stack. Baedeker does not execute that stack machine directly. Instead, lower_module translates it into a register IR — a representation where values live in named registers and control flow is explicit.

Why lower to registers

A register IR separates what a value is from how it flows:

• Phi-copy joins. When control flow converges (end of an if/else, a loop back-edge, a br_table target), each branch produces its values into registers, and the join copies them into the continuation’s expected locations. This makes polymorphic-stack and multi-value joins mechanical.
• No implicit stack. The interpreter never reconstructs operand-stack depths; each instruction reads its inputs from explicit register slots.
• Type-checked once. Validation runs over the stack machine; the register IR inherits well-typedness, so execution trusts the IR shape.

Branch values and block types

WebAssembly blocks carry result types. Baedeker carries branch values through the register IR: a br to a target with arity n copies n registers into the target’s incoming slots. br_table joins require a consistent arity across all targets and per-target subtype conformance — a property the official br_table spec tests exercise heavily.

Funcref identity

Reference values carry an (instance, function) pair rather than a bare index, so funcref identity is meaningful across linked modules. Instance 0 is the default for unlinked execution; linking rewrites references to the resolved instance.

The register IR is serializable (behind the serde feature), which is what the ahead-of-time pipeline builds on.