Lesson 29 · Declared locals, local.set and local.tee

Declared locals, local.set and local.tee

Beyond its parameters, a function can declare working locals, and it writes them with local.set and local.tee. Today you initialize the declared locals and add the two instructions that assign to them.

The goal

Initialize declared locals to zero and execute local.set and local.tee.

Start here - the target

TO DO

Scenario: Writing to locals

Givena function whose declared locals follow its parameters, each starting at 0

Whenlocal.set and local.tee execute

Thenlocal.set pops a value and stores it, so setting local 0 to 7 then local.get 0 pushes 7

Andlocal.tee pops, stores, and pushes the value back, so tee 7 into a local leaves 7 on the stack and stored in the local

Background

Parameters are only the first locals; a function also declares its own working locals in the code section - the run-length list you decoded back in lesson 11. Those slots come right after the parameters in the same index space, and WebAssembly guarantees they start at zero. So when you build the frame, seed it with the arguments, then append one zero-initialized slot per declared local. Now local.get n can read any of them.

Writing is two instructions that differ by one detail. local.set (0x21) pops the top of the stack and stores it into the local - a plain assignment. local.tee (0x22) does the same store but leaves a copy on the stack, so the value is both saved and still available to the next instruction. That copy-and-keep is exactly what you want for the update-and-test idiom a loop uses - local.tee a counter, then immediately compare it - which is why real loops lean on tee. With locals readable and writable, the engine has everything the control-flow chapter needs to build actual loops.

Make it work

// Declared locals (from the code section) follow the parameters and start at
// zero. set (0x21) pops and stores; tee (0x22) stores but keeps a copy.
case 0x21: // local.set
  idx := readVarU32(body, &pc); frame.Locals[idx] = stack.Pop()
case 0x22: // local.tee
  idx := readVarU32(body, &pc); v := stack.Peek(); frame.Locals[idx] = v

CheckpointDONE

The engine can declare, read, and write local variables. Commit and stop here.