Lesson 34 · if and else - Build a WASM Runtime

if and else

if and else are the structured conditional: run one branch or the other based on a popped value. Today you add them, built on the same label machinery as block.

The goal

Execute if and else, running the then-branch when the condition is nonzero and the else-branch when it is zero.

Start here - the target

TO DO

Scenario: Choosing a branch by condition

Giventhe interpreter running a body that contains an if

Whenif pops its condition and runs the matching branch

Then41 01 04 7F 41 2A 05 41 07 0B 0B (i32.const 1, if result i32, i32.const 42, else, i32.const 7, end) runs the then-branch and leaves 42

Andwith a zero condition - 41 00 04 7F 41 2A 05 41 07 0B 0B - the else-branch runs instead and the result is 7

Background

if (0x04) is the structured conditional, and it reuses everything you built for block. It carries a blocktype and pushes a label whose target is just past the matching end, so a branch can escape it like any other region. What is new is that if pops an i32 condition the moment it runs: if the value is nonzero, execution enters the then-branch; if it is zero, execution jumps forward to the else branch (0x05), or to the end if there is no else.

The else opcode marks the boundary between the two branches. When the then-branch runs and reaches else by falling through, it must skip the else-branch and continue at the end - you do not run both halves. So i32.const 1 then if ... else ... end takes the then-branch and yields 42, while i32.const 0 takes the else-branch and yields 7. If the blocktype declares a result, both branches must leave one value, which is how if/else doubles as an expression that produces a value. With if in place, the engine has the full structured-control vocabulary except the multi-way branch.

Make it work

// if (0x04) pops an i32 and pushes a label like block. Nonzero -> run the
// then-branch; zero -> skip to just after else (0x05). end (0x0B) closes it.
case 0x04: // if
  bt := readBlockType(body, &pc)
  labels = append(labels, Label{Arity: bt.results, Target: matchingEnd(body, pc)})
  if stack.PopI32() == 0 { pc = elseOrEnd(body, pc) } // jump to else/end
case 0x05: // else: reached only by falling out of the then-branch; skip to end
  pc = matchingEnd(...)

CheckpointDONE

The engine can choose between two branches with if and else. Commit and stop here.