Module wasm::core::sidetable

This module contains a data structure to allow in-place interpretation

Control-flow in WASM is denoted in labels. To avoid linear search through the WASM binary or stack for the respective label of a branch, a sidetable is generated during validation, which stores the offset on the current instruction pointer for the branch. A sidetable entry hence allows to translate the implicit control flow information (“jump to the next else”) to explicit modifications of the instruction pointer (instruction_pointer += 13).

Branches in WASM can only go outwards, they either break out of a block or continue to the head of a loob block. Put differently, a label can only be referenced from within its associated structured control instruction.

Noteworthy, branching can also have side-effects on the operand stack:

• Taking a branch unwinds the operand stack, down to where the targeted structured control flow instruction was entered. SidetableEntry::popcnt holds information on how many values to pop from the operand stack when a branch is taken.
• When a branch is taken, it may consume arguments from the operand stack. These are pushed back on the operand stack after unwinding. This behavior can be emulated by copying the uppermost SidetableEntry::valcnt operands on the operand stack before taking a branch into a structured control instruction.

Reference

• Core / Syntax / Instructions / Control Instructions, WASM Spec, https://webassembly.github.io/spec/core/syntax/instructions.html#control-instructions
• “A fast in-place interpreter for WebAssembly”, Ben L. Titzer, https://arxiv.org/abs/2205.01183

Structs

• SidetableEntry
  Entry to translate the current branches implicit target into an explicit offset to the instruction pointer, as well as the side table pointer

Type Aliases

• Sidetable