In binaryen, each pass is constructed by several visitors and walkers.

This page is about how to select suitable visitors and walkers for your passes.

in wasm-traversal.h, we can find all basic visitors and walkers.

There are two layers of abstraction here, visitors and walkers.

visitor

The visitor provides a set of interfaces for accessing expressions.

• Visitor: will call visit<OpCode> for each expression.
• UnifiedExpressionVisitor: will call visitExpression for each expression.

walker

The walker is decoupled with visitors, is used to control the order of access expressions.

• PostWalker: will call visitor in post-order, i.e., children first. Its access order is related to the execution order of the wasm bytecode.
• ExpressionStackWalker: based on PostWalker, and using a stack to store the parent expression.

hook point

Each walker maintains a stack to store operation (static method pointer and Expression *). Hook point can be found by searching self->pushTask(SubType::... in base class. These static methods need to be overridden in derived classes to hook.

A common used hook point:

scan can be used to change the visit behavior. We can insert specific methods into the scan or adjust the traversal order.