Consider that if the bytecode is just a flattened representation, then instead of generating and traversing the tree, you can just traverse what would have been turned into the tree in the same traversal order directly.
E.g. imagine you're in a leaf production for some simple grammar for an operator that can only take integers, w/all error handling ignored:
parse_fooexpr() {
left = parse_int
emit_int(left)
expect_token(FOO_OP)
right = parse_int
emit_int(right)
emit_foo_op()
}
Whether emit_int outputs a "push <someint>" VM instruction to a bytecode buffer, or constructs an IntNode and pushes it to an argument stack, and whether "emit_foo_op" emits a "foo_op" bytecode instruction or pops the argument stack and constructs a FooOpNode(...argument stack values) AST node is an implementation detail in terms of the parser.
Wirth's compilers usually didn't use an AST, and his book on compiler construction contains a thorough walkthrough on generating code as you parse, if you want to see it explained in a lot more detail and w/tricks to generate better code than the most naive approaches will.
The difficulty with this approach comes when you want to apply optimisations or where it's inconvenient to generate the code in parse-order because the language tries to smart about what feels better to write, but you can get pretty far with this method.
E.g. imagine you're in a leaf production for some simple grammar for an operator that can only take integers, w/all error handling ignored:
Whether emit_int outputs a "push <someint>" VM instruction to a bytecode buffer, or constructs an IntNode and pushes it to an argument stack, and whether "emit_foo_op" emits a "foo_op" bytecode instruction or pops the argument stack and constructs a FooOpNode(...argument stack values) AST node is an implementation detail in terms of the parser.Wirth's compilers usually didn't use an AST, and his book on compiler construction contains a thorough walkthrough on generating code as you parse, if you want to see it explained in a lot more detail and w/tricks to generate better code than the most naive approaches will.
The difficulty with this approach comes when you want to apply optimisations or where it's inconvenient to generate the code in parse-order because the language tries to smart about what feels better to write, but you can get pretty far with this method.