Package com.dylibso.chicory.wasm

Class ControlTree

java.lang.Object

com.dylibso.chicory.wasm.ControlTree

public class ControlTree
extends Object

The purpose of this class is to provide a control structure to, in-line, label branches in a list of instructions.

Wasm has a structured form of control flow. There is no `jmp` to an address. So instead, each branching point can only "jump" to a few points internal to the function we are executing in. This algorithm mimics the control flow rules and annotates each branching instruction with the labels (here the jumping point is the index of the instruction in the body) that it can jump to. Branching instructions can only jump to the beginning of a block if this target is a loop, or the end of a block for every other target block.

It's up to the Machine to decide, based on what is on the stack, which label to choose.

Here is an example on how to label some code. The left side contains indexes. Comments show where the labels go.

 0     (local i32)
 1     (block
 2       (block
 3          (block
                 ;; x == 0
 4               local.get 0
 5               i32.eqz
 6               br_if 0 ;; true=14 false=7

                 ;; x == 1
 7               local.get 0
 8               i32.const 1
 9               i32.eq
 10              br_if 1 ;; true=17 false=11

                 ;; the `else` case
 11              i32.const 7
 12              local.set 1
 13              br 2    ;; true=19
            )
 14         i32.const 42
 15         local.set 1
 16         br 1         ;; true=19
            )
 17     i32.const 99
 18     local.set 1
        )
 19   local.get 1)
 

Constructor Summary

Constructors

Constructor	Description
ControlTree()

Method Summary

Modifier and Type	Method	Description
void	addCallback(Consumer<Integer> callback)
void	addNested(ControlTree nested)
Instruction	instruction()
int	instructionNumber()
boolean	isRoot()
List<ControlTree>	nested()
ControlTree	parent()
void	setFinalInstructionNumber(int finalInstructionNumber, Instruction end)
ControlTree	spawn(int initialInstructionNumber, Instruction instruction)

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

ControlTree

public ControlTree()

Method Detail

spawn

public ControlTree spawn(int initialInstructionNumber,
                         Instruction instruction)

isRoot

public boolean isRoot()

instruction

public Instruction instruction()

instructionNumber

public int instructionNumber()

addNested

public void addNested(ControlTree nested)

parent

public ControlTree parent()

nested

public List<ControlTree> nested()

addCallback

public void addCallback(Consumer<Integer> callback)

setFinalInstructionNumber

public void setFinalInstructionNumber(int finalInstructionNumber,
                                      Instruction end)