Package ghidra.pcode.emu.jit.gen.var

Enum Class MissingVarGen

java.lang.Object

java.lang.Enum<MissingVarGen>

ghidra.pcode.emu.jit.gen.var.MissingVarGen

All Implemented Interfaces:

ValGen<JitMissingVar>, VarGen<JitMissingVar>, Serializable, Comparable<MissingVarGen>, Constable

public enum MissingVarGen
extends Enum<MissingVarGen>
implements VarGen<JitMissingVar>

The generator for a missing (local) variable.

In principle, a JitMissingVar should never show up in the use-def graph, since they should all be replaced by phi outputs. We can be certain these should never show up as an output, so we prohibit any attempt to generate code that writes to a missing variable. However, we wait until run time to make that assertion about reads. In theory, it's possible the generator will generate unreachable code that reads from a variable; however, that code is unreachable. First how does this happen? Second, what if it does?

To answer the first question, we note that the passage decoder should never decode any statically unreachable instructions. However, the p-code emitted by those instructions may technically contain unreachable ops.

To answer the second, we note that the ASM library has a built-in control-flow analyzer, and it ought to detect the unreachable code. In my observation, it replaces that code with nop and/or athrow. Still, in case it doesn't, or in case something changes in a later version (or if/when we port this to the JDK's upcoming classfile API), we emit our own bytecode to throw an AssertionError.

Nested Class Summary

Nested classes/interfaces inherited from class java.lang.Enum

Enum.EnumDesc<E extends Enum<E>>

Nested classes/interfaces inherited from interface ghidra.pcode.emu.jit.gen.var.VarGen

VarGen.BlockTransition

Enum Constant Summary

Enum Constants

Enum Constant	Description
GEN	Singleton

Method Summary

Modifier and Type	Method	Description
void	generateValInitCode(JitCodeGenerator gen, JitMissingVar v, org.objectweb.asm.MethodVisitor iv)	Prepare any class-level items required to use this variable
JitType	generateValReadCode(JitCodeGenerator gen, JitMissingVar v, JitTypeBehavior typeReq, org.objectweb.asm.MethodVisitor rv)	Read the value onto the stack
void	generateVarWriteCode(JitCodeGenerator gen, JitMissingVar v, JitType type, org.objectweb.asm.MethodVisitor rv)	Write a value from the stack into the given variable
static MissingVarGen	valueOf(String name)	Returns the enum constant of this class with the specified name.
static MissingVarGen[]	values()	Returns an array containing the constants of this enum class, in the order they are declared.

Methods inherited from class java.lang.Enum

clone, compareTo, describeConstable, equals, finalize, getDeclaringClass, hashCode, name, ordinal, toString, valueOf

Methods inherited from class java.lang.Object

getClass, notify, notifyAll, wait, wait, wait

Enum Constant Details

GEN

public static final MissingVarGen GEN

Singleton

Method Details

values

public static MissingVarGen[] values()

Returns an array containing the constants of this enum class, in the order they are declared.

Returns:

an array containing the constants of this enum class, in the order they are declared

valueOf

public static MissingVarGen valueOf(String name)

Returns the enum constant of this class with the specified name. The string must match exactly an identifier used to declare an enum constant in this class. (Extraneous whitespace characters are not permitted.)

Parameters:

name - the name of the enum constant to be returned.

Returns:

the enum constant with the specified name

Throws:

IllegalArgumentException - if this enum class has no constant with the specified name

NullPointerException - if the argument is null

generateValInitCode

public void generateValInitCode(JitCodeGenerator gen,
 JitMissingVar v,
 org.objectweb.asm.MethodVisitor iv)

Description copied from interface: ValGen

Prepare any class-level items required to use this variable

For example, if this represents a direct memory variable, then this can prepare a reference to the portion of the state involved, allowing it to access it readily.

Specified by:

generateValInitCode in interface ValGen<JitMissingVar>

Parameters:

gen - the code generator

v - the value

iv - the constructor visitor

generateValReadCode

public JitType generateValReadCode(JitCodeGenerator gen,
 JitMissingVar v,
 JitTypeBehavior typeReq,
 org.objectweb.asm.MethodVisitor rv)

Description copied from interface: ValGen

Read the value onto the stack

Specified by:

generateValReadCode in interface ValGen<JitMissingVar>

Parameters:

gen - the code generator

v - the value to read

typeReq - the required type of the value

rv - the visitor for the run method

Returns:

the actual p-code type (which determines the JVM type) of the value on the stack

generateVarWriteCode

public void generateVarWriteCode(JitCodeGenerator gen,
 JitMissingVar v,
 JitType type,
 org.objectweb.asm.MethodVisitor rv)

Description copied from interface: VarGen

Write a value from the stack into the given variable

Specified by:

generateVarWriteCode in interface VarGen<JitMissingVar>

Parameters:

gen - the code generator

v - the variable to write

type - the p-code type (which also determines the expected JVM type) of the value on the stack

rv - the visitor for the run method