modelverse/kernel/modelverse_jit/cfg_ir.py

"""Defines control flow graph IR data structures."""

from collections import defaultdict

# Let's just agree to disagree on map vs list comprehensions, pylint.
# pylint: disable=I0011,W0141

class SharedCounter(object):
    """Defines a shared counter."""
    def __init__(self):
        self.index = 0

    def next_value(self):
        """Gets the next value for this counter."""
        result = self.index
        self.index += 1
        return result

class BasicBlock(object):
    """Represents a basic block."""
    def __init__(self, counter):
        self.parameters = []
        self.definitions = []
        self.counter = counter
        self.index = counter.next_value()
        self.definition_counter = SharedCounter()
        self.flow = UnreachableFlow()

    def append_parameter(self, parameter):
        """Appends a parameter to this basic block."""
        result = self.create_definition(parameter)
        self.parameters.append(result)
        if len(self.definitions) > 0:
            self.renumber_definitions()
        return result

    def remove_parameter(self, parameter):
        """Removes the given parameter definition from this basic block."""
        return self.parameters.remove(parameter)

    def prepend_definition(self, value):
        """Defines the given value in this basic block."""
        result = self.create_definition(value)
        self.definitions.insert(0, result)
        self.renumber_definitions()
        return result

    def insert_definition_before(self, anchor, value):
        """Inserts the second definition or value before the first definition."""
        index = None
        for i, definition in enumerate(self.definitions):
            if definition.definition_index == anchor.definition_index:
                index = i

        if index is None:
            raise ValueError(
                'Cannot insert a definition because the anchor '
                'is not defined in this block.')

        result = self.create_definition(value)
        self.definitions.insert(index, result)
        self.renumber_definitions()
        return result

    def append_definition(self, value):
        """Defines the given value in this basic block."""
        result = self.create_definition(value)
        self.definitions.append(result)
        return result

    def create_definition(self, value=None):
        """Creates a definition, but does not assign it to this block yet."""
        if isinstance(value, Definition):
            value.block = self
            value.renumber(self.definition_counter.next_value())
            return value
        else:
            assert isinstance(value, Value) or value is None
            return Definition(
                self.counter.next_value(),
                self,
                self.definition_counter.next_value(),
                value)

    def remove_definition(self, definition):
        """Removes the given definition from this basic block."""
        return self.definitions.remove(definition)

    def renumber_definitions(self):
        """Re-numbers all definitions in this basic block."""
        self.definition_counter = SharedCounter()
        for definition in self.parameters:
            definition.renumber(self.definition_counter.next_value())
        for definition in self.definitions:
            definition.renumber(self.definition_counter.next_value())

    def __str__(self):
        prefix = '!%d(%s):' % (self.index, ', '.join(map(str, self.parameters)))
        return '\n'.join(
            [prefix] +
            [' ' * 4 + str(item) for item in self.definitions + [self.flow]])

class Definition(object):
    """Maps a value to a variable."""
    def __init__(self, index, block, definition_index, value):
        self.index = index
        self.block = block
        self.definition_index = definition_index
        self.value = value
        if value is not None:
            assert isinstance(value, Value) or isinstance(value, Definition)

    def redefine(self, new_value):
        """Tweaks this definition to take on the given new value."""
        self.value = new_value
        if new_value is not None:
            assert isinstance(new_value, Value) or isinstance(new_value, Definition)

    def renumber(self, new_definition_index):
        """Updates this definition's index in the block that defines it."""
        self.definition_index = new_definition_index

    def get_all_dependencies(self):
        """Gets all definitions and instructions on which this definition depends,
           along with any dependencies of instruction dependencies."""
        if isinstance(self.value, Definition):
            return [self.value]
        else:
            return self.value.get_all_dependencies()

    def has_side_effects(self):
        """Tests if this definition produces any side-effects."""
        return self.value.has_side_effects()

    def has_value(self):
        """Tells if this definition produces a result that is not None."""
        return self.value.has_value()

    def insert_before(self, value):
        """Inserts the given value or definition before this definition."""
        return self.block.insert_definition_before(self, value)

    def ref_str(self):
        """Gets a string that represents a reference to this definition."""
        return '$%d' % self.index

    def __str__(self):
        return '$%d = %s' % (self.index, self.value.ref_str())

class Instruction(object):
    """Represents an instruction."""
    def get_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends."""
        raise NotImplementedError()

    def create(self, new_dependencies):
        """Creates an instruction of this type from the given set of dependencies."""
        raise NotImplementedError()

    def get_all_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends,
           along with any dependencies of instruction dependencies."""
        results = list(self.get_dependencies())
        for item in results:
            if not isinstance(item, Definition):
                results.extend(item.get_all_dependencies())

        return results

class Branch(Instruction):
    """Represents a branch from one basic block to another."""
    def __init__(self, block, arguments=None):
        self.block = block
        assert isinstance(block, BasicBlock)
        if arguments is None:
            arguments = []
        self.arguments = arguments
        assert all([isinstance(arg, Definition) for arg in arguments])

    def get_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends."""
        return self.arguments

    def create(self, new_dependencies):
        """Creates an instruction of this type from the given set of dependencies."""
        return Branch(self.block, new_dependencies)

    def __str__(self):
        return '!%d(%s)' % (self.block.index, ', '.join([arg.ref_str() for arg in self.arguments]))

class FlowInstruction(Instruction):
    """Represents a control flow instruction which terminates a basic block."""
    def branches(self):
        """Gets a list of basic blocks targeted by this flow instruction."""
        raise NotImplementedError()

    def has_side_effects(self):
        """Tells if this instruction has side-effects."""
        # All flow-instructions have side-effects!
        return True

class JumpFlow(FlowInstruction):
    """Represents a control flow instruction which jumps directly to a basic block."""
    def __init__(self, branch):
        FlowInstruction.__init__(self)
        self.branch = branch
        assert isinstance(branch, Branch)

    def get_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends."""
        return self.branches()

    def create(self, new_dependencies):
        """Creates an instruction of this type from the given set of dependencies."""
        return JumpFlow(*new_dependencies)

    def branches(self):
        """Gets a list of basic blocks targeted by this flow instruction."""
        return [self.branch]

    def __str__(self):
        return 'jump %s' % self.branch

class SelectFlow(FlowInstruction):
    """Represents a control flow instruction which jumps to one of two basic blocks depending
       on whether a condition is truthy or not."""
    def __init__(self, condition, if_branch, else_branch):
        FlowInstruction.__init__(self)
        self.condition = condition
        assert isinstance(condition, Definition)
        self.if_branch = if_branch
        assert isinstance(if_branch, Branch)
        self.else_branch = else_branch
        assert isinstance(else_branch, Branch)

    def get_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends."""
        return [self.condition] + self.branches()

    def create(self, new_dependencies):
        """Creates an instruction of this type from the given set of dependencies."""
        return SelectFlow(*new_dependencies)

    def branches(self):
        """Gets a list of basic blocks targeted by this flow instruction."""
        return [self.if_branch, self.else_branch]

    def __str__(self):
        return 'select %s, %s, %s' % (self.condition.ref_str(), self.if_branch, self.else_branch)

class ReturnFlow(FlowInstruction):
    """Represents a control flow instruction which terminates the execution of the current
       function and returns a value."""
    def __init__(self, value):
        FlowInstruction.__init__(self)
        self.value = value
        assert isinstance(value, Definition)

    def get_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends."""
        return [self.value]

    def create(self, new_dependencies):
        """Creates an instruction of this type from the given set of dependencies."""
        return ReturnFlow(*new_dependencies)

    def branches(self):
        """Gets a list of basic blocks targeted by this flow instruction."""
        return []

    def __str__(self):
        return 'return %s' % self.value.ref_str()

class ThrowFlow(FlowInstruction):
    """Represents a control flow instruction which throws an exception."""
    def __init__(self, exception):
        FlowInstruction.__init__(self)
        self.exception = exception
        assert isinstance(exception, Definition)

    def get_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends."""
        return [self.exception]

    def create(self, new_dependencies):
        """Creates an instruction of this type from the given set of dependencies."""
        return ThrowFlow(*new_dependencies)

    def branches(self):
        """Gets a list of basic blocks targeted by this flow instruction."""
        return []

    def __str__(self):
        return 'throw %s' % self.exception.ref_str()

class UnreachableFlow(FlowInstruction):
    """Represents a control flow instruction which is unreachable."""
    def get_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends."""
        return []

    def create(self, new_dependencies):
        """Creates an instruction of this type from the given set of dependencies."""
        assert len(new_dependencies) == 0
        return self

    def branches(self):
        """Gets a list of basic blocks targeted by this flow instruction."""
        return []

    def __str__(self):
        return 'unreachable'

class Value(Instruction):
    """A value: an instruction that produces some result."""
    def get_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends."""
        raise NotImplementedError()

    def has_value(self):
        """Tells if this value produces a result that is not None."""
        return True

    def has_side_effects(self):
        """Tells if this instruction has side-effects."""
        return False

    def ref_str(self):
        """Gets a string that represents this value."""
        return str(self)

class BlockParameter(Value):
    """A basic block parameter."""
    def get_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends."""
        return []

    def create(self, new_dependencies):
        """Creates an instruction of this type from the given set of dependencies."""
        assert len(new_dependencies) == 0
        return BlockParameter()

    def __str__(self):
        return 'block-parameter'

class FunctionParameter(Value):
    """A function parameter."""
    def __init__(self, name):
        Value.__init__(self)
        self.name = name

    def create(self, new_dependencies):
        """Creates an instruction of this type from the given set of dependencies."""
        assert len(new_dependencies) == 0
        return FunctionParameter(self.name)

    def get_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends."""
        return []

    def __str__(self):
        return 'func-parameter %s' % self.name

class Literal(Value):
    """A literal value."""
    def __init__(self, literal):
        Value.__init__(self)
        self.literal = literal

    def create(self, new_dependencies):
        """Creates an instruction of this type from the given set of dependencies."""
        assert len(new_dependencies) == 0
        return Literal(self.literal)

    def get_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends."""
        return []

    def has_value(self):
        """Tells if this value produces a result that is not None."""
        return self.literal is not None

    def __str__(self):
        return 'literal %r' % self.literal

class IndirectFunctionCall(Value):
    """A value that is the result of an indirect function call."""
    def __init__(self, target, argument_list):
        Value.__init__(self)
        assert isinstance(target, Definition)
        self.target = target
        assert all([isinstance(val, Definition) for _, val in argument_list])
        self.argument_list = argument_list

    def has_side_effects(self):
        """Tells if this instruction has side-effects."""
        return True

    def get_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends."""
        return [self.target] + [val for _, val in self.argument_list]

    def create(self, new_dependencies):
        """Creates an instruction of this type from the given set of dependencies."""
        return IndirectFunctionCall(
            new_dependencies[0],
            [(name, new_val)
             for new_val, (name, _) in zip(new_dependencies[1:], self.argument_list)])

    def __str__(self):
        return 'indirect-call %s(%s)' % (
            self.target.ref_str(),
            ', '.join(['%s=%s' % (key, val.ref_str()) for key, val in self.argument_list]))

SIMPLE_POSITIONAL_CALLING_CONVENTION = 'simple-positional'
"""The calling convention for functions that use 'return' statements to return.
   Arguments are matched to parameters based on position."""

JIT_CALLING_CONVENTION = 'jit'
"""The calling convention for jitted functions."""

MACRO_POSITIONAL_CALLING_CONVENTION = 'macro-positional'
"""The calling convention for well-known functions that are expanded as macros during codegen."""

PRINT_MACRO_NAME = 'print'
"""The name of the 'print' macro."""

class DirectFunctionCall(Value):
    """A value that is the result of a direct function call."""
    def __init__(
            self, target_name, argument_list,
            calling_convention=JIT_CALLING_CONVENTION,
            has_value=True,
            has_side_effects=True):
        Value.__init__(self)
        self.target_name = target_name
        assert all([isinstance(val, Definition) for _, val in argument_list])
        self.argument_list = argument_list
        self.calling_convention = calling_convention
        self.has_value_val = has_value
        self.has_side_effects_val = has_side_effects

    def has_side_effects(self):
        """Tells if this instruction has side-effects."""
        return self.has_side_effects_val

    def has_value(self):
        """Tells if this value produces a result that is not None."""
        return self.has_value_val

    def create(self, new_dependencies):
        """Creates an instruction of this type from the given set of dependencies."""
        return DirectFunctionCall(
            self.target_name,
            [(name, new_val)
             for new_val, (name, _) in zip(new_dependencies, self.argument_list)],
            self.calling_convention, self.has_value_val, self.has_side_effects_val)

    def get_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends."""
        return [val for _, val in self.argument_list]

    def __str__(self):
        return 'direct-call %r %s(%s)' % (
            self.calling_convention,
            self.target_name,
            ', '.join(['%s=%s' % (key, val.ref_str()) for key, val in self.argument_list]))

class AllocateRootNode(Value):
    """A value that produces a new root node. Typically used in function prologs."""
    def __init__(self):
        Value.__init__(self)

    def get_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends."""
        return []

    def create(self, new_dependencies):
        """Creates an instruction of this type from the given set of dependencies."""
        assert len(new_dependencies) == 0
        return AllocateRootNode()

    def __str__(self):
        return 'alloc-root-node'

class DeallocateRootNode(Value):
    """A value that deallocates a root node. Typically used in function epilogs."""
    def __init__(self, root_node):
        Value.__init__(self)
        assert isinstance(root_node, Definition)
        self.root_node = root_node

    def get_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends."""
        return [self.root_node]

    def create(self, new_dependencies):
        """Creates an instruction of this type from the given set of dependencies."""
        return DeallocateRootNode(*new_dependencies)

    def has_value(self):
        """Tells if this value produces a result that is not None."""
        return False

    def has_side_effects(self):
        """Tells if this instruction has side-effects."""
        return True

    def __str__(self):
        return 'free-root-node %s' % self.root_node.ref_str()

class DeclareLocal(Value):
    """A value that declares a local variable."""
    def __init__(self, variable, root_node):
        Value.__init__(self)
        self.variable = variable
        self.root_node = root_node

    def get_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends."""
        return [self.root_node]

    def create(self, new_dependencies):
        """Creates an instruction of this type from the given set of dependencies."""
        root_node, = new_dependencies
        return DeclareLocal(self.variable, root_node)

    def has_value(self):
        """Tells if this value produces a result that is not None."""
        return False

    def has_side_effects(self):
        """Tells if this instruction has side-effects."""
        return True

    def __str__(self):
        return 'declare-local %s, %s' % (self.variable, self.root_node.ref_str())

class DeclareGlobal(Value):
    """A value that declares a global variable."""
    def __init__(self, variable):
        Value.__init__(self)
        self.variable = variable

    def get_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends."""
        return []

    def create(self, new_dependencies):
        """Creates an instruction of this type from the given set of dependencies."""
        return DeclareGlobal(self.variable)

    def has_value(self):
        """Tells if this value produces a result that is not None."""
        return False

    def has_side_effects(self):
        """Tells if this instruction has side-effects."""
        return True

    def __str__(self):
        return 'declare-global %s' % self.variable.name

class CheckLocalExists(Value):
    """A value that checks if a local value has been defined (yet)."""
    def __init__(self, variable):
        Value.__init__(self)
        self.variable = variable

    def get_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends."""
        return []

    def create(self, new_dependencies):
        """Creates an instruction of this type from the given set of dependencies."""
        return CheckLocalExists(self.variable)

    def __str__(self):
        return 'check-local-exists %s' % self.variable

class ResolveLocal(Value):
    """A value that resolves a local as a pointer."""
    def __init__(self, variable):
        Value.__init__(self)
        self.variable = variable

    def get_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends."""
        return []

    def create(self, new_dependencies):
        """Creates an instruction of this type from the given set of dependencies."""
        return ResolveLocal(self.variable)

    def __str__(self):
        return 'resolve-local %s' % self.variable

class ResolveGlobal(Value):
    """A value that resolves a global as a pointer."""
    def __init__(self, variable):
        Value.__init__(self)
        self.variable = variable

    def get_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends."""
        return []

    def create(self, new_dependencies):
        """Creates an instruction of this type from the given set of dependencies."""
        return ResolveGlobal(self.variable)

    def __str__(self):
        return 'resolve-global %s' % self.variable.name

class LoadPointer(Value):
    """A value that loads the value assigned to a pointer."""
    def __init__(self, pointer):
        Value.__init__(self)
        self.pointer = pointer
        assert isinstance(pointer, Definition)

    def get_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends."""
        return [self.pointer]

    def create(self, new_dependencies):
        """Creates an instruction of this type from the given set of dependencies."""
        return LoadPointer(*new_dependencies)

    def __str__(self):
        return 'load %s' % self.pointer.ref_str()

class StoreAtPointer(Value):
    """A value that assigns a value to a pointer."""
    def __init__(self, pointer, value):
        Value.__init__(self)
        self.pointer = pointer
        assert isinstance(pointer, Definition)
        self.value = value
        assert isinstance(value, Definition)

    def get_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends."""
        return [self.pointer, self.value]

    def create(self, new_dependencies):
        """Creates an instruction of this type from the given set of dependencies."""
        return StoreAtPointer(*new_dependencies)

    def has_value(self):
        """Tells if this value produces a result that is not None."""
        return False

    def has_side_effects(self):
        """Tells if this instruction has side-effects."""
        return True

    def __str__(self):
        return 'store %s, %s' % (self.pointer.ref_str(), self.value.ref_str())

class Read(Value):
    """A value that reads the value stored in a node."""
    def __init__(self, node):
        Value.__init__(self)
        self.node = node
        assert isinstance(node, Definition)

    def get_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends."""
        return [self.node]

    def create(self, new_dependencies):
        """Creates an instruction of this type from the given set of dependencies."""
        return Read(*new_dependencies)

    def __str__(self):
        return 'read %s' % (self.node.ref_str())

class CreateNode(Value):
    """A value that creates a new node."""
    def __init__(self, value):
        Value.__init__(self)
        self.value = value
        assert isinstance(value, Definition)

    def get_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends."""
        return [self.value]

    def create(self, new_dependencies):
        """Creates an instruction of this type from the given set of dependencies."""
        return CreateNode(*new_dependencies)

    def __str__(self):
        return 'create-node %s' % (self.value.ref_str())

class Input(Value):
    """A value that pops a node from the input queue."""
    def get_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends."""
        return []

    def create(self, new_dependencies):
        """Creates an instruction of this type from the given set of dependencies."""
        assert len(new_dependencies) == 0
        return Input()

    def has_side_effects(self):
        """Tells if this instruction has side-effects."""
        return True

    def __str__(self):
        return 'input'

class Output(Value):
    """A value that pushes a node onto the output queue."""
    def __init__(self, value):
        Value.__init__(self)
        self.value = value
        assert isinstance(value, Definition)

    def get_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends."""
        return [self.value]

    def create(self, new_dependencies):
        """Creates an instruction of this type from the given set of dependencies."""
        return Output(*new_dependencies)

    def has_value(self):
        """Tells if this value produces a result that is not None."""
        return False

    def has_side_effects(self):
        """Tells if this instruction has side-effects."""
        return True

    def __str__(self):
        return 'output %s' % self.value.ref_str()

class Binary(Value):
    """A value that applies a binary operator to two other values."""
    def __init__(self, lhs, operator, rhs):
        Value.__init__(self)
        self.lhs = lhs
        assert isinstance(lhs, Definition)
        self.operator = operator
        self.rhs = rhs
        assert isinstance(rhs, Definition)

    def get_dependencies(self):
        """Gets all definitions and instructions on which this instruction depends."""
        return [self.lhs, self.rhs]

    def create(self, new_dependencies):
        """Creates an instruction of this type from the given set of dependencies."""
        lhs, rhs = new_dependencies
        return Binary(lhs, self.operator, rhs)

    def __str__(self):
        return 'binary %s, %r, %s' % (self.lhs.ref_str(), self.operator, self.rhs.ref_str())

def create_jump(block, arguments=None):
    """Creates a jump to the given block with the given argument list."""
    return JumpFlow(Branch(block, arguments))

def create_print(argument):
    """Creates a value that prints the specified argument."""
    return DirectFunctionCall(
        PRINT_MACRO_NAME, [('argument', argument)],
        calling_convention=MACRO_POSITIONAL_CALLING_CONVENTION,
        has_value=False)

def get_def_value(def_or_value):
    """Returns the given value, or the underlying value of the given definition, whichever is
       appropriate."""
    if isinstance(def_or_value, Definition):
        return get_def_value(def_or_value.value)
    else:
        return def_or_value

def apply_to_value(function, def_or_value):
    """Applies the given function to the specified value, or the underlying value of the
       given definition."""
    return function(get_def_value(def_or_value))

def is_literal(value):
    """Tests if the given value is a literal."""
    return isinstance(value, Literal)

def is_literal_def(def_or_value):
    """Tests if the given value is a literal or a definition with an underlying literal."""
    return apply_to_value(is_literal, def_or_value)

def is_value_def(def_or_value, class_or_type_or_tuple=Value):
    """Tests if the given definition or value is a value of the given type."""
    return isinstance(get_def_value(def_or_value), class_or_type_or_tuple)

def get_def_variable(def_or_value):
    """Gets the 'variable' attribute of the given value, or the underlying value of the given
       definition, whichever is appropriate."""
    return get_def_value(def_or_value).variable

def get_literal_value(value):
    """Gets the value of the given literal value."""
    return value.literal

def get_literal_def_value(def_or_value):
    """Gets the value of the given literal value or definition with an underlying literal."""
    return apply_to_value(get_literal_value, def_or_value)

def get_all_predecessor_blocks(entry_point):
    """Creates a mapping of blocks to their direct predecessors for every block in the control-flow
       graph defined by the given entry point."""
    results = defaultdict(set)
    processed = set()
    def __find_predecessors_step(block):
        if block in processed:
            return

        processed.add(block)
        for branch in block.flow.branches():
            target_block = branch.block
            results[target_block].add(block)
            __find_predecessors_step(target_block)

    __find_predecessors_step(entry_point)
    return results

def get_directly_reachable_blocks(block):
    """Gets the set of all blocks that can be reached by taking a single branch from the
       given block."""
    return [branch.block for branch in block.flow.branches()]

def get_reachable_blocks(entry_point):
    """Constructs the set of all reachable vertices from the given block."""
    # This is a simple O(n^2) algorithm. Maybe a faster algorithm is more appropriate here.
    def __add_block_children(block, results):
        for child in get_directly_reachable_blocks(block):
            if child not in results:
                results.add(child)
                __add_block_children(child, results)

        return results

    return __add_block_children(entry_point, set())

def get_all_reachable_blocks(entry_point):
    """Constructs the set of all reachable vertices, for every block that is
       reachable from the given entry point."""
    # This is a simple O(n^3) algorithm. Maybe a faster algorithm is more appropriate here.
    results = {}
    all_blocks = get_reachable_blocks(entry_point)
    results[entry_point] = all_blocks
    for block in all_blocks:
        if block not in results:
            results[block] = get_reachable_blocks(block)
    return results

def get_all_blocks(entry_point):
    """Gets all basic blocks in the control-flow graph defined by the given entry point."""
    yield entry_point
    for block in get_reachable_blocks(entry_point):
        yield block

def get_trivial_phi_value(parameter_def, values):
    """Tests if the given parameter definition is an alias for another definition.
       If so, then the other definition is returned; otherwise, None."""
    result = None
    for elem in values:
        if elem is not parameter_def:
            if result is None:
                result = elem
            else:
                return None

    return result