fortran.py

# -----------------------------------------------------------------------------
# BSD 3-Clause License
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# Copyright (c) 2019-2020, Science and Technology Facilities Council
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# -----------------------------------------------------------------------------
# Authors R. W. Ford and S. Siso, STFC Daresbury Lab.
# Modified J. Henrichs, Bureau of Meteorology
# Modified A. R. Porter, STFC Daresbury Lab.

'''Fortran PSyIR backend. Generates Fortran code from PSyIR
nodes. Currently limited to PSyIR Kernel and NemoInvoke schedules as
PSy-layer PSyIR already has a gen() method to generate Fortran.

'''

from __future__ import absolute_import
from fparser.two import Fortran2003
from psyclone.psyir.frontend.fparser2 import Fparser2Reader, \
    TYPE_MAP_FROM_FORTRAN
from psyclone.psyir.symbols import DataSymbol, ArgumentInterface, \
    ContainerSymbol, ScalarType, ArrayType, SymbolTable, RoutineSymbol, \
    GlobalInterface
from psyclone.psyir.nodes import UnaryOperation, BinaryOperation, Operation, \
    Reference, Literal
from psyclone.psyir.backend.visitor import PSyIRVisitor, VisitorError

# The list of Fortran instrinsic functions that we know about (and can
# therefore distinguish from array accesses). These are taken from
# fparser.
FORTRAN_INTRINSICS = Fortran2003.Intrinsic_Name.function_names

# Mapping from PSyIR types to Fortran data types. Simply reverse the
# map from the frontend, removing the special case of "double
# precision", which is captured as a REAL intrinsic in the PSyIR.
TYPE_MAP_TO_FORTRAN = {}
for key, item in TYPE_MAP_FROM_FORTRAN.items():
    if key != "double precision":
        TYPE_MAP_TO_FORTRAN[item] = key


def gen_intent(symbol):
    '''Given a DataSymbol instance as input, determine the Fortran intent that
    the DataSymbol should have and return the value as a string.

    :param symbol: the symbol instance.
    :type symbol: :py:class:`psyclone.psyir.symbols.DataSymbol`

    :returns: the Fortran intent of the symbol instance in lower case, \
    or None if the access is unknown or if this is a local variable.
    :rtype: str or NoneType

    '''
    mapping = {ArgumentInterface.Access.UNKNOWN: None,
               ArgumentInterface.Access.READ: "in",
               ArgumentInterface.Access.WRITE: "out",
               ArgumentInterface.Access.READWRITE: "inout"}

    if symbol.is_argument:
        try:
            return mapping[symbol.interface.access]
        except KeyError as excinfo:
            raise VisitorError("Unsupported access '{0}' found."
                               "".format(str(excinfo)))
    else:
        return None  # non-Arguments do not have intent


def gen_dims(symbol):
    '''Given a DataSymbol instance as input, return a list of strings
    representing the symbol's array dimensions.

    :param symbol: the symbol instance.
    :type symbol: :py:class:`psyclone.psyir.symbols.DataSymbol`

    :returns: the Fortran representation of the symbol's dimensions as \
    a list.
    :rtype: list of str

    :raises NotImplementedError: if the format of the dimension is not \
    supported.

    '''
    dims = []
    for index in symbol.shape:
        if isinstance(index, DataSymbol):
            # references another symbol
            dims.append(index.name)
        elif isinstance(index, int):
            # literal constant
            dims.append(str(index))
        elif isinstance(index, ArrayType.Extent):
            # unknown extent
            dims.append(":")
        else:
            raise NotImplementedError(
                "unsupported gen_dims index '{0}'".format(str(index)))
    return dims


def gen_datatype(symbol):
    '''Given a DataSymbol instance as input, return the datatype of the
    symbol including any specific precision properties.

    :param symbol: the symbol instance.
    :type symbol: :py:class:`psyclone.psyir.symbols.DataSymbol`

    :returns: the Fortran representation of the symbol's datatype \
              including any precision properties.
    :rtype: str

    :raises NotImplementedError: if the symbol has an unsupported \
    datatype.
    :raises VisitorError: if the symbol specifies explicit precision \
    and this is not supported for the datatype.
    :raises VisitorError: if the size of the explicit precision is not \
    supported for the datatype.
    :raises VisitorError: if the size of the symbol is specified by \
    another variable and the datatype is not one that supports the \
    Fortran KIND option.
    :raises NotImplementedError: if the type of the precision object \
    is an unsupported type.

    '''
    try:
        fortrantype = TYPE_MAP_TO_FORTRAN[symbol.datatype.intrinsic]
    except KeyError:
        raise NotImplementedError(
            "Unsupported datatype '{0}' for symbol '{1}' found in "
            "gen_datatype().".format(symbol.datatype.intrinsic, symbol.name))

    precision = symbol.datatype.precision

    if isinstance(precision, int):
        if fortrantype not in ['real', 'integer', 'logical']:
            raise VisitorError("Explicit precision not supported for datatype "
                               "'{0}' in symbol '{1}' in Fortran backend."
                               "".format(fortrantype, symbol.name))
        if fortrantype == 'real' and precision not in [4, 8, 16]:
            raise VisitorError(
                "Datatype 'real' in symbol '{0}' supports fixed precision of "
                "[4, 8, 16] but found '{1}'.".format(symbol.name,
                                                     precision))
        if fortrantype in ['integer', 'logical'] and precision not in \
           [1, 2, 4, 8, 16]:
            raise VisitorError(
                "Datatype '{0}' in symbol '{1}' supports fixed precision of "
                "[1, 2, 4, 8, 16] but found '{2}'."
                "".format(fortrantype, symbol.name, precision))
        # Precision has an an explicit size. Use the "type*size" Fortran
        # extension for simplicity. We could have used
        # type(kind=selected_int|real_kind(size)) or, for Fortran 2008,
        # ISO_FORTRAN_ENV; type(type64) :: MyType.
        return "{0}*{1}".format(fortrantype, precision)

    if isinstance(precision, ScalarType.Precision):
        # The precision information is not absolute so is either
        # machine specific or is specified via the compiler. Fortran
        # only distinguishes relative precision for single and double
        # precision reals.
        if fortrantype.lower() == "real" and \
           precision == ScalarType.Precision.DOUBLE:
            return "double precision"
        # This logging warning can be added when issue #11 is
        # addressed.
        # import logging
        # logging.warning(
        #      "Fortran does not support relative precision for the '%s' "
        #      "datatype but '%s' was specified for variable '%s'.",
        #      datatype, str(symbol.precision), symbol.name)
        return fortrantype

    if isinstance(precision, DataSymbol):
        if fortrantype not in ["real", "integer", "logical"]:
            raise VisitorError(
                "kind not supported for datatype '{0}' in symbol '{1}' in "
                "Fortran backend.".format(fortrantype, symbol.name))
        # The precision information is provided by a parameter, so use KIND.
        return "{0}(kind={1})".format(fortrantype, precision.name)

    raise VisitorError(
        "Unsupported precision type '{0}' found for symbol '{1}' in Fortran "
        "backend.".format(type(precision).__name__, symbol.name))


def _reverse_map(op_map):
    '''
    Reverses the supplied fortran2psyir mapping to make a psyir2fortran
    mapping.

    :param op_map: mapping from string representation of operator to \
                   enumerated type.
    :type op_map: :py:class:`collections.OrderedDict`

    :returns: a mapping from PSyIR operation to the equivalent Fortran string.
    :rtype: dict with :py:class:`psyclone.psyir.nodes.Operation.Operator` \
            keys and str values.

    '''
    mapping = {}
    for operator in op_map:
        mapping_key = op_map[operator]
        mapping_value = operator
        # Only choose the first mapping value when there is more
        # than one.
        if mapping_key not in mapping:
            mapping[mapping_key] = mapping_value
    return mapping


def get_fortran_operator(operator):
    '''Determine the Fortran operator that is equivalent to the provided
    PSyIR operator. This is achieved by reversing the Fparser2Reader
    maps that are used to convert from Fortran operator names to PSyIR
    operator names.

    :param operator: a PSyIR operator.
    :type operator: :py:class:`psyclone.psyir.nodes.Operation.Operator`

    :returns: the Fortran operator.
    :rtype: str

    :raises KeyError: if the supplied operator is not known.

    '''
    unary_mapping = _reverse_map(Fparser2Reader.unary_operators)
    if operator in unary_mapping:
        return unary_mapping[operator].upper()

    binary_mapping = _reverse_map(Fparser2Reader.binary_operators)
    if operator in binary_mapping:
        return binary_mapping[operator].upper()

    nary_mapping = _reverse_map(Fparser2Reader.nary_operators)
    if operator in nary_mapping:
        return nary_mapping[operator].upper()
    raise KeyError()


def is_fortran_intrinsic(fortran_operator):
    '''Determine whether the supplied Fortran operator is an intrinsic
    Fortran function or not.

    :param str fortran_operator: the supplied Fortran operator.

    :returns: true if the supplied Fortran operator is a Fortran \
        intrinsic and false otherwise.

    '''
    return fortran_operator in FORTRAN_INTRINSICS


def precedence(fortran_operator):
    '''Determine the relative precedence of the supplied Fortran operator.
    Relative Operator precedence is taken from the Fortran 2008
    specification document and encoded as a list.

    :param str fortran_operator: the supplied Fortran operator.

    :returns: an integer indicating the relative precedence of the \
        supplied Fortran operator. The higher the value, the higher \
        the precedence.

    :raises KeyError: if the supplied operator is not in the \
        precedence list.

    '''
    # The index of the fortran_precedence list indicates relative
    # precedence. Strings within sub-lists have the same precendence
    # apart from the following two caveats. 1) unary + and - have
    # a higher precedence than binary + and -, e.g. -(a-b) !=
    # -a-b and 2) floating point operations are not actually
    # associative due to rounding errors, e.g. potentially (a * b) / c
    # != a * (b / c). Therefore, if a particular ordering is specified
    # then it should be respected. These issues are dealt with in the
    # binaryoperation handler.
    fortran_precedence = [
        ['.EQV.', 'NEQV'],
        ['.OR.'],
        ['.AND.'],
        ['.NOT.'],
        ['.EQ.', '.NE.', '.LT.', '.LE.', '.GT.', '.GE.', '==', '/=', '<',
         '<=', '>', '>='],
        ['//'],
        ['+', '-'],
        ['*', '/'],
        ['**']]
    for oper_list in fortran_precedence:
        if fortran_operator in oper_list:
            return fortran_precedence.index(oper_list)
    raise KeyError()


class FortranWriter(PSyIRVisitor):
    '''Implements a PSyIR-to-Fortran back end for PSyIR kernel code (not
    currently PSyIR algorithm code which has its own gen method for
    generating Fortran).

    '''
    def gen_use(self, symbol, symbol_table):
        ''' Performs consistency checks and then creates and returns the
        Fortran use statement(s) for this ContainerSymbol as required for
        the supplied symbol table. If this symbol has both a wildcard import
        and explicit imports then two use statements are generated. (This
        means that when generating Fortran from PSyIR created from Fortran
        code, we replicate the structure of the original.)

        :param symbol: the container symbol instance.
        :type symbol: :py:class:`psyclone.psyir.symbols.ContainerSymbol`
        :param symbol_table: the symbol table containing this container symbol.
        :type symbol_table: :py:class:`psyclone.psyir.symbols.SymbolTable`

        :returns: the Fortran use statement(s) as a string.
        :rtype: str

        :raises VisitorError: if the symbol argument is not a ContainerSymbol.
        :raises VisitorError: if the symbol_table argument is not a \
                            SymbolTable.
        :raises VisitorError: if the supplied symbol is not in the supplied \
                            SymbolTable.
        :raises VisitorError: if the supplied symbol has the same name as an \
                            entry in the SymbolTable but is a different object.
        '''
        if not isinstance(symbol, ContainerSymbol):
            raise VisitorError(
                "gen_use() expects a ContainerSymbol as its first argument "
                "but got '{0}'".format(type(symbol).__name__))
        if not isinstance(symbol_table, SymbolTable):
            raise VisitorError(
                "gen_use() expects a SymbolTable as its second argument but "
                "got '{0}'".format(type(symbol_table).__name__))
        if symbol.name not in symbol_table:
            raise VisitorError("gen_use() - the supplied symbol ('{0}') is not"
                               " in the supplied SymbolTable.".format(
                                   symbol.name))
        if symbol_table.lookup(symbol.name) is not symbol:
            raise VisitorError(
                "gen_use() - the supplied symbol ('{0}') is not the same "
                "object as the entry with that name in the supplied "
                "SymbolTable.".format(symbol.name))

        # Construct the list of symbol names for the ONLY clause
        only_list = [dsym.name for dsym in
                     symbol_table.imported_symbols(symbol)]

        # Finally construct the use statements for this Container (module)
        if not only_list and not symbol.wildcard_import:
            # We have a "use xxx, only:" - i.e. an empty only list
            return "{0}use {1}, only :\n".format(self._nindent, symbol.name)
        use_stmts = ""
        if only_list:
            use_stmts = "{0}use {1}, only : {2}\n".format(
                self._nindent, symbol.name, ", ".join(sorted(only_list)))
        # It's possible to have both explicit and wildcard imports from the
        # same Fortran module.
        if symbol.wildcard_import:
            use_stmts += "{0}use {1}\n".format(self._nindent, symbol.name)
        return use_stmts

    def gen_vardecl(self, symbol):
        '''Create and return the Fortran variable declaration for this Symbol.

        :param symbol: the symbol instance.
        :type symbol: :py:class:`psyclone.psyir.symbols.DataSymbol`

        :returns: the Fortran variable declaration as a string.
        :rtype: str

        :raises VisitorError: if the symbol does not specify a \
            variable declaration (it is not a local declaration or an \
            argument declaration).
        :raises VisitorError: if the symbol has an array with a shape \
            containing a mixture of DEFERRED and other extents.
        '''
        if not (symbol.is_local or symbol.is_argument):
            raise VisitorError(
                "gen_vardecl requires the symbol '{0}' to have a Local or "
                "an Argument interface but found a '{1}' interface."
                "".format(symbol.name, type(symbol.interface).__name__))

        datatype = gen_datatype(symbol)
        result = "{0}{1}".format(self._nindent, datatype)
        if ArrayType.Extent.DEFERRED in symbol.shape:
            if not all(dim == ArrayType.Extent.DEFERRED
                       for dim in symbol.shape):
                raise VisitorError(
                    "A Fortran declaration of an allocatable array must have"
                    " the extent of every dimension as 'DEFERRED' but "
                    "symbol '{0}' has shape: {1}".format(symbol.name,
                                                         symbol.shape))
            # A 'deferred' array extent means this is an allocatable array
            result += ", allocatable"
        if ArrayType.Extent.ATTRIBUTE in symbol.shape:
            if not all(dim == ArrayType.Extent.ATTRIBUTE
                       for dim in symbol.shape):
                # If we have an 'assumed-size' array then only the last
                # dimension is permitted to have an 'ATTRIBUTE' extent
                if symbol.shape.count(ArrayType.Extent.ATTRIBUTE) != 1 or \
                   symbol.shape[-1] != ArrayType.Extent.ATTRIBUTE:
                    raise VisitorError(
                        "An assumed-size Fortran array must only have its "
                        "last dimension unspecified (as 'ATTRIBUTE') but "
                        "symbol '{0}' has shape: {1}".format(symbol.name,
                                                             symbol.shape))
        dims = gen_dims(symbol)
        if dims:
            result += ", dimension({0})".format(",".join(dims))
        intent = gen_intent(symbol)
        if intent:
            result += ", intent({0})".format(intent)
        if symbol.is_constant:
            result += ", parameter"
        result += " :: {0}".format(symbol.name)
        if symbol.is_constant:
            result += " = {0}".format(self._visit(symbol.constant_value))
        result += "\n"
        return result

    def gen_decls(self, symbol_table, args_allowed=True):
        '''Create and return the Fortran declarations for the supplied
        SymbolTable.

        :param symbol_table: the SymbolTable instance.
        :type symbol: :py:class:`psyclone.psyir.symbols.SymbolTable`
        :param bool args_allowed: if False then one or more argument
        declarations in symbol_table will cause this method to raise
        an exception. Defaults to True.

        :returns: the Fortran declarations as a string.
        :rtype: str

        :raises VisitorError: if one of the symbols is a RoutineSymbol \
            and it does not have a GlobalInterface interface as this \
            is not supported by this backend.
        :raises VisitorError: if args_allowed is False and one or more \
                              argument declarations exist in symbol_table.
        :raises VisitorError: if any symbols representing variables (i.e. \
            not kind parameters) without an explicit declaration or 'use' \
            are encountered.

        '''
        declarations = ""

        if not all([isinstance(symbol.interface, GlobalInterface)
                    for symbol in symbol_table.symbols
                    if isinstance(symbol, RoutineSymbol)]):
            raise VisitorError(
                "Routine symbols without a global interface are unsupported "
                "by the Fortran back-end.")

        # Does the symbol table contain any symbols with a deferred
        # interface (i.e. we don't know how they are brought into scope) that
        # are not KIND parameters?
        unresolved_datasymbols = symbol_table.get_unresolved_datasymbols(
            ignore_precision=True)
        if unresolved_datasymbols:
            symbols_txt = ", ".join(
                ["'" + sym + "'" for sym in unresolved_datasymbols])
            raise VisitorError(
                "The following symbols are not explicitly declared or imported"
                " from a module (in the local scope) and are not KIND "
                "parameters: {0}".format(symbols_txt))

        # Fortran requires use statements to be specified before
        # variable declarations. As a convention, this method also
        # declares any argument variables before local variables.

        # 1: Use statements
        for symbol in symbol_table.containersymbols:
            declarations += self.gen_use(symbol, symbol_table)

        # 2: Argument variable declarations
        if symbol_table.argument_datasymbols and not args_allowed:
            raise VisitorError(
                "Arguments are not allowed in this context but this symbol "
                "table contains argument(s): '{0}'."
                "".format([symbol.name for symbol in
                           symbol_table.argument_datasymbols]))
        for symbol in symbol_table.argument_datasymbols:
            declarations += self.gen_vardecl(symbol)

        # 3: Local variable declarations
        for symbol in symbol_table.local_datasymbols:
            declarations += self.gen_vardecl(symbol)

        return declarations

    def container_node(self, node):
        '''This method is called when a Container instance is found in
        the PSyIR tree.

        A container node is mapped to a module in the Fortran back end.

        :param node: a Container PSyIR node.
        :type node: :py:class:`psyclone.psyir.nodes.Container`

        :returns: the Fortran code as a string.
        :rtype: str

        :raises VisitorError: if the name attribute of the supplied \
        node is empty or None.
        :raises VisitorError: if any of the children of the supplied \
        Container node are not KernelSchedules.

        '''
        if not node.name:
            raise VisitorError("Expected Container node name to have a value.")

        # All children must be KernelSchedules as modules within
        # modules are not supported.
        from psyclone.psyGen import KernelSchedule
        if not all([isinstance(child, KernelSchedule)
                    for child in node.children]):
            raise VisitorError(
                "The Fortran back-end requires all children of a Container "
                "to be KernelSchedules.")

        result = "{0}module {1}\n".format(self._nindent, node.name)

        self._depth += 1

        # Declare the Container's data and specify that Containers do
        # not allow argument declarations.
        declarations = self.gen_decls(node.symbol_table, args_allowed=False)

        # Get the subroutine statements.
        subroutines = ""
        for child in node.children:
            subroutines += self._visit(child)

        result += (
            "{1}\n"
            "{0}contains\n"
            "{2}\n"
            "".format(self._nindent, declarations, subroutines))

        self._depth -= 1
        result += "{0}end module {1}\n".format(self._nindent, node.name)
        return result

    def kernelschedule_node(self, node):
        '''This method is called when a KernelSchedule instance is found in
        the PSyIR tree.

        The constants_mod module is currently hardcoded into the
        output as it is required for LFRic code. When issue #375 has
        been addressed this module can be added only when required.

        :param node: a KernelSchedule PSyIR node.
        :type node: :py:class:`psyclone.psyGen.KernelSchedule`

        :returns: the Fortran code as a string.
        :rtype: str

        :raises VisitorError: if the name attribute of the supplied \
        node is empty or None.

        '''
        if not node.name:
            raise VisitorError("Expected node name to have a value.")

        args = [symbol.name for symbol in node.symbol_table.argument_list]
        result = (
            "{0}subroutine {1}({2})\n"
            "".format(self._nindent, node.name, ",".join(args)))

        self._depth += 1
        # Declare the kernel data.
        declarations = self.gen_decls(node.symbol_table)
        # Get the executable statements.
        exec_statements = ""
        for child in node.children:
            exec_statements += self._visit(child)
        result += (
            "{0}\n"
            "{1}\n"
            "".format(declarations, exec_statements))

        self._depth -= 1
        result += (
            "{0}end subroutine {1}\n"
            "".format(self._nindent, node.name))

        return result

    def assignment_node(self, node):
        '''This method is called when an Assignment instance is found in the
        PSyIR tree.

        :param node: an Assignment PSyIR node.
        :type node: :py:class:`psyclone.psyGen.Assigment`

        :returns: the Fortran code as a string.
        :rtype: str

        '''
        lhs = self._visit(node.lhs)
        rhs = self._visit(node.rhs)
        result = "{0}{1}={2}\n".format(self._nindent, lhs, rhs)
        return result

    def binaryoperation_node(self, node):
        '''This method is called when a BinaryOperation instance is found in
        the PSyIR tree.

        :param node: a BinaryOperation PSyIR node.
        :type node: :py:class:`psyclone.psyir.nodes.BinaryOperation`

        :returns: the Fortran code as a string.
        :rtype: str

        '''
        lhs = self._visit(node.children[0])
        rhs = self._visit(node.children[1])
        try:
            fort_oper = get_fortran_operator(node.operator)
            if is_fortran_intrinsic(fort_oper):
                # This is a binary intrinsic function.
                return "{0}({1}, {2})".format(fort_oper, lhs, rhs)
            parent = node.parent
            if isinstance(parent, Operation):
                # We may need to enforce precedence
                parent_fort_oper = get_fortran_operator(parent.operator)
                if not is_fortran_intrinsic(parent_fort_oper):
                    # We still may need to enforce precedence
                    if precedence(fort_oper) < precedence(parent_fort_oper):
                        # We need brackets to enforce precedence
                        return "({0} {1} {2})".format(lhs, fort_oper, rhs)
                    if precedence(fort_oper) == precedence(parent_fort_oper):
                        # We still may need to enforce precedence
                        if (isinstance(parent, UnaryOperation) or
                                (isinstance(parent, BinaryOperation) and
                                 parent.children[1] == node)):
                            # We need brackets to enforce precedence
                            # as a) a unary operator is performed
                            # before a binary operator and b) floating
                            # point operations are not actually
                            # associative due to rounding errors.
                            return "({0} {1} {2})".format(lhs, fort_oper, rhs)
            return "{0} {1} {2}".format(lhs, fort_oper, rhs)
        except KeyError:
            raise VisitorError("Unexpected binary op '{0}'."
                               "".format(node.operator))

    def naryoperation_node(self, node):
        '''This method is called when an NaryOperation instance is found in
        the PSyIR tree.

        :param node: an NaryOperation PSyIR node.
        :type node: :py:class:`psyclone.psyir.nodes.NaryOperation`

        :returns: the Fortran code as a string.
        :rtype: str

        :raises VisitorError: if an unexpected N-ary operator is found.

        '''
        arg_list = []
        for child in node.children:
            arg_list.append(self._visit(child))
        try:
            fort_oper = get_fortran_operator(node.operator)
            return "{0}({1})".format(fort_oper, ", ".join(arg_list))
        except KeyError:
            raise VisitorError("Unexpected N-ary op '{0}'".
                               format(node.operator))

    def array_node(self, node):
        '''This method is called when an Array instance is found in the PSyIR
        tree.

        :param node: an Array PSyIR node.
        :type node: :py:class:`psyclone.psyir.nodes.Array`

        :returns: the Fortran code as a string.
        :rtype: str

        '''
        args = []
        for child in node.children:
            args.append(str(self._visit(child)))
        result = "{0}({1})".format(node.name, ",".join(args))
        return result

    def range_node(self, node):
        '''This method is called when a Range instance is found in the PSyIR
        tree.

        :param node: a Range PSyIR node.
        :type node: :py:class:`psyclone.psyir.nodes.Range`

        :returns: the Fortran code as a string.
        :rtype: str

        '''
        def _full_extent(node, operator):
            '''Utility function that returns True if the supplied node
            represents the first index of an array dimension (via the LBOUND
            operator) or the last index of an array dimension (via the
            UBOUND operator).

            This function is required as, whilst Fortran supports an
            implicit lower and/or upper bound e.g. a(:), the PSyIR
            does not. Therefore the a(:) example is represented as
            a(lbound(a,1):ubound(a,1):1). In order to output implicit
            upper and/or lower bounds (so that we output e.g. a(:), we
            must therefore recognise when the lbound and/or ubound
            matches the above pattern.

            :param node: the node to check.
            :type node: :py:class:`psyclone.psyir.nodes.Range`
            :param operator: an lbound or ubound operator.
            :type operator: either :py:class:`Operator.LBOUND` or \
                :py:class:`Operator.UBOUND` from \
                :py:class:`psyclone.psyir.nodes.BinaryOperation`

            '''
            my_range = node.parent
            array = my_range.parent
            array_index = array.children.index(my_range) + 1
            # pylint: disable=too-many-boolean-expressions
            if isinstance(node, BinaryOperation) and \
               node.operator == operator and \
               isinstance(node.children[0], Reference) and \
               node.children[0].name == array.name and \
               isinstance(node.children[1], Literal) and \
               node.children[1].datatype.intrinsic == \
               ScalarType.Intrinsic.INTEGER and \
               node.children[1].value == str(array_index):
                return True
            return False

        if _full_extent(node.start, BinaryOperation.Operator.LBOUND):
            # The range starts for the first element in this
            # dimension. This is the default in Fortran so no need to
            # output anything.
            start = ""
        else:
            start = self._visit(node.start)

        if _full_extent(node.stop, BinaryOperation.Operator.UBOUND):
            # The range ends with the last element in this
            # dimension. This is the default in Fortran so no need to
            # output anything.
            stop = ""
        else:
            stop = self._visit(node.stop)
        result = "{0}:{1}".format(start, stop)

        if isinstance(node.step, Literal) and \
           node.step.datatype.intrinsic == ScalarType.Intrinsic.INTEGER and \
           node.step.value == "1":
            # Step is 1. This is the default in Fortran so no need to
            # output any text.
            pass
        else:
            step = self._visit(node.step)
            result += ":{0}".format(step)
        return result

    # pylint: disable=no-self-use
    def literal_node(self, node):
        '''This method is called when a Literal instance is found in the PSyIR
        tree.

        :param node: a Literal PSyIR node.
        :type node: :py:class:`psyclone.psyir.nodes.Literal`

        :returns: the Fortran code as a string.
        :rtype: str

        '''
        if node.datatype.intrinsic == ScalarType.Intrinsic.BOOLEAN:
            # Booleans need to be converted to Fortran format
            result = '.' + node.value + '.'
        elif node.datatype.intrinsic == ScalarType.Intrinsic.CHARACTER:
            result = "'{0}'".format(node.value)
        else:
            result = node.value
        precision = node.datatype.precision
        if isinstance(precision, DataSymbol):
            # A KIND variable has been specified
            if node.datatype.intrinsic == ScalarType.Intrinsic.CHARACTER:
                result = "{0}_{1}".format(precision.name, result)
            else:
                result = "{0}_{1}".format(result, precision.name)
        if isinstance(precision, int):
            # A KIND value has been specified
            if node.datatype.intrinsic == ScalarType.Intrinsic.CHARACTER:
                result = "{0}_{1}".format(precision, result)
            else:
                result = "{0}_{1}".format(result, precision)
        return result

    # pylint: enable=no-self-use
    def ifblock_node(self, node):
        '''This method is called when an IfBlock instance is found in the
        PSyIR tree.

        :param node: an IfBlock PSyIR node.
        :type node: :py:class:`psyclone.psyir.nodes.IfBlock`

        :returns: the Fortran code as a string.
        :rtype: str

        '''
        condition = self._visit(node.children[0])

        self._depth += 1
        if_body = ""
        for child in node.if_body:
            if_body += self._visit(child)
        else_body = ""
        # node.else_body is None if there is no else clause.
        if node.else_body:
            for child in node.else_body:
                else_body += self._visit(child)
        self._depth -= 1

        if else_body:
            result = (
                "{0}if ({1}) then\n"
                "{2}"
                "{0}else\n"
                "{3}"
                "{0}end if\n"
                "".format(self._nindent, condition, if_body, else_body))
        else:
            result = (
                "{0}if ({1}) then\n"
                "{2}"
                "{0}end if\n"
                "".format(self._nindent, condition, if_body))
        return result

    def loop_node(self, node):
        '''This method is called when a Loop instance is found in the
        PSyIR tree.

        :param node: a Loop PSyIR node.
        :type node: :py:class:`psyclone.psyir.nodes.Loop`

        :returns: the loop node converted into a (language specific) string.
        :rtype: str

        '''
        start = self._visit(node.start_expr)
        stop = self._visit(node.stop_expr)
        step = self._visit(node.step_expr)
        variable_name = node.variable.name

        self._depth += 1
        body = ""
        for child in node.loop_body:
            body += self._visit(child)
        self._depth -= 1

        return "{0}do {1} = {2}, {3}, {4}\n"\
               "{5}"\
               "{0}enddo\n".format(self._nindent, variable_name,
                                   start, stop, step, body)

    def unaryoperation_node(self, node):
        '''This method is called when a UnaryOperation instance is found in
        the PSyIR tree.

        :param node: a UnaryOperation PSyIR node.
        :type node: :py:class:`psyclone.psyir.nodes.UnaryOperation`

        :returns: the Fortran code as a string.
        :rtype: str

        :raises VisitorError: if an unexpected Unary op is encountered.

        '''
        content = self._visit(node.children[0])
        try:
            fort_oper = get_fortran_operator(node.operator)
            if is_fortran_intrinsic(fort_oper):
                # This is a unary intrinsic function.
                return "{0}({1})".format(fort_oper, content)
            return "{0}{1}".format(fort_oper, content)
        except KeyError:
            raise VisitorError("Unexpected unary op '{0}'.".format(
                node.operator))

    def return_node(self, _):
        '''This method is called when a Return instance is found in
        the PSyIR tree.

        :param node: a Return PSyIR node.
        :type node: :py:class:`psyclone.psyir.nodes.Return`

        :returns: the Fortran code as a string.
        :rtype: str

        '''
        return "{0}return\n".format(self._nindent)

    def codeblock_node(self, node):
        '''This method is called when a CodeBlock instance is found in the
        PSyIR tree. It returns the content of the CodeBlock as a
        Fortran string, indenting as appropriate.

        At the moment it is not possible to distinguish between a
        codeblock that is one or more full lines (and therefore needs
        a newline added) and a codeblock that is part of a line (and
        therefore does not need a newline). The current implementation
        adds a newline irrespective. This is the subject of issue
        #388.

        :param node: a CodeBlock PSyIR node.
        :type node: :py:class:`psyclone.psyir.nodes.CodeBlock`

        :returns: the Fortran code as a string.
        :rtype: str

        '''
        from psyclone.psyir.nodes import CodeBlock
        result = ""
        if node.structure == CodeBlock.Structure.STATEMENT:
            # indent and newlines required
            for ast_node in node.get_ast_nodes:
                result += "{0}{1}\n".format(self._nindent, str(ast_node))
        elif node.structure == CodeBlock.Structure.EXPRESSION:
            for ast_node in node.get_ast_nodes:
                result += str(ast_node)
        else:
            raise VisitorError(
                ("Unsupported CodeBlock Structure '{0}' found."
                 "".format(node.structure)))
        return result

    def nemoinvokeschedule_node(self, node):
        '''A NEMO invoke schedule is the top level node in a PSyIR
        representation of a NEMO program unit (program, subroutine
        etc). It does not represent any code itself so all it needs to
        to is call its children and return the result.

        :param node: a NemoInvokeSchedule PSyIR node.
        :type node: :py:class:`psyclone.nemo.NemoInvokeSchedule`

        :returns: the Fortran code as a string.
        :rtype: str

        '''
        result = ""
        for child in node.children:
            result += self._visit(child)
        return result

    def nemokern_node(self, node):
        '''NEMO kernels are a group of nodes collected into a schedule
        so simply call the nodes in the schedule.

        :param node: a NemoKern PSyIR node.
        :type node: :py:class:`psyclone.nemo.NemoKern`

        :returns: the Fortran code as a string.
        :rtype: str

        '''
        result = ""
        schedule = node.get_kernel_schedule()
        for child in schedule.children:
            result += self._visit(child)
        return result

    def nemoimplicitloop_node(self, node):
        '''Fortran implicit loops are currently captured in the PSyIR as a
        NemoImplicitLoop node. This is a temporary solution while the
        best way to capture their behaviour is decided. This method
        outputs the Fortran representation of such a loop by simply
        using the original Fortran ast (i.e. acting in a similar way
        to a code block). As it is a temporary solution we don't
        bother fixing the _ast internal access.

        :param node: a NemoImplicitLoop PSyIR node.
        :type node: :py:class:`psyclone.psyGen.NemoImplicitLoop`

        :returns: the Fortran code as a string.
        :rtype: str

        '''
        return "{0}{1}\n".format(self._nindent, str(node.ast))

    def ompdirective_node(self, node):
        '''This method is called when an OMPDirective instance is found in
        the PSyIR tree. It returns the opening and closing directives, and
        the statements in between as a string (depending on the language).

        :param node: a Directive PSyIR node.
        :type node: :py:class:`psyclone.psyGen.Directive`

        :returns: the Fortran code as a string.
        :rtype: str

        '''
        result_list = ["!${0}\n".format(node.begin_string())]
        self._depth += 1
        for child in node.dir_body:
            result_list.append(self._visit(child))
        self._depth -= 1
        result_list.append("!${0}\n".format(node.end_string()))
        return "".join(result_list)

    def call_node(self, node):
        '''Translate the PSyIR call node to Fortran.

        :param node: a Call PSyIR node.
        :type node: :py:class:`psyclone.psyir.nodes.Call`

        :returns: the Fortran code as a string.
        :rtype: str

        '''

        result_list = []
        for child in node.children:
            result_list.append(self._visit(child))
        args = ", ".join(result_list)
        return "{0}call {1}({2})\n".format(self._nindent, node.routine.name,
                                           args)