LlvmGenerator.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

"""
    OpenGEODE - A tiny SDL Editor for TASTE

    This module generates LLVM IR code from SDL process models, allowing
    generation of a binary application without an intermediate language.
    LLVM also allows for various code verification, analysis, and optimization.

    The design is based on the Ada code generator. Check it for details.

    Copyright (c) 2012-2013 European Space Agency

    Designed and implemented by Maxime Perrotin

    Contact: maxime.perrotin@esa.int
"""

import logging

from singledispatch import singledispatch
from llvm import core, ee

import ogAST
import Helper

LOG = logging.getLogger(__name__)

__all__ = ['generate']


# Global context
ctx = None


class Context():
    def __init__(self, process):
        self.name = str(process.processName)
        self.module = core.Module.new(self.name)
        self.target_data = ee.TargetData.new(self.module.data_layout)
        self.dataview = process.dataview

        self.scope = Scope()
        self.global_scope = self.scope
        self.states = {}
        self.enums = {}
        self.structs = {}
        self.unions = {}
        self.strings = {}
        self.funcs = {}
        self.lltypes = {}

        # Initialize built-in types
        self.i1 = core.Type.int(1)
        self.i8 = core.Type.int(8)
        self.i32 = core.Type.int(32)
        self.i64 = core.Type.int(64)
        self.void = core.Type.void()
        self.double = core.Type.double()
        self.i1_ptr = core.Type.pointer(self.i1)
        self.i8_ptr = core.Type.pointer(self.i8)
        self.i32_ptr = core.Type.pointer(self.i32)
        self.i64_ptr = core.Type.pointer(self.i64)
        self.double_ptr = core.Type.pointer(self.double)

        # Initialize common constants
        self.zero = core.Constant.int(self.i32, 0)
        self.one = core.Constant.int(self.i32, 1)

        # Intialize built-in functions
        ty = core.Type.function(self.void, [self.i8_ptr], True)
        self.funcs['printf'] = self.module.add_function(ty, 'printf')

        self.funcs['memcpy'] = core.Function.intrinsic(
            self.module,
            core.INTR_MEMCPY,
            [self.i8_ptr, self.i8_ptr, self.i64]
        )

        self.funcs['powi'] = core.Function.intrinsic(
            self.module,
            core.INTR_POWI,
            [self.double]
        )

        self.funcs['fabs'] = core.Function.intrinsic(
            self.module,
            core.INTR_FABS,
            [self.double]
        )

    def open_scope(self):
        ''' Open a scope '''
        self.scope = Scope(self.scope)

    def close_scope(self):
        ''' Close the current scope '''
        self.scope = self.scope.parent

    def type_of(self, asn1ty):
        ''' Return the LL type of a ASN.1 type '''
        try:
            name = asn1ty.ReferencedTypeName.replace('-', '_')
        except AttributeError:
            name = None

        if name and name in self.lltypes:
            return self.lltypes[name]

        basic_asn1ty = find_basic_type(asn1ty)

        if basic_asn1ty.kind in ['IntegerType', 'Integer32Type']:
            llty = ctx.i32
        elif basic_asn1ty.kind == 'BooleanType':
            llty = ctx.i1
        elif basic_asn1ty.kind == 'RealType':
            llty = ctx.double
        elif basic_asn1ty.kind == 'SequenceOfType':
            llty = self._type_of_sequenceof(name, basic_asn1ty)
        elif basic_asn1ty.kind == 'SequenceType':
            llty = self._type_of_sequence(name, basic_asn1ty)
        elif basic_asn1ty.kind == 'EnumeratedType':
            llty = ctx.i32
        elif basic_asn1ty.kind == 'ChoiceType':
            llty = self._type_of_choice(name, basic_asn1ty)
        elif basic_asn1ty.kind == 'OctetStringType':
            llty = self._type_of_octetstring(name, basic_asn1ty)
        else:
            raise NotImplementedError

        if name:
            self.lltypes[name] = llty

        return llty

    def _type_of_sequenceof(self, name, sequenceof_ty):
        ''' Return the LL type of a SequenceOf ASN.1 type '''
        min_size = int(sequenceof_ty.Min)
        max_size = int(sequenceof_ty.Max)
        is_variable_size = min_size != max_size

        elem_ty = self.type_of(sequenceof_ty.type)
        array_ty = core.Type.array(elem_ty, max_size)

        if is_variable_size:
            struct = self.decl_struct(['nCount', 'arr'], [ctx.i32, array_ty], name)
        else:
            struct = self.decl_struct(['arr'], [array_ty], name)

        return struct.ty

    def _type_of_sequence(self, name, sequence_ty):
        ''' Return the LL type of a Sequence ASN.1 type '''
        field_names = []
        field_types = []
        for field_name in Helper.sorted_fields(sequence_ty):
            field_names.append(field_name.replace('-', '_'))
            field_types.append(self.type_of(sequence_ty.Children[field_name].type))
        struct = self.decl_struct(field_names, field_types, name)

        return struct.ty

    def _type_of_choice(self, name, choice_ty):
        ''' Return the equivalent LL type of a Choice ASN.1 type '''
        field_names = []
        field_types = []

        for idx, field_name in enumerate(Helper.sorted_fields(choice_ty)):
            # enum values used in choice determinant/present
            self.enums[field_name.replace('-', '_')] = core.Constant.int(self.i32, idx)

            field_names.append(field_name.replace('-', '_'))
            field_types.append(self.type_of(choice_ty.Children[field_name].type))

        union = self.decl_union(field_names, field_types, name)
        return union.ty

    def _type_of_octetstring(self, name, octetstring_ty):
        ''' Return the equivalent LL type of a OcterString ASN.1 type '''
        max_size = int(octetstring_ty.Max)
        arr_ty = core.Type.array(ctx.i8, max_size)
        struct = self.decl_struct(['nCount', 'arr'], [ctx.i32, arr_ty], name)
        return struct.ty

    def string_ptr(self, str):
        ''' Returns a pointer to a global string with the given value '''
        if str in self.strings:
            return self.strings[str].gep([self.zero, self.zero])

        str_val = core.Constant.stringz(str)
        var_name = '.str%s' % len(self.strings)
        var_ptr = self.module.add_global_variable(str_val.type, var_name)
        var_ptr.initializer = str_val
        self.strings[str] = var_ptr
        return var_ptr.gep([self.zero, self.zero])

    def decl_func(self, name, return_ty, param_tys, extern=False):
        ''' Declare a function '''
        func_ty = core.Type.function(return_ty, param_tys)
        func_name = ("%s_RI_%s" % (self.name, name)) if extern else name
        func = core.Function.new(self.module, func_ty, func_name)
        self.funcs[name.lower()] = func
        return func

    def decl_struct(self, field_names, field_types, name=None):
        ''' Declare a struct '''
        name = name if name else "struct.%s" % len(self.structs)
        name = name.replace('-', '_')
        struct = StructType(name, field_names, field_types)
        self.structs[name] = struct
        return struct

    def resolve_struct(self, name):
        ''' Return the struct associated to a name '''
        return self.structs[name.replace('-', '_')]

    def decl_union(self, field_names, field_types, name=None):
        name = name if name else "union.%s" % len(self.structs)
        name = name.replace('-', '_')
        union = UnionType(name, field_names, field_types)
        self.unions[name] = union
        return union

    def resolve_union(self, name):
        ''' Return the union associated to a name '''
        return self.unions[name.replace('-', '_')]


class StructType():
    def __init__(self, name, field_names, field_types):
        self.name = name
        self.field_names = field_names
        self.ty = core.Type.struct(field_types, self.name)

    def idx(self, field_name):
        return self.field_names.index(field_name)


class UnionType():
    def __init__(self, name, field_names, field_types):
        self.name = name
        self.field_names = field_names
        self.field_types = field_types
        # Unions are represented a struct with a field indicating the index of its type
        # and a byte array with the size of the biggest type in the union
        self.size = max([ctx.target_data.size(ty) for ty in field_types])
        self.ty = core.Type.struct([ctx.i32, core.Type.array(ctx.i8, self.size)], name)

    def kind(self, name):
        idx = self.field_names.index(name)
        return (idx, self.field_types[idx])


class Scope:
    def __init__(self, parent=None):
        self.vars = {}
        self.labels = {}
        self.parent = parent

    def define(self, name, var):
        self.vars[name.lower()] = var

    def resolve(self, name):
        var = self.vars.get(name.lower())
        if var:
            return var
        if self.parent:
            return self.parent.resolve(name)
        else:
            raise NameError("name '%s' is not defined" % name)

    def label(self, name):
        name = name.lower()
        label_block = self.labels.get(name)
        if not label_block:
            func = ctx.builder.basic_block.function
            label_block = func.append_basic_block(name)
            self.labels[name] = label_block
        return label_block


@singledispatch
def generate(ast):
    ''' Generate the code for an item of the AST '''
    raise TypeError('[Backend] Unsupported AST construct')


# Processing of the AST
@generate.register(ogAST.Process)
def _process(process):
    ''' Generate LLVM IR code '''
    process_name = str(process.processName)
    LOG.info('Generating LLVM IR code for process ' + process_name)

    global ctx
    ctx = Context(process)

    # In case model has nested states, flatten everything
    Helper.flatten(process)

    # Make an maping {input: {state: transition...}} in order to easily
    # generate the lookup tables for the state machine runtime
    mapping = Helper.map_input_state(process)

    # Initialize states
    for name, val in process.mapping.viewitems():
        if not name.endswith('START'):
            cons_val = core.Constant.int(ctx.i32, len(ctx.states))
            ctx.states[name] = cons_val
        elif name != 'START':
            cons_val = core.Constant.int(ctx.i32, val)
            ctx.states[name] = cons_val

    # Generate state var
    state_cons = ctx.module.add_global_variable(ctx.i32, 'state')
    state_cons.initializer = core.Constant.int(ctx.i32, -1)
    ctx.scope.define('state', state_cons)

    # Generare process-level vars
    for name, (ty, expr) in process.variables.viewitems():
        var_ty = ctx.type_of(ty)
        global_var = ctx.module.add_global_variable(var_ty, str(name))
        global_var.initializer = core.Constant.null(var_ty)
        ctx.scope.define(str(name).lower(), global_var)

    # Declare timer set/reset functions
    for timer in process.timers:
        # TODO: Should be uint?
        ctx.decl_func("set_%s" % str(timer), ctx.void, [ctx.i32_ptr], True)
        ctx.decl_func("reset_%s" % str(timer), ctx.void, [], True)

    # Declare output signal functions
    for signal in process.output_signals:
        if 'type' in signal:
            param_tys = [core.Type.pointer(ctx.type_of(signal['type']))]
        else:
            param_tys = []
        ctx.decl_func(str(signal['name']), ctx.void, param_tys, True)

    # Declare external procedures functions
    for proc in [proc for proc in process.procedures if proc.external]:
        param_tys = [core.Type.pointer(ctx.type_of(p['type'])) for p in proc.fpar]
        ctx.decl_func(str(proc.inputString), ctx.void, param_tys, True)

    # Generate internal procedures
    for proc in process.content.inner_procedures:
        generate(proc)

    # Generate process functions
    generate_runtr_func(process)
    generate_startup_func(process)

    # Generate input signals
    for signal in process.input_signals:
        generate_input_signal(signal, mapping[signal['name']])

    ctx.module.verify()

    with open(ctx.name + '.ll', 'w') as ll_file:
        ll_file.write(str(ctx.module))


def generate_runtr_func(process):
    ''' Generate code for the run_transition function '''
    func = ctx.decl_func('run_transition', ctx.void, [ctx.i32])

    ctx.open_scope()

    entry_block = func.append_basic_block('entry')
    cond_block = func.append_basic_block('cond')
    body_block = func.append_basic_block('body')
    exit_block = func.append_basic_block('exit')

    ctx.builder = core.Builder.new(entry_block)

    # entry
    id_ptr = ctx.builder.alloca(ctx.i32, None, 'id')
    ctx.scope.define('id', id_ptr)
    ctx.builder.store(func.args[0], id_ptr)
    ctx.builder.branch(cond_block)

    # cond
    ctx.builder.position_at_end(cond_block)
    no_tr_cons = core.Constant.int(ctx.i32, -1)
    id_val = ctx.builder.load(id_ptr)
    cond_val = ctx.builder.icmp(core.ICMP_NE, id_val, no_tr_cons, 'cond')
    ctx.builder.cbranch(cond_val, body_block, exit_block)

    # body
    ctx.builder.position_at_end(body_block)
    switch = ctx.builder.switch(func.args[0], exit_block)

    # transitions
    for idx, tr in enumerate(process.transitions):
        tr_block = func.append_basic_block('tr%d' % idx)
        const = core.Constant.int(ctx.i32, idx)
        switch.add_case(const, tr_block)
        ctx.builder.position_at_end(tr_block)
        generate(tr)
        if not ctx.builder.basic_block.terminator:
            ctx.builder.branch(cond_block)

    # exit
    ctx.builder.position_at_end(exit_block)
    ctx.builder.ret_void()

    Helper.inner_labels_to_floating(process)
    for label in process.content.floating_labels:
        generate(label)

    # TODO: Use defined cond_block instead?
    next_tr_label_block = ctx.scope.label('next_transition')
    ctx.builder.position_at_end(next_tr_label_block)
    ctx.builder.branch(cond_block)

    ctx.close_scope()

    func.verify()
    return func


def generate_startup_func(process):
    ''' Generate code for the startup function '''
    func = ctx.decl_func(ctx.name + '_startup', ctx.void, [])

    ctx.open_scope()

    entry_block = func.append_basic_block('entry')
    ctx.builder = core.Builder.new(entry_block)

    # Initialize process level variables
    for name, (ty, expr) in process.variables.viewitems():
        if expr:
            global_var = ctx.scope.resolve(str(name))
            generate_assign(global_var, expression(expr))

    ctx.builder.call(ctx.funcs['run_transition'], [core.Constant.int(ctx.i32, 0)])
    ctx.builder.ret_void()

    ctx.close_scope()

    func.verify()
    return func


def generate_input_signal(signal, inputs):
    ''' Generate code for an input signal '''
    func_name = ctx.name + "_" + str(signal['name'])
    param_tys = []
    if 'type' in signal:
        param_tys.append(core.Type.pointer(ctx.type_of(signal['type'])))

    func = ctx.decl_func(func_name, ctx.void, param_tys)

    ctx.open_scope()

    entry_block = func.append_basic_block('entry')
    exit_block = func.append_basic_block('exit')
    ctx.builder = core.Builder.new(entry_block)

    g_state_val = ctx.builder.load(ctx.global_scope.resolve('state'))
    switch = ctx.builder.switch(g_state_val, exit_block)

    for state_name, state_id in ctx.states.iteritems():
        if state_name.endswith('START'):
            continue
        state_block = func.append_basic_block('state_%s' % str(state_name))
        switch.add_case(state_id, state_block)
        ctx.builder.position_at_end(state_block)

        # TODO: Nested states

        input = inputs.get(state_name)
        if input:
            for var_name in input.parameters:
                var_ptr = ctx.scope.resolve(str(var_name))
                if is_struct_ptr(var_ptr):
                    generate_assign(var_ptr, func.args[0])
                else:
                    generate_assign(var_ptr, ctx.builder.load(func.args[0]))
            if input.transition:
                id_val = core.Constant.int(ctx.i32, input.transition_id)
                ctx.builder.call(ctx.funcs['run_transition'], [id_val])

        ctx.builder.ret_void()

    ctx.builder.position_at_end(exit_block)
    ctx.builder.ret_void()

    ctx.close_scope()

    func.verify()


@generate.register(ogAST.Output)
@generate.register(ogAST.ProcedureCall)
def _call_external_function(output):
    ''' Generate the code of a set of output or procedure call statement '''
    for out in output.output:
        name = out['outputName'].lower()

        if name == 'write':
            generate_write(out['params'])
            continue
        elif name == 'writeln':
            generate_writeln(out['params'])
            continue
        elif name == 'reset_timer':
            generate_reset_timer(out['params'])
            continue
        elif name == 'set_timer':
            generate_set_timer(out['params'])
            continue

        func = ctx.funcs[str(name).lower()]

        params = []
        for p in out.get('params', []):
            p_val = expression(p)
            # Pass by reference
            if p_val.type.kind != core.TYPE_POINTER:
                p_var = ctx.builder.alloca(p_val.type, None)
                ctx.builder.store(p_val, p_var)
                params.append(p_var)
            else:
                params.append(p_val)

        ctx.builder.call(func, params)


def generate_write(params):
    ''' Generate the code for the write operator '''
    for param in params:
        basic_ty = find_basic_type(param.exprType)
        expr_val = expression(param)

        if basic_ty.kind in ['IntegerType', 'Integer32Type']:
            fmt_str_ptr = ctx.string_ptr('% d')
            ctx.builder.call(ctx.funcs['printf'], [fmt_str_ptr, expr_val])
        elif basic_ty.kind == 'RealType':
            fmt_str_ptr = ctx.string_ptr('% .14E')
            ctx.builder.call(ctx.funcs['printf'], [fmt_str_ptr, expr_val])
        elif basic_ty.kind == 'BooleanType':
            true_str_ptr = ctx.string_ptr('TRUE')
            false_str_ptr = ctx.string_ptr('FALSE')
            str_ptr = ctx.builder.select(expr_val, true_str_ptr, false_str_ptr)
            ctx.builder.call(ctx.funcs['printf'], [str_ptr])
        elif basic_ty.kind in ['StringType', 'OctetStringType']:
            fmt_str_ptr = ctx.string_ptr('%s')
            arr_ptr = ctx.builder.gep(expr_val, [ctx.zero, ctx.one])
            ctx.builder.call(ctx.funcs['printf'], [fmt_str_ptr, arr_ptr])
        else:
            raise NotImplementedError


def generate_writeln(params):
    ''' Generate the code for the writeln operator '''
    generate_write(params)

    str_ptr = ctx.string_ptr('\n')
    ctx.builder.call(ctx.funcs['printf'], [str_ptr])


def generate_reset_timer(params):
    ''' Generate the code for the reset timer operator '''
    timer_id = params[0]
    reset_func_name = 'reset_%s' % timer_id.value[0]
    reset_func = ctx.funcs[reset_func_name.lower()]

    ctx.builder.call(reset_func, [])


def generate_set_timer(params):
    ''' Generate the code for the set timer operator '''
    timer_expr, timer_id = params
    set_func_name = 'set_%s' % timer_id.value[0]
    set_func = ctx.funcs[set_func_name.lower()]

    expr_val = expression(timer_expr)

    tmp_ptr = ctx.builder.alloca(expr_val.type)
    ctx.builder.store(expr_val, tmp_ptr)

    ctx.builder.call(set_func, [tmp_ptr])


@generate.register(ogAST.TaskAssign)
def _task_assign(task):
    ''' Generate the code of a list of assignments '''
    for expr in task.elems:
        expression(expr)


@generate.register(ogAST.TaskInformalText)
def _task_informal_text(task):
    ''' Generate comments for informal text '''
    pass


@generate.register(ogAST.TaskForLoop)
def _task_forloop(task):
    ''' Generate the code for a for loop '''
    for loop in task.elems:
        if loop['range']:
            generate_for_range(loop)
        else:
            generate_for_iterable(loop)


def generate_for_range(loop):
    ''' Generate the code for a for x in range loop '''
    func = ctx.builder.basic_block.function
    cond_block = func.append_basic_block('for:cond')
    body_block = func.append_basic_block('for:body')
    inc_block = func.append_basic_block('for:inc')
    end_block = func.append_basic_block('')

    ctx.open_scope()

    loop_var = ctx.builder.alloca(ctx.i32, None, str(loop['var']))
    ctx.scope.define(str(loop['var']), loop_var)

    if loop['range']['start']:
        start_val = expression(loop['range']['start'])
        ctx.builder.store(start_val, loop_var)
    else:
        ctx.builder.store(core.Constant.int(ctx.i32, 0), loop_var)

    stop_val = expression(loop['range']['stop'])
    ctx.builder.branch(cond_block)

    ctx.builder.position_at_end(cond_block)
    loop_val = ctx.builder.load(loop_var)
    cond_val = ctx.builder.icmp(core.ICMP_SLT, loop_val, stop_val)
    ctx.builder.cbranch(cond_val, body_block, end_block)

    ctx.builder.position_at_end(body_block)
    generate(loop['transition'])
    ctx.builder.branch(inc_block)

    ctx.builder.position_at_end(inc_block)
    step_val = core.Constant.int(ctx.i32, loop['range']['step'])
    loop_val = ctx.builder.load(loop_var)
    temp_val = ctx.builder.add(loop_val, step_val)
    ctx.builder.store(temp_val, loop_var)
    ctx.builder.branch(cond_block)

    ctx.builder.position_at_end(end_block)

    ctx.close_scope()


def generate_for_iterable(loop):
    ''' Generate the code for a for x in iterable loop'''
    seqof_asn1ty = find_basic_type(loop['list'].exprType)
    is_variable_size = seqof_asn1ty.Min != seqof_asn1ty.Max

    func = ctx.builder.basic_block.function

    # block for loading the value from the secuence
    # at the current index, incrementing the index afterwards
    load_block = func.append_basic_block('forin:load')
    # block for the body of the loop
    body_block = func.append_basic_block('forin:body')
    # block for checking if should loop again or terminate
    cond_block = func.append_basic_block('forin:cond')
    end_block = func.append_basic_block('')

    ctx.open_scope()

    idx_ptr = ctx.builder.alloca(ctx.i32)
    ctx.builder.store(core.Constant.int(ctx.i32, 0), idx_ptr)
    seqof_struct_ptr = expression(loop['list'])

    if is_variable_size:
        # In variable size SequenceOfs the array values are in the second field
        array_ptr = ctx.builder.gep(seqof_struct_ptr, [ctx.zero, ctx.one])
    else:
        array_ptr = ctx.builder.gep(seqof_struct_ptr, [ctx.zero, ctx.zero])

    element_typ = array_ptr.type.pointee.element

    if is_variable_size:
        # load the current number of elements that is on the first field
        end_idx = ctx.builder.load(ctx.builder.gep(seqof_struct_ptr, [ctx.zero, ctx.zero]))
    else:
        end_idx = core.Constant.int(ctx.i32, array_ptr.type.pointee.count)

    var_ptr = ctx.builder.alloca(element_typ, None, str(loop['var']))
    ctx.scope.define(str(loop['var']), var_ptr)

    ctx.builder.branch(load_block)

    # load block
    ctx.builder.position_at_end(load_block)
    idx_var = ctx.builder.load(idx_ptr)
    if element_typ.kind == core.TYPE_STRUCT:
        generate_assign(var_ptr, ctx.builder.gep(array_ptr, [ctx.zero, idx_var]))
    else:
        generate_assign(var_ptr, ctx.builder.load(ctx.builder.gep(array_ptr, [ctx.zero, idx_var])))
    ctx.builder.branch(body_block)

    # body block
    ctx.builder.position_at_end(body_block)
    generate(loop['transition'])
    ctx.builder.branch(cond_block)

    # cond block
    ctx.builder.position_at_end(cond_block)
    tmp_val = ctx.builder.add(idx_var, ctx.one)
    ctx.builder.store(tmp_val, idx_ptr)
    cond_val = ctx.builder.icmp(core.ICMP_SLT, tmp_val, end_idx)
    ctx.builder.cbranch(cond_val, load_block, end_block)

    ctx.builder.position_at_end(end_block)

    ctx.close_scope()


@singledispatch
def reference(prim):
    ''' Generate a variable reference '''
    raise TypeError('Unsupported reference: ' + str(prim))


@reference.register(ogAST.PrimVariable)
def _prim_var_reference(prim):
    ''' Generate a primary variable reference '''
    return ctx.scope.resolve(str(prim.value[0]))


@reference.register(ogAST.PrimPath)
def _prim_path_reference(prim):
    ''' Generate a primary path reference '''
    var_name = prim.value[0].lower()
    var_ptr = ctx.scope.resolve(str(var_name))

    if not prim.value:
        return var_ptr

    for elem in prim.value[1:]:
        if type(elem) == dict:
            if 'index' in elem:
                idx_val = expression(elem['index'][0])
                array_ptr = ctx.builder.gep(var_ptr, [ctx.zero, ctx.zero])
                # TODO: Refactor this
                if array_ptr.type.pointee.kind != core.TYPE_ARRAY:
                    # If is not an array this is a pointer to a variable size SeqOf
                    # The array is in the second field of the struct
                    var_ptr = ctx.builder.gep(var_ptr, [ctx.zero, ctx.one, idx_val])
                else:
                    var_ptr = ctx.builder.gep(var_ptr, [ctx.zero, ctx.zero, idx_val])
            else:
                raise NotImplementedError
        else:
            var_ty = var_ptr.type
            if var_ty.pointee.name in ctx.structs:
                struct = ctx.structs[var_ty.pointee.name]
                field_idx_cons = core.Constant.int(ctx.i32, struct.idx(elem))
                field_ptr = ctx.builder.gep(var_ptr, [ctx.zero, field_idx_cons])
                var_ptr = field_ptr
            elif var_ty.pointee.name in ctx.unions:
                union = ctx.unions[var_ty.pointee.name]
                _, field_ty = union.kind(elem)
                field_ptr = ctx.builder.gep(var_ptr, [ctx.zero, ctx.one])
                var_ptr = ctx.builder.bitcast(field_ptr, core.Type.pointer(field_ty))
            else:
                raise NotImplementedError
    return var_ptr


@singledispatch
def expression(expr):
    ''' Generate the code for Expression-classes '''
    raise TypeError('Unsupported expression: ' + str(expr))


@expression.register(ogAST.PrimVariable)
def _primary_variable(prim):
    ''' Generate the code for a variable expression '''
    var_ptr = reference(prim)
    return var_ptr if is_struct_ptr(var_ptr) else ctx.builder.load(var_ptr)


@expression.register(ogAST.PrimPath)
def _prim_path(prim):
    ''' Generate the code for an of path expression '''
    specops_generators = {
        'length': generate_length,
        'present': generate_present,
        'abs': generate_abs,
        'fix': generate_fix,
        'float': generate_float,
        'power': generate_power
    }

    name = prim.value[0].lower()
    if name in specops_generators:
        generator = specops_generators[name]
        return generator(prim.value[1]['procParams'])

    return generate_access(prim)


def generate_access(prim):
    ''' Generate the code for an access '''
    var_ptr = reference(prim)
    return var_ptr if is_struct_ptr(var_ptr) else ctx.builder.load(var_ptr)


def generate_length(params):
    ''' Generate the code for the built-in length operation'''
    seq_ptr = reference(params[0])
    arr_ty = seq_ptr.type.pointee.elements[0]
    if arr_ty.kind != core.TYPE_ARRAY:
        # If is not an array this is a pointer to a variable size SeqOf
        # The array is in the second field of the struct
        arr_ty = seq_ptr.type.pointee.elements[1]
    return core.Constant.int(ctx.i32, arr_ty.count)


def generate_present(params):
    ''' Generate the code for the built-in present operation'''
    expr_val = expression(params[0])
    kind_ptr = ctx.builder.gep(expr_val, [ctx.zero, ctx.zero])
    return ctx.builder.load(kind_ptr)


def generate_abs(params):
    ''' Generate the code for the built-in abs operation'''
    expr_val = expression(params[0])

    if expr_val.type.kind == core.TYPE_INTEGER:
        expr_conv = ctx.builder.sitofp(expr_val, ctx.double)
        res_val = ctx.builder.call(ctx.funcs['fabs'], [expr_conv])
        return ctx.builder.fptosi(res_val, ctx.i32)
    else:
        return ctx.builder.call(ctx.funcs['fabs'], [expr_val])


def generate_fix(params):
    ''' Generate the code for the built-in fix operation'''
    expr_val = expression(params[0])
    return ctx.builder.fptosi(expr_val, ctx.i32)


def generate_float(params):
    ''' Generate the code for the built-in float operation'''
    expr_val = expression(params[0])
    return ctx.builder.sitofp(expr_val, ctx.double)


def generate_power(params):
    ''' Generate the code for the built-in power operation'''
    left_val = expression(params[0])
    right_val = expression(params[1])

    if left_val.type.kind == core.TYPE_INTEGER:
        left_conv = ctx.builder.sitofp(left_val, ctx.double)
        res_val = ctx.builder.call(ctx.funcs['powi'], [left_conv, right_val])
        return ctx.builder.fptosi(res_val, ctx.i32)
    else:
        return ctx.builder.call(ctx.funcs['powi'], [left_val, right_val])


@expression.register(ogAST.ExprPlus)
@expression.register(ogAST.ExprMul)
@expression.register(ogAST.ExprMinus)
@expression.register(ogAST.ExprEq)
@expression.register(ogAST.ExprNeq)
@expression.register(ogAST.ExprGt)
@expression.register(ogAST.ExprGe)
@expression.register(ogAST.ExprLt)
@expression.register(ogAST.ExprLe)
@expression.register(ogAST.ExprDiv)
@expression.register(ogAST.ExprMod)
@expression.register(ogAST.ExprRem)
def _basic(expr):
    ''' Generate the code for an arithmetic of relational expression '''
    lefttmp = expression(expr.left)
    righttmp = expression(expr.right)

    if lefttmp.type.kind == core.TYPE_INTEGER:
        if expr.operand == '+':
            return ctx.builder.add(lefttmp, righttmp, 'addtmp')
        elif expr.operand == '-':
            return ctx.builder.sub(lefttmp, righttmp, 'subtmp')