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 state
g = None


class GlobalState():
    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.structs = {}
        self.unions = {}
        self.strings = {}
        self.funcs = {}

        # 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)

        # Intialize built-in functions
        ty = core.Type.function(self.void, [core.Type.pointer(self.i8)], 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]
        )


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([g.target_data.size(ty) for ty in field_types])
        self.ty = core.Type.struct([g.i32, core.Type.array(g.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 = g.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 g
    g = GlobalState(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 enum
    for name in process.mapping.iterkeys():
        if not name.endswith('START'):
            cons = core.Constant.int(g.i32, len(g.states))
            g.states[name] = cons

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

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

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

    # Declare output signal functions
    for signal in process.output_signals:
        if 'type' in signal:
            param_tys = [core.Type.pointer(generate_type(signal['type']))]
        else:
            param_tys = []
        decl_func(str(signal['name']), g.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(generate_type(p['type'])) for p in proc.fpar]
        decl_func(str(proc.inputString), g.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']])

    g.module.verify()

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


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

    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')

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

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

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

    # body
    g.builder.position_at_end(body_block)
    switch = g.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(g.i32, idx)
        switch.add_case(const, tr_block)
        g.builder.position_at_end(tr_block)
        generate(tr)
        if not g.builder.basic_block.terminator:
            g.builder.branch(cond_block)

    # exit
    g.builder.position_at_end(exit_block)
    g.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 = g.scope.label('next_transition')
    g.builder.position_at_end(next_tr_label_block)
    g.builder.branch(cond_block)

    close_scope()

    func.verify()
    return func


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

    open_scope()

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

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

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

    close_scope()

    func.verify()
    return func


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

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

    open_scope()

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

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

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

        # TODO: Nested states

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

        g.builder.ret_void()

    g.builder.position_at_end(exit_block)
    g.builder.ret_void()

    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 = g.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 = g.builder.alloca(p_val.type, None)
                g.builder.store(p_val, p_var)
                params.append(p_var)
            else:
                params.append(p_val)

        g.builder.call(func, params)


def generate_write(params):
    ''' Generate the code for the write operator '''
    zero = core.Constant.int(g.i32, 0)
    for param in params:
        basic_ty = find_basic_type(param.exprType)
        expr_val = expression(param)

        if basic_ty.kind in ['IntegerType', 'Integer32Type']:
            fmt_val = get_string_cons('% d')
            fmt_ptr = g.builder.gep(fmt_val, [zero, zero])
            g.builder.call(g.funcs['printf'], [fmt_ptr, expr_val])
        elif basic_ty.kind == 'RealType':
            fmt_val = get_string_cons('% .14E')
            fmt_ptr = g.builder.gep(fmt_val, [zero, zero])
            g.builder.call(g.funcs['printf'], [fmt_ptr, expr_val])
        elif basic_ty.kind == 'BooleanType':
            true_str_val = get_string_cons('TRUE')
            true_str_ptr = g.builder.gep(true_str_val, [zero, zero])
            false_str_val = get_string_cons('FALSE')
            false_str_ptr = g.builder.gep(false_str_val, [zero, zero])
            str_ptr = g.builder.select(expr_val, true_str_ptr, false_str_ptr)
            g.builder.call(g.funcs['printf'], [str_ptr])
        elif basic_ty.kind == 'StringType':
            expr_ptr = g.builder.gep(expr_val, [zero, zero])
            g.builder.call(g.funcs['printf'], [expr_ptr])
        else:
            raise NotImplementedError


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

    zero = core.Constant.int(g.i32, 0)
    str_cons = get_string_cons('\n')
    str_ptr = g.builder.gep(str_cons, [zero, zero])
    g.builder.call(g.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 = g.funcs[reset_func_name.lower()]

    g.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 = g.funcs[set_func_name.lower()]

    expr_val = expression(timer_expr)

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

    g.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 = g.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('')

    open_scope()

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

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

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

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

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

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

    g.builder.position_at_end(end_block)

    close_scope()


def generate_for_iterable(loop):
    ''' Generate the code for a for x in iterable loop'''
    raise NotImplementedError


@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 g.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 = g.scope.resolve(str(var_name))

    if not prim.value:
        return var_ptr

    zero_cons = core.Constant.int(g.i32, 0)

    for elem in prim.value[1:]:
        if type(elem) == dict:
            if 'index' in elem:
                idx_val = expression(elem['index'][0])
                var_ptr = g.builder.gep(var_ptr, [zero_cons, zero_cons, idx_val])
            else:
                raise NotImplementedError
        else:
            var_ty = var_ptr.type
            if var_ty.pointee.name in g.structs:
                struct = g.structs[var_ty.pointee.name]
                field_idx_cons = core.Constant.int(g.i32, struct.idx(elem))
                field_ptr = g.builder.gep(var_ptr, [zero_cons, field_idx_cons])
                var_ptr = field_ptr
            elif var_ty.pointee.name in g.unions:
                union = g.unions[var_ty.pointee.name]
                _, field_ty = union.kind(elem)
                field_ptr = g.builder.gep(var_ptr, [zero_cons, core.Constant.int(g.i32, 1)])
                var_ptr = g.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 g.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 g.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]
    return core.Constant.int(g.i32, arr_ty.count)


def generate_present(params):
    ''' Generate the code for the built-in present operation'''
    raise NotImplementedError


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 = g.builder.sitofp(expr_val, g.double)
        res_val = g.builder.call(g.funcs['fabs'], [expr_conv])
        return g.builder.fptosi(res_val, g.i32)
    else:
        return g.builder.call(g.funcs['fabs'], [expr_val])


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


def generate_float(params):
    ''' Generate the code for the built-in float operation'''
    expr_val = expression(params[0])
    return g.builder.sitofp(expr_val, g.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 = g.builder.sitofp(left_val, g.double)
        res_val = g.builder.call(g.funcs['powi'], [left_conv, right_val])
        return g.builder.fptosi(res_val, g.i32)
    else:
        return g.builder.call(g.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 g.builder.add(lefttmp, righttmp, 'addtmp')
        elif expr.operand == '-':
            return g.builder.sub(lefttmp, righttmp, 'subtmp')
        elif expr.operand == '*':
            return g.builder.mul(lefttmp, righttmp, 'multmp')
        elif expr.operand == '/':
            return g.builder.sdiv(lefttmp, righttmp, 'divtmp')
        elif expr.operand == 'mod':
            # l mod r == (((l rem r) + r) rem r)
            remtmp = g.builder.srem(lefttmp, righttmp)
            addtmp = g.builder.add(remtmp, righttmp)
            return g.builder.srem(addtmp, righttmp, 'modtmp')
        elif expr.operand == 'rem':
            return g.builder.srem(lefttmp, righttmp, 'remtmp')
        elif expr.operand == '<':
            return g.builder.icmp(core.ICMP_SLT, lefttmp, righttmp, 'lttmp')
        elif expr.operand == '<=':
            return g.builder.icmp(core.ICMP_SLE, lefttmp, righttmp, 'letmp')
        elif expr.operand == '=':
            return g.builder.icmp(core.ICMP_EQ, lefttmp, righttmp, 'eqtmp')
        elif expr.operand == '/=':
            return g.builder.icmp(core.ICMP_NE, lefttmp, righttmp, 'netmp')
        elif expr.operand == '>=':
            return g.builder.icmp(core.ICMP_SGE, lefttmp, righttmp, 'getmp')
        elif expr.operand == '>':
            return g.builder.icmp(core.ICMP_SGT, lefttmp, righttmp, 'gttmp')
        else:
            raise NotImplementedError
    elif lefttmp.type.kind == core.TYPE_DOUBLE:
        if expr.operand == '+':
            return g.builder.fadd(lefttmp, righttmp, 'addtmp')
        elif expr.operand == '-':
            return g.builder.fsub(lefttmp, righttmp, 'subtmp')
        elif expr.operand == '*':
            return g.builder.fmul(lefttmp, righttmp, 'multmp')
        elif expr.operand == '/':
            return g.builder.fdiv(lefttmp, righttmp, 'divtmp')
        elif expr.operand == '<':
            return g.builder.fcmp(core.FCMP_OLT, lefttmp, righttmp, 'lttmp')
        elif expr.operand == '<=':
            return g.builder.fcmp(core.FCMP_OLE, lefttmp, righttmp, 'letmp')
        elif expr.operand == '=':
            return g.builder.fcmp(core.FCMP_OEQ, lefttmp, righttmp, 'eqtmp')
        elif expr.operand == '/=':
            return g.builder.fcmp(core.FCMP_ONE, lefttmp, righttmp, 'netmp')
        elif expr.operand == '>=':
            return g.builder.fcmp(core.FCMP_OGE, lefttmp, righttmp, 'getmp')
        elif expr.operand == '>':
            return g.builder.fcmp(core.FCMP_OGT, lefttmp, righttmp, 'gttmp')
        else:
            raise NotImplementedError
    else:
        raise NotImplementedError


@expression.register(ogAST.ExprAssign)
def _assign(expr):
    ''' Generate the code for an assign expression '''
    generate_assign(reference(expr.left), expression(expr.right))


def generate_assign(left, right):
    ''' Generate code for an assign from two LLVM values'''
    # This is extracted as an standalone function because is used by
    # multiple generation rules
    if is_struct_ptr(left):
        size = core.Constant.sizeof(left.type.pointee)
        align = core.Constant.int(g.i32, 0)
        volatile = core.Constant.int(g.i1, 0)

        right_ptr = g.builder.bitcast(right, g.i8_ptr)
        left_ptr = g.builder.bitcast(left, g.i8_ptr)

        g.builder.call(g.funcs['memcpy'], [left_ptr, right_ptr, size, align, volatile])
    else:
        g.builder.store(right, left)


@expression.register(ogAST.ExprOr)
@expression.register(ogAST.ExprAnd)
@expression.register(ogAST.ExprXor)
def _logical(expr):
    ''' Generate the code for a logical expression '''
    lefttmp = expression(expr.left)
    righttmp = expression(expr.right)

    ty = find_basic_type(expr.exprType)
    if ty.kind != 'BooleanType':
        raise NotImplementedError

    if expr.operand == 'and':
        return g.builder.and_(lefttmp, righttmp, 'andtmp')
    elif expr.operand == 'or':
        return g.builder.or_(lefttmp, righttmp, 'ortmp')
    else:
        return g.builder.xor(lefttmp, righttmp, 'xortmp')


@expression.register(ogAST.ExprAppend)
def _append(expr):
    ''' Generate code for the APPEND construct: a // b '''
    raise NotImplementedError


@expression.register(ogAST.ExprIn)
def _expr_in(expr):
    ''' Generate the code for an in expression '''
    func = g.builder.basic_block.function

    next_block = func.append_basic_block('in:next')
    check_block = func.append_basic_block('in:check')
    end_block = func.append_basic_block('in:end')

    seq_asn1_ty = find_basic_type(expr.left.exprType)
    if seq_asn1_ty.Min != seq_asn1_ty.Max:
        # variable size sequence
        raise NotImplementedError

    idx_ptr = g.builder.alloca(g.i32)
    g.builder.store(core.Constant.int(g.i32, 0), idx_ptr)

    # TODO: Should be 'left' in 'right'?
    value_val = expression(expr.right)
    array_ptr = expression(expr.left)

    array_ty = array_ptr.type.pointee.elements[0]
    array_size = core.Constant.int(g.i32, array_ty.count)
    zero_cons = core.Constant.int(g.i32, 0)

    g.builder.branch(check_block)

    g.builder.position_at_end(check_block)
    idx_val = g.builder.load(idx_ptr)
    elem_val = g.builder.load(g.builder.gep(array_ptr, [zero_cons, zero_cons, idx_val]))
    if value_val.type.kind == core.TYPE_INTEGER:
        cond_val = g.builder.icmp(core.ICMP_EQ, value_val, elem_val)
    elif value_val.type.kind == core.TYPE_DOUBLE:
        cond_val = g.builder.fcmp(core.FCMP_OEQ, value_val, elem_val)
    else:
        raise NotImplementedError
    g.builder.cbranch(cond_val, end_block, next_block)

    g.builder.position_at_end(next_block)
    idx_tmp_val = g.builder.add(idx_val, core.Constant.int(g.i32, 1))
    g.builder.store(idx_tmp_val, idx_ptr)
    end_cond_val = g.builder.icmp(core.ICMP_SGE, idx_tmp_val, array_size)
    g.builder.cbranch(end_cond_val, end_block, check_block)

    g.builder.position_at_end(end_block)
    return cond_val


@expression.register(ogAST.PrimEnumeratedValue)
def _enumerated_value(primary):
    ''' Generate code for an enumerated value '''
    enumerant = primary.value[0].replace('_', '-')
    basic_ty = find_basic_type(primary.exprType)
    return core.Constant.int(g.i32, basic_ty.EnumValues[enumerant].IntValue)


@expression.register(ogAST.PrimChoiceDeterminant)
def _choice_determinant(primary):
    ''' Generate code for a choice determinant (enumerated) '''
    raise NotImplementedError


@expression.register(ogAST.PrimInteger)
def _integer(primary):
    ''' Generate code for a raw integer value  '''
    return core.Constant.int(g.i32, primary.value[0])


@expression.register(ogAST.PrimReal)
def _real(primary):
    ''' Generate code for a raw real value  '''
    return core.Constant.real(g.double, primary.value[0])


@expression.register(ogAST.PrimBoolean)
def _boolean(primary):
    ''' Generate code for a raw boolean value  '''
    if primary.value[0].lower() == 'true':
        return core.Constant.int(g.i1, 1)
    else:
        return core.Constant.int(g.i1, 0)


@expression.register(ogAST.PrimEmptyString)
def _empty_string(primary):
    ''' Generate code for an empty SEQUENCE OF: {} '''
    raise NotImplementedError


@expression.register(ogAST.PrimStringLiteral)
def _string_literal(primary):
    ''' Generate code for a string (Octet String) '''
    return get_string_cons(str(primary.value[1:-1]))


@expression.register(ogAST.PrimConstant)
def _constant(primary):
    ''' Generate code for a reference to an ASN.1 constant '''
    raise NotImplementedError


@expression.register(ogAST.PrimMantissaBaseExp)
def _mantissa_base_exp(primary):
    ''' Generate code for a Real with Mantissa-base-Exponent representation '''
    raise NotImplementedError


@expression.register(ogAST.PrimIfThenElse)
def _if_then_else(ifthen):
    ''' Generate the code for ternary operator '''
    raise NotImplementedError


@expression.register(ogAST.PrimSequence)
def _sequence(seq):
    ''' Generate the code for an ASN.1 SEQUENCE '''
    struct = g.structs[seq.exprType.ReferencedTypeName]
    struct_ptr = g.builder.alloca(struct.ty)
    zero_cons = core.Constant.int(g.i32, 0)

    for field_name, field_expr in seq.value.viewitems():
        field_idx_cons = core.Constant.int(g.i32, struct.idx(field_name))
        field_ptr = g.builder.gep(struct_ptr, [zero_cons, field_idx_cons])