[RELAY][Frontend][TF] decompile tf control flow (#2830)

* decompile tf control flow

* Add docs

* remove import relay

* move tests under tensorflow frontend

* minor fix

python/tvm/relay/frontend/tensorflow.py

@@ -5,6 +5,7 @@
 
 import logging
 import warnings
+from collections import defaultdict
 # Numpy support
 import numpy as np
 
@@ -1270,6 +1271,220 @@ def _get_abs_layer_name(node):
                                                      params, num_layers)
         return sym
 
+# An internal list to contain all the control flow primitives used in Tensorflow
+# 1.x.
+_control_flow_nodes = ['Merge', 'Switch', 'NextIteration', 'Exit', 'Enter', 'LoopCond']
+
+def _in_while_loop(control_flow_node_map, op_name):
+    """
+    Check if a given control flow operator is part of a while loop execution
+    frame. This is based on the fact that there is only one occurrence of
+    `LoopCond` for a loop execution frame and it is only presented in the loop
+    construct.
+
+    Parameters
+    ----------
+    control_flow_node_map : Dict[str, Set[str]]
+        A dictionay contains the unqiue control flow execution frame name to
+        a set of primitive operators mapping.
+
+    op_name : str
+        The name of a control flow primitive.
+
+    Returns
+    -------
+    ret : bool
+        Return true if the operator is in a while loop execution frame,
+    otherwise, return false.
+    """
+    return op_name in control_flow_node_map and \
+            "LoopCond" in control_flow_node_map[op_name]
+
+
+class Branch:
+    """A class contains the components that are used to build up a Relay if
+    node.
+
+    Parameters
+    ----------
+    cond : tvm.relay.Expr
+        The condition of a if node.
+
+    true_branch : tvm.relay.Expr
+        The body of the true branch of a if expression.
+
+    false_branch: tvm.relay.Expr
+        The body of the false branch of a if expression.
+
+    _if : tvm.relay.Expr
+        An internal variable indicates where an if expression is already created
+        for a matched TF condition construct.
+
+    Examples
+    --------
+    The following is a cond statement written in TensorFlow:
+
+    .. code-block:: python
+
+        def vanilla_cond():
+            i = tf.constant(1)
+            j = tf.constant(4)
+
+             def f1():
+                return tf.multiply(1, 17)
+
+             def f2():
+                return tf.add(4, 23)
+            r = tf.cond(tf.less(i, j), f1, f2)
+
+    This condition statement should be coverted into Relay in the following
+    form:
+
+    .. code-block:: python
+
+        fn (%Const: Tensor[(1,), int32],
+            %Const_1: Tensor[(1,), int32],
+            %cond/Mul/x: Tensor[(1,), int32],
+            %cond/Mul/y: Tensor[(1,), int32],
+            %cond/Add/x: Tensor[(1,), int32],
+            %cond/Add/y: Tensor[(1,), int32]) {
+          %0 = less(%Const, %Const_1) # ty=Tensor[(1,), bool]
+          %1 = min(%0)
+          if (%1) {
+            %2 = multiply(%cond/Mul/x, %cond/Mul/y)
+            %2
+          }  else {
+            %3 = add(%cond/Add/x, %cond/Add/y)
+            %3
+          }
+        }
+    """
+    def __init__(self):
+        self._if = None
+        self.cond = None
+        self.true_branch = None
+        self.false_branch = None
+
+    def _if_node(self):
+        """An internal API to create a relay if node from the matched TF
+        condition construct.
+        """
+        # `cond`  returns a tensor that contains boolean values. We add a `min`
+        # operator to checks if there is any false value. If so, this condition
+        # doesn't not hold.
+        cond = tvm.relay.op.min(self.cond)
+        return tvm.relay.If(cond, self.true_branch, self.false_branch)
+
+    def if_node(self):
+        """Create an tvm.relay.If node if it hasn't been created yet."""
+        if self._if is None:
+            self._if = self._if_node()
+        return self._if
+
+
+class Loop:
+    """
+    A class contains the components that are used to build up a Relay
+    recursive call.
+
+    Parameters
+    ----------
+    loop_vars : List[tvm.relay.Expr]
+        The loop variables that used in a while loop.
+
+    cond : tvm.relay.Expr
+        The condition of a while loop.
+
+    body : tvm.relay.Expr
+        The body of a matched while loop.
+
+    _loop : tvm.relay.Expr
+        An internal variable indicates where a recursive call is already created
+        for a matched TF while loop construct.
+
+    Examples
+    --------
+    The following is a vanilla loop from TensorFlow:
+
+    .. code-block:: python
+
+        i = tf.constant(0)
+        c = lambda i: tf.less(i, 10)
+        b = lambda i: tf.add(i, 1)
+        r = tf.while_loop(c, b, [i])
+
+    It will be converted to the following recursive call in Relay:
+
+    .. code-block:: python
+
+        fn (%while/Less/y: Tensor[(1,), int32],
+            %while/Add/y: Tensor[(1,), int32],
+            %Const: Tensor[(1,), int32]) {
+          %0 = fn(%loop_var0: Tensor[(1,), int32]) {
+            %1 = less(%loop_var0, %while/Less/y)
+            %2 = min(%1)
+            if (%2) {
+              %3 = add(%loop_var0, %while/Add/y)
+              free_var %while_loop
+              %4 = %while_loop(%3)
+              %4
+            }    else {
+              %5 = (%loop_var0,)
+              %5
+            }
+          }
+          let %while_loop1 = %0
+          %6 = %while_loop1(%Const)
+          %6
+        }
+    """
+    def __init__(self):
+        self.loop_vars = []
+        self.cond = None
+        self.body = []
+        self._loop = None
+
+    def _while_loop(self):
+        """An internal API to create a Relay recurisve call for a matched TF
+        `while_loop` construct.
+        """
+        wl = tvm.relay.var('while_loop')
+
+        sb = tvm.relay.scope_builder.ScopeBuilder()
+
+        loop_vars = []
+        bind_map = {}
+        for i, var in enumerate(self.loop_vars):
+            assert isinstance(var, _expr.Var), repr(var)
+            v = tvm.relay.var("loop_var" + str(i),
+                              type_annotation=var.type_annotation)
+            loop_vars.append(v)
+            bind_map[var] = v
+
+        self.cond = tvm.relay.bind(self.cond, bind_map)
+        self.body = [tvm.relay.bind(b, bind_map) for b in self.body]
+
+        cond = tvm.relay.op.min(self.cond)
+
+        with sb.if_scope(cond):
+            sb.ret(wl(*self.body))
+        with sb.else_scope():
+            sb.ret(tvm.relay.Tuple(loop_vars))
+
+        loop_fn = tvm.relay.Function(loop_vars, sb.get())
+        sb = tvm.relay.scope_builder.ScopeBuilder()
+        sb.let(wl, loop_fn)
+        sb.ret(wl(*self.loop_vars))
+        return sb.get()
+
+    def while_loop(self):
+        """Instantiate a while loop if it has not been created yet."""
+        if self._loop is None:
+            self._loop = self._while_loop()
+            return self._loop
+        return self._loop
+
+
 class GraphProto(object):
     """ A helper class for handling relay graph copying from Tensorflow GraphDef.
     Definition:
@@ -1284,6 +1499,8 @@ def __init__(self):
         self._num_rnn_layer = False
         self._outputs_are_0d = {}
         self._input_shapes = {}
+        self._loops = {}
+        self._branches = {}
 
     def from_tensorflow(self, graph, layout="NHWC", shape=None, outputs=None):
         """Construct relay nodes from tensorflow  graph definition - GraphDef.
@@ -1332,7 +1549,10 @@ def from_tensorflow(self, graph, layout="NHWC", shape=None, outputs=None):
             raise NotImplementedError( \
                 "The following operators are not implemented: {}".format(missing_operators))
 
+        control_flow_node_map = defaultdict(set)
         for node in graph.node:
+            node_name_prefix = node.name.rsplit('/', 1)[0]
+            control_flow_node_map[node_name_prefix].add(node.op)
             if node.op == 'Placeholder':
                 if shape and node.name in shape:
                     self._input_shapes[node.name] = list(shape[node.name])
@@ -1447,12 +1667,17 @@ def from_tensorflow(self, graph, layout="NHWC", shape=None, outputs=None):
                         # This means the node is 1d in Relay and 0d in TF.
                         # See `_expand_dims_0d_aware`.
                         if self._outputs_are_0d[node_name][tensor_slot] and input_shape:
-                            input_0d_mismatch.add(in_sym)
+                            input_0d_mismatch.add(in_sym[0])
 
                 attr['_input_shapes'] = input_shapes
                 attr['_input_0d_mismatch'] = input_0d_mismatch
 
-                op = self._convert_operator(node.op, inputs, attr, graph)
+                if node.op in _control_flow_nodes:
+                    op = self._convert_control_flow_operator(node, inputs,
+                                                             attr,
+                                                             control_flow_node_map)
+                else:
+                    op = self._convert_operator(node.op, inputs, attr, graph)
 
                 # Check if op is converted to param
                 if isinstance(op, np.ndarray):
@@ -1493,7 +1718,10 @@ def from_tensorflow(self, graph, layout="NHWC", shape=None, outputs=None):
 
         out = []
         if outputs is None:
-            out = op
+            if node.op == "Exit":
+                out = [op[0].tuple_value]
+            else:
+                out = op
         else:
             for out_name in outputs:
                 if ":" in out_name:
@@ -1529,7 +1757,9 @@ def _parse_import_prerequisites(self, graph):
             elif node.op == "Const":
                 pass
             else:
-                if any([node.op in t for t in [_identity_list, _convert_map, _convert_map_rnn]]):
+                if any([node.op in t for t in [_identity_list, _convert_map,
+                                               _convert_map_rnn,
+                                               _control_flow_nodes]]):
                     pass
                 else:
                     missing_operators.add(node.op)
@@ -1656,6 +1886,89 @@ def _convert_rnn_operator(self, op_name, inputs,
         sym = self.rnn.process_op(op_name, inputs, attrs, params)
         return sym
 
+    def _convert_control_flow_operator(self, node, inputs, attrs, control_flow_node_map):
+        """
+        Convert the Relay control flow primitive into corresponding component
+        of a Relay control flow construct, i.e. `tf.cond` and `tf.while_loop`
+        are converted in Relay `If` and recusrive call, respectively.
+
+        Parameters
+        ----------
+        node: TensorFlow graph node object.
+            A TensorFlow graph node object.
+
+        inputs : List[tvm.relay.Expr]
+            List of input symbols.
+
+        attrs : Dict[tvm.Attrs]
+            Dict of operator attributes.
+
+        control_flow_node_map : Dict[str, Set[str]]
+            A dictionary contains the execution frame name to primitives
+            mapping.
+
+        Returns
+        -------
+        op : tvm.relay.Expr
+            Converted relay expression.
+        """
+        node_name_prefix = node.name.rsplit('/', 1)[0]
+        if node.op == "Merge":
+            if _in_while_loop(control_flow_node_map, node_name_prefix):
+                op = self._nodes[node.input[0]]
+                self._loops[node_name_prefix] = Loop()
+            else:
+                if len(self._branches) == 0:
+                    raise RuntimeError("Cannot find a created "
+                                       "conditional for merge node")
+                branch = self._branches[node_name_prefix]
+                false_br = self._nodes[node.input[0]]
+                true_br = self._nodes[node.input[1]]
+                assert len(true_br) == 1
+                assert len(false_br) == 1
+                branch.true_branch = true_br[0]
+                branch.false_branch = false_br[0]
+                op = [branch.if_node()]
+        elif node.op == "Exit":
+            loop = self._loops[node_name_prefix]
+            exit_name = node.name.split('/')[-1]
+            assert str.startswith(exit_name, 'Exit')
+
+            # TensorFlow has differen naming convention on different
+            # versions.
+            if '_' in exit_name:
+                exit_number = int("0" + exit_name[5:])
+            else:
+                exit_number = int("0" + exit_name[4:])
+
+            expr = loop.while_loop()
+            op = _expr.TupleGetItem(expr, exit_number)
+        elif node.op == "Enter":
+            op = self._nodes[node.input[0]]
+        elif node.op == "LoopCond":
+            op = self._nodes[node.input[0]]
+            assert len(op) == 1
+            self._loops[node_name_prefix].cond = op[0]
+        elif node.op == "Switch":
+            op = self._nodes[node.input[0]]
+            assert len(op) == 1
+            if _in_while_loop(control_flow_node_map, node_name_prefix):
+                self._loops[node_name_prefix].loop_vars.append(op[0])
+            else:
+                if node_name_prefix not in self._branches:
+                    self._branches[node_name_prefix] = Branch()
+                self._branches[node_name_prefix].cond = ir_pass.infer_type(op[0])
+        elif node.op == "NextIteration":
+            op = self._nodes[node.input[0]]
+            assert len(op) == 1
+            self._loops[node_name_prefix].body.append(op[0])
+        else:
+            raise Exception("Cannot identify control flow operator: " +
+                            "{}".format(node.op))
+
+        return op
+
+
     def _convert_operator(self, op_name, inputs, attrs,
                           graph, identity_list=None, convert_map=None):
         """Convert from Tensorflow operator to relay operator.