vllm.compilation.fix_functionalization ¶

class FixFunctionalizationPass(VllmInductorPass):
    """
    This pass defunctionalizes certain nodes to avoid redundant tensor copies.
    After this pass, DCE (dead-code elimination) should never be run,
    as de-functionalized nodes may appear as dead code.

    To add new nodes to defunctionalize, add to the if-elif chain in __call__.
    """

    @VllmInductorPass.time_and_log
    def __call__(self, graph: torch.fx.Graph):
        # XPU does not support auto-functionalization yet.
        # Will enable this when switch to vllm-xpu-kernels.
        if current_platform.is_xpu():
            logger.debug(
                "XPU platform does not support fix functionalizationpass currently."
            )
            return

        self.nodes_to_remove: list[torch.fx.Node] = []
        count = 0
        for node in graph.nodes:
            if not is_func(node, auto_functionalized):
                continue  # Avoid deep if-elif nesting

            kwargs = node.kwargs
            at_target = node.args[0]

            if at_target == torch.ops._C.rotary_embedding.default:
                query = kwargs["query"]
                key = kwargs["key"]
                getitem_nodes = self.getitem_users(node)

                if (
                    is_func(query, operator.getitem)
                    and is_func(key, operator.getitem)
                    and query.args[0] == key.args[0]
                    and is_func(query.args[0], torch.ops.aten.split_with_sizes.default)
                    and all(
                        is_func(user, torch.ops.aten.slice_scatter.default)
                        for getitem_node in getitem_nodes.values()
                        for user in getitem_node.users
                    )
                ):
                    # Pattern where query and key are slices of an mm_node.
                    # While functionalized, results at [1] and [2] are scattered
                    # back into mm_node. So after de-functionalization, we can
                    # just use mm_node directly.

                    mm_node = query.args[0].args[0]
                    for user in getitem_nodes.values():
                        for user_of_getitem in user.users:
                            if is_func(
                                user_of_getitem, torch.ops.aten.slice_scatter.default
                            ):
                                user_of_getitem.replace_all_uses_with(mm_node)
                                self._remove(user_of_getitem)
                        self._remove(user)

                    self.insert_defunctionalized(graph, node)
                    self._remove(node)

                else:
                    # Directly replace the auto_functionalize(rotary_embedding)
                    # with the inplace rotary_embedding. In theory, we shouldn't
                    # do this blindly, but in practice in vLLM it's ok. The best
                    # solution is to use auto_functionalization_v2 and then use
                    # inductor's builtin defunctionalization (reinplacing) pass.
                    mutated_args = {1: "query", 2: "key"}
                    self.defunctionalize(graph, node, mutated_args)

            # rms_norm replacements avoid the most copies for LLaMa.
            elif at_target == torch.ops._C.fused_add_rms_norm.default:
                mutated_args = {1: "input", 2: "residual"}
                self.defunctionalize(graph, node, mutated_args)
            elif at_target == torch.ops._C.fused_add_rms_norm_static_fp8_quant.default:  # noqa: E501
                mutated_args = {1: "result", 2: "residual"}
                self.defunctionalize(graph, node, mutated_args)
            elif at_target == torch.ops._C.rms_norm_dynamic_per_token_quant.default:  # noqa: E501
                mutated_args = {1: "result", 2: "scale", 3: "residual"}
                self.defunctionalize(graph, node, mutated_args)
            elif at_target in [
                torch.ops._C.rms_norm.default,
                torch.ops._C.rms_norm_static_fp8_quant.default,
            ]:
                mutated_args = {1: "result"}
                self.defunctionalize(graph, node, mutated_args)
            # For some reason we need to specify the args for both
            # silu_and_mul and silu_and_mul_quant. The kwargs
            # pathway gets the wrong answer.
            elif at_target == torch.ops._C.silu_and_mul.default:
                mutated_args = {1: "result"}
                self.defunctionalize(
                    graph, node, mutated_args, args=("result", "input")
                )
            elif at_target == torch.ops._C.silu_and_mul_quant.default:
                mutated_args = {1: "result"}
                self.defunctionalize(
                    graph, node, mutated_args, args=("result", "input", "scale")
                )
            elif (
                hasattr(torch.ops._C, "silu_and_mul_nvfp4_quant")
                and at_target == torch.ops._C.silu_and_mul_nvfp4_quant.default
            ):
                mutated_args = {1: "result", 2: "result_block_scale"}
                self.defunctionalize(
                    graph,
                    node,
                    mutated_args,
                    args=(
                        "result",
                        "result_block_scale",
                        "input",
                        "input_global_scale",
                    ),
                )
            else:
                continue  # skip the count

            count += 1

        self.dump_graph(graph, "before_cleanup")

        # Remove the nodes all at once
        count_removed = len(self.nodes_to_remove)
        for node in self.nodes_to_remove:
            graph.erase_node(node)

        logger.debug(
            "De-functionalized %s nodes, removed %s nodes", count, count_removed
        )
        self.nodes_to_remove.clear()

    def _remove(self, node_or_nodes: Union[torch.fx.Node, Iterable[torch.fx.Node]]):
        """
        Stage a node (or nodes) for removal at the end of the pass.
        """
        if isinstance(node_or_nodes, torch.fx.Node):
            self.nodes_to_remove.append(node_or_nodes)
        else:
            self.nodes_to_remove.extend(node_or_nodes)

    def defunctionalize(
        self,
        graph: torch.fx.Graph,
        node: torch.fx.Node,
        mutated_args: dict[int, Union[torch.fx.Node, str]],
        args: Optional[tuple[Union[torch.fx.Node, str], ...]] = None,
    ):
        """
        De-functionalize a node by replacing it with a call to the original.
        It also replaces the getitem users with the mutated arguments.
        See replace_users_with_mutated_args and insert_defunctionalized.
        """
        self.replace_users_with_mutated_args(node, mutated_args)
        self.insert_defunctionalized(graph, node, args=args)
        self._remove(node)

    def replace_users_with_mutated_args(
        self, node: torch.fx.Node, mutated_args: dict[int, Union[torch.fx.Node, str]]
    ):
        """
        Replace all getitem users of the auto-functionalized node with the
        mutated arguments.
        :param node: The auto-functionalized node
        :param mutated_args: The mutated arguments, indexed by getitem index.
        If the value of an arg is a string, `node.kwargs[arg]` is used.
        """
        for idx, user in self.getitem_users(node).items():
            arg = mutated_args[idx]
            arg = node.kwargs[arg] if isinstance(arg, str) else arg
            user.replace_all_uses_with(arg)
            self._remove(user)

    def getitem_users(self, node: torch.fx.Node) -> dict[int, torch.fx.Node]:
        """
        Returns the operator.getitem users of the auto-functionalized node,
        indexed by the index they are getting.
        """
        users = {}
        for user in node.users:
            if is_func(user, operator.getitem):
                idx = user.args[1]
                users[idx] = user
        return users

    def insert_defunctionalized(
        self,
        graph: torch.fx.Graph,
        node: torch.fx.Node,
        args: Optional[tuple[Union[torch.fx.Node, str], ...]] = None,
    ):
        """
        Insert a new defunctionalized node into the graph before node.
        If one of the kwargs is 'out', provide args directly,
        as node.kwargs cannot be used.
        See https://github.com/pytorch/pytorch/blob/a00faf440888ffb724bad413f329a49e2b6388e7/torch/_inductor/lowering.py#L351

        :param graph: Graph to insert the defunctionalized node into
        :param node: The auto-functionalized node to defunctionalize
        :param args: If we cannot use kwargs, specify args directly.
        If an arg is a string, `node.kwargs[arg]` is used.
        """  # noqa: E501
        assert is_func(node, auto_functionalized), (
            f"node must be auto-functionalized, is {node} instead"
        )

        # Create a new call to the original function
        with graph.inserting_before(node):
            function = node.args[0]
            if args is None:
                graph.call_function(function, kwargs=node.kwargs)
            else:
                # Args passed as strings refer to items in node.kwargs
                args = tuple(
                    node.kwargs[arg] if isinstance(arg, str) else arg for arg in args
                )
                graph.call_function(function, args=args)