Relay Arm® Compute Library Integration

Author: Luke Hutton


Arm Compute Library (ACL) is an open source project that provides accelerated kernels for Arm CPU’s and GPU’s. Currently the integration offloads operators to ACL to use hand-crafted assembler routines in the library. By offloading select operators from a relay graph to ACL we can achieve a performance boost on such devices.

Installing Arm Compute Library

Before installing Arm Compute Library, it is important to know what architecture to build for. One way to determine this is to use lscpu and look for the “Model name” of the CPU. You can then use this to determine the architecture by looking online.

TVM only supports a single version of ACL, currently this is v21.08, there are two recommended ways to build and install the required libraries:

  • Use the script located at docker/install/ You can use this script for downloading ACL binaries for the architecture and extensions specified in target_lib, these will be installed to the location denoted by install_path.

  • Alternatively, you can download the pre-built binaries from: When using this package, you will need to select the binaries for the architecture and extensions you require, then make sure they are visible to CMake:

    cd <acl-prebuilt-package>/lib
    mv ./<architecture-and-extensions-required>/* .

In both cases you will need to set USE_ARM_COMPUTE_LIB_GRAPH_EXECUTOR to the path where the ACL package is located. CMake will look in /path-to-acl/ along with /path-to-acl/lib and /path-to-acl/build for the required binaries. See the section below for more information on how to use these configuration options.

Building with ACL support

The current implementation has two separate build options in CMake. The reason for this split is because ACL cannot be used on an x86 machine. However, we still want to be able compile an ACL runtime module on an x86 machine.

  • USE_ARM_COMPUTE_LIB=ON/OFF - Enabling this flag will add support for compiling an ACL runtime module.

  • USE_ARM_COMPUTE_LIB_GRAPH_EXECUTOR=ON/OFF/path-to-acl - Enabling this flag will allow the graph executor to compute the ACL offloaded functions.

These flags can be used in different scenarios depending on your setup. For example, if you want to compile an ACL module on an x86 machine and then run the module on a remote Arm device via RPC, you will need to use USE_ARM_COMPUTE_LIB=ON on the x86 machine and USE_ARM_COMPUTE_LIB_GRAPH_EXECUTOR=ON on the remote AArch64 device.

By default both options are set to OFF. Using USE_ARM_COMPUTE_LIB_GRAPH_EXECUTOR=ON will mean that ACL binaries are searched for by CMake in the default locations (see In addition to this, /path-to-tvm-project/acl/ will also be searched. It is likely that you will need to set your own path to locate ACL. This can be done by specifying a path in the place of ON.

These flags should be set in your config.cmake file. For example:




This section may not stay up-to-date with changes to the API.

Create a relay graph. This may be a single operator or a whole graph. The intention is that any relay graph can be input. The ACL integration will only pick supported operators to be offloaded whilst the rest will be computed via TVM. (For this example we will use a single max_pool2d operator).

import tvm
from tvm import relay

data_type = "float32"
data_shape = (1, 14, 14, 512)
strides = (2, 2)
padding = (0, 0, 0, 0)
pool_size = (2, 2)
layout = "NHWC"
output_shape = (1, 7, 7, 512)

data = relay.var('data', shape=data_shape, dtype=data_type)
out = relay.nn.max_pool2d(data, pool_size=pool_size, strides=strides, layout=layout, padding=padding)
module = tvm.IRModule.from_expr(out)

Annotate and partition the graph for ACL.

from tvm.relay.op.contrib.arm_compute_lib import partition_for_arm_compute_lib
module = partition_for_arm_compute_lib(module)

Build the Relay graph.

target = "llvm -mtriple=aarch64-linux-gnu -mattr=+neon"
with tvm.transform.PassContext(opt_level=3, disabled_pass=["AlterOpLayout"]):
    lib =, target=target)

Export the module.

lib_path = '~/'
cross_compile = 'aarch64-linux-gnu-c++'
lib.export_library(lib_path, cc=cross_compile)

Run Inference. This must be on an Arm device. If compiling on x86 device and running on AArch64, consider using the RPC mechanism. Tutorials for using the RPC mechanism

dev = tvm.cpu(0)
loaded_lib = tvm.runtime.load_module('')
gen_module = tvm.contrib.graph_executor.GraphModule(loaded_lib['default'](dev))
d_data = np.random.uniform(0, 1, data_shape).astype(data_type)
map_inputs = {'data': d_data}

More examples

The example above only shows a basic example of how ACL can be used for offloading a single Maxpool2D. If you would like to see more examples for each implemented operator and for networks refer to the tests: tests/python/contrib/test_arm_compute_lib. Here you can modify test_config.json to configure how a remote device is created in and, as a result, how runtime tests will be run.

An example configuration for test_config.json:

  • connection_type - The type of RPC connection. Options: local, tracker, remote.

  • host - The host device to connect to.

  • port - The port to use when connecting.

  • target - The target to use for compilation.

  • device_key - The device key when connecting via a tracker.

  • cross_compile - Path to cross compiler when connecting from a non-arm platform e.g. aarch64-linux-gnu-g++.

  "connection_type": "local",
  "host": "",
  "port": 9090,
  "target": "llvm -mtriple=aarch64-linux-gnu -mattr=+neon",
  "device_key": "",
  "cross_compile": ""

Operator support

Relay Node




Simple: nn.conv2d Composite: nn.pad?, nn.conv2d, nn.bias_add?, nn.relu?

Normal and depth-wise (when kernel is 3x3 or 5x5 and strides are 1x1 or 2x2) convolution supported. Grouped convolution is not supported.



Composite: nn.pad?, nn.conv2d, nn.bias_add?, nn.relu?, qnn.requantize

Normal and depth-wise (when kernel is 3x3 or 5x5 and strides are 1x1 or 2x2) convolution supported. Grouped convolution is not supported.



Simple: nn.dense Composite: nn.dense, nn.bias_add?



Composite: qnn.dense, nn.bias_add?, qnn.requantize


fp32, uint8


fp32, uint8



Simple: nn.avg_pool2d


Composite: cast(int32), nn.avg_pool2d, cast(uint8)



Simple: nn.global_avg_pool2d


Composite: cast(int32), nn.avg_pool2d, cast(uint8)

power(of 2) + nn.avg_pool2d + sqrt

A special case for L2 pooling.


Composite: power(of 2), nn.avg_pool2d, sqrt


fp32, uint8








A composite operator is a series of operators that map to a single Arm Compute Library operator. You can view this as being a single fused operator from the view point of Arm Compute Library. ‘?’ denotes an optional operator in the series of operators that make up a composite operator.

Adding a new operator

Adding a new operator requires changes to a series of places. This section will give a hint on what needs to be changed and where, it will not however dive into the complexities for an individual operator. This is left to the developer.

There are a series of files we need to make changes to:

  • python/relay/op/contrib/ In this file we define the operators we wish to offload using the op.register decorator. This will mean the annotation pass recognizes this operator as ACL offloadable.

  • src/relay/backend/contrib/arm_compute_lib/ Implement Create[OpName]JSONNode method. This is where we declare how the operator should be represented by JSON. This will be used to create the ACL module.

  • src/runtime/contrib/arm_compute_lib/ Implement Create[OpName]Layer method. This is where we define how the JSON representation can be used to create an ACL function. We simply define how to translate from the JSON representation to ACL API.

  • tests/python/contrib/test_arm_compute_lib Add unit tests for the given operator.