Markdown for CMSIS-NN integration

Change-Id: I3b0954f3fdb4d54b3e38a84de0ab649c1e79bca8

rfcs/0013_Arm_CMSIS-NN_Integration.md

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+- Feature Name: Arm(R) CMSIS-NN Integration for Cortex-M
+- Start Date: July 2021
+- RFC PR: [apache/tvm-rfcs#0000](https://github.com/apache/tvm-rfcs/pull/0000)
+- GitHub Issue: [apache/tvm#0000](https://github.com/apache/tvm/issues/0000)
+
+# Summary
+
+This RFC introduces plan of integration of CMSIS-NN library into TVM. It consists of efficient kernels targetted for Arm's Cortex-M architecture.
+
+Please refer to the following pages for more details on CMSIS-NN.
+https://arm-software.github.io/CMSIS_5/NN/html/index.html
+https://github.com/ARM-software/CMSIS_5/tree/develop/CMSIS/NN
+
+First PR in the series of PRs to fullfill this integration would be graph partitioner for softmax int8. Detailed plan can found below in this RFC.
+
+
+# Motivation
+
+CMSIS-NN library consists of hand-tuned kernels that are suitable for Cortex-M and are compliant with the quantization scheme used in Tensorflow Lite. They have been optimized for better performance and small memory footprint which is required on these embedded devices and it would make sense for TVM to reuse these while generating code for Cortex-M. They have been integrated with the TensorFlow Lite Micro project.
+
+
+# Guide-level explanation
+
+TVM's external code generation infrastructure allows for the automatic partitoning and code generation using the external compiler. Partitioned subgraphs containing operator(s) targetted for Cortex-M can then be translated into the CMSIS-NN C APIs which eventually become part of MLF. For this integration, we are heavily dependent on the TVM's infrastructure for external code generation.
+
+If a user runs tvmc, they will get a MLF format archive which calls out to the CMSIS operators.
+
+```
+tvmc --target=c,cmsisnn --output-format=mlf --executor=aot
+```
+
+
+# Reference-level explanation
+
+We will enable this integration by considering TFLite networks, but is equally applicable for all other networks that can be translated into Relay IR. TFLite test that contains just a quantized (int8) softmax is first converted as a sequence of following relay operations: *dequantize -> softmax -> quantize* by the TFLite frontend. Please refer to the code snippet below.
+
+```python
+def @main(%a: Tensor[(1, 16, 16, 3), int8]) -> Tensor[(1, 16, 16, 3), int8] {
+  %0 = qnn.dequantize(%a, 0.02f /* ty=float32 */, 64 /* ty=int32 */) /* ty=Tensor[(1, 16, 16, 3), float32] */;
+  %1 = nn.softmax(%0) /* ty=Tensor[(1, 16, 16, 3), float32] */;
+  qnn.quantize(%1, 0.02f /* ty=float32 */, 64 /* ty=int32 */, out_dtype="int8") /* ty=Tensor[(1, 16, 16, 3), int8] */
+}
+```
+
+Following code block shows result of the graph partitioning for cmsisnn target.
+
+```python
+def @main(%a: Tensor[(1, 16, 16, 3), int8]) -> Tensor[(1, 16, 16, 3), int8] {
+  @tvmgen_default_cmsisnn_0(%a) /* ty=Tensor[(1, 16, 16, 3), int8] */
+}
+
+def @tvmgen_default_cmsisnn_0(%cmsisnn_0_i0: Tensor[(1, 16, 16, 3), int8], Inline=1, Compiler="cmsisnn", global_symbol="tvmgen_default_cmsisnn_0", Primitive=1) -> Tensor[(1, 16, 16, 3), int8] {
+  %2 = fn (%FunctionVar_0_0: Tensor[(1, 16, 16, 3), int8], PartitionedFromPattern="qnn.dequantize_nn.softmax_qnn.quantize_", Composite="cmsisnn.qnn_softmax") -> Tensor[(1, 16, 16, 3), int8] {
+    %0 = qnn.dequantize(%FunctionVar_0_0, 0.02f /* ty=float32 */, 64 /* ty=int32 */) /* ty=Tensor[(1, 16, 16, 3), float32] */;
+    %1 = nn.softmax(%0) /* ty=Tensor[(1, 16, 16, 3), float32] */;
+    qnn.quantize(%1, 0.02f /* ty=float32 */, 64 /* ty=int32 */, out_dtype="int8") /* ty=Tensor[(1, 16, 16, 3), int8] */
+  };
+  %2(%cmsisnn_0_i0) /* ty=Tensor[(1, 16, 16, 3), int8] */
+}
+```
+
+Target hooks for `relay_to_tir` implemented as part of https://github.com/apache/tvm-rfcs/pull/10 is used to obtain the following tir for graph with softmax. These hooks provide us with the flexibility to reuse memory planning and much of the TVM's code generation capabilities.
+
+```python
+primfn(placeholder_1: handle, out_write_1: handle) -> ()
+    attr = {"global_symbol": "main", "tir.noalias": True}
+    buffers = {placeholder: Buffer(placeholder_1: Pointer(int8), int8, [1, 300, 300, 3], []),
+                out_write: Buffer(out_write_1: Pointer(int8), int8, [1, 300, 300, 3], [])}
+    buffer_map = {placeholder_1: placeholder_1, out_write_1: out_write_1} {
+    ...
+    allocate(placeholder.d.global, uint8, [1,300,300,3]) {
+        @tir.call_extern("cmsisnn_softmax_s8", ..., dtype=handle)
+    }
+}
+```
+
+At last, code generator identifies the extern_call and generates code for softmax with the CMSIS-NN API for softmax int8.
+
+For more complex operations, CMSIS-NN structures will need to be used. For this purpose, `tir_to_runtime` will be used to extend the existing C Codegen and produce C code with the appropriate headers and calling patterns. Please refer to the [Additional Target Hooks RFC] (https://github.com/apache/tvm-rfcs/pull/10).
+
+# Testing
+
+As we introduce the operators, we will keep on adding individual unit tests. Once the operator support is partially completed, we will start adding network tests. We are planning to use [Arm® Corestone™-300 Fixed Virtual Platform] (https://developer.arm.com/ip-products/subsystem/corstone/corstone-300) to run these tests in the CI. Reference: [Arm Ethos-U Integration] (https://github.com/apache/tvm-rfcs/pull/11/files)
+
+# Drawbacks
+
+CMSIS-NN APIs provide hand coded kernels. Therefore, code generation skips the auto tuning capabilities of TVM. In future, we wish to make use of full power of TVM's auto scheduling.
+
+# Upstreaming Plan
+
+Before adding other operators from CMSIS-NN, the integration will be enabled only for softmax.
+
+P1: Graph partitioner for CMSIS-NN target
+P2: Code generation using existing BYOC
+P3: tvmc support to generate code for CMSIS-NN
+P4: Move this implementation using `tir_to_runtime` from target hooks
+P5: Use of CMSIS-NN data structures while supporting depthwise convolution
+P6: Support for Convolution
+P7: Support for Fully connected
+P8: Support for Max Pooling
+P9: Support for Avg Pooling
+P10: Support for MatMul
+
+
+# Prior art
+
+CMSIS-NN integration into TVM builds on top of ACL's integration into TVM. Existing infrastructure of BYOC allows for graph partitioning to detach the operators or chain of operations as a separate subgraph that then can be compiled for Cortex-M.
+
+Reference: [Arm Compute Lib] (https://tvm.apache.org/docs/deploy/arm_compute_lib.html)
+
+Code generation for CMSIS-NN will use the newly introduced target hooks.
+
+Reference: [Additional Target Hooks] (https://github.com/apache/tvm-rfcs/pull/10/files)