Tensorflow AOT (tfcompile)の使い方

# この記事について
Tensorflow AOT (Ahead-of-Time)のツールであるtfcompileの使い方のメモです。tfcompileの詳細については[公式サイト(Using AOT compilation)](https://www.tensorflow.org/performance/xla/tfcompile)に説明が記載されています。

# tfcompileの使い方

tfcompileで使えるフラグ一覧は以下の通りです。

```bash

tfcompile performs ahead-of-time compilation of a TensorFlow graph,
resulting in an object file compiled for your target architecture, and a
header file that gives access to the functionality in the object file.
A typical invocation looks like this:

   $ tfcompile --graph=mygraph.pb --config=myfile.pbtxt --cpp_class="mynamespace::MyComputation"

usage: ./tfcompile
Flags:
        --graph=""                              string  Input GraphDef file.  If the file ends in '.pbtxt' it is expected to be in the human-readable proto text format, otherwise it is expected to be in the proto binary format.
        --config=""                             string  Input file containing Config proto.  If the file ends in '.pbtxt' it is expected to be in the human-readable proto text format, otherwise it is expected to be in the proto binary format.
        --dump_fetch_nodes=false                bool    If set, only flags related to fetches are processed, and the resulting fetch nodes will be dumped to stdout in a comma-separated list.  Typically used to format arguments for other tools, e.g. freeze_graph.
        --target_triple="x86_64-pc-linux"       string  Target platform, similar to the clang -target flag.  The general format is <arch><sub>-<vendor>-<sys>-<abi>.  http://clang.llvm.org/docs/CrossCompilation.html#target-triple.
        --target_cpu=""                         string  Target cpu, similar to the clang -mcpu flag.  http://clang.llvm.org/docs/CrossCompilation.html#cpu-fpu-abi
        --target_features=""                    string  Target features, e.g. +avx2, +neon, etc.
        --entry_point="entry"                   string  Name of the generated function.  If multiple generated object files will be linked into the same binary, each will need a unique entry point.
        --cpp_class=""                          string  Name of the generated C++ class, wrapping the generated function.  The syntax of this flag is [[<optional_namespace>::],...]<class_name>.  This mirrors the C++ syntax for referring to a class, where multiple namespaces may precede the class name, separated by double-colons.  The class will be generated in the given namespace(s), or if no namespaces are given, within the global namespace.
        --out_function_object="out_model.o"     string  Output object file containing the generated function for the TensorFlow model.
        --out_header="out.h"                    string  Output header file name.
        --out_metadata_object="out_helper.o"    string  Output object file name containing optional metadata for the generated function.
        --out_session_module=""                 string  Output session module proto.
        --gen_name_to_index=false               bool    Generate name-to-index data for Lookup{Arg,Result}Index methods.
        --gen_program_shape=false               bool    Generate program shape data for the ProgramShape method.
        --xla_generate_hlo_graph=""             string  HLO modules matching this regex will be dumped to a .dot file throughout various stages in compilation.
        --xla_hlo_graph_addresses=false         bool    With xla_generate_hlo_graph, show addresses of HLO ops in graph dump.
        --xla_hlo_graph_path=""                 string  With xla_generate_hlo_graph, dump the graphs into this path.
        --xla_hlo_dump_as_graphdef=false        bool    Dump HLO graphs as TensorFlow GraphDefs.
        --xla_hlo_graph_sharding_color=false    bool    Assign colors based on sharding assignments when generating the HLO graphs.
        --xla_hlo_tfgraph_device_scopes=false   bool    When generating TensorFlow HLO graphs, if the HLO instructions are assigned to a specific device, prefix the name scope with "devX" with X being the device ordinal.
        --xla_log_hlo_text=""                   string  HLO modules matching this regex will be dumped to LOG(INFO).
        --xla_generate_hlo_text_to=""           string  Dump all HLO modules as text into the provided directory path.
        --xla_enable_fast_math=true             bool    Enable unsafe fast-math optimizations in the compiler; this may produce faster code at the expense of some accuracy.
        --xla_llvm_enable_alias_scope_metadata=true     bool    In LLVM-based backends, enable the emission of !alias.scope metadata in the generated IR.
        --xla_llvm_enable_noalias_metadata=true bool    In LLVM-based backends, enable the emission of !noalias metadata in the generated IR.
        --xla_llvm_enable_invariant_load_metadata=true  bool    In LLVM-based backends, enable the emission of !invariant.load metadata in the generated IR.
        --xla_llvm_disable_expensive_passes=false       bool    In LLVM-based backends, disable a custom set of expensive optimization passes.
        --xla_backend_optimization_level=3      int32   Numerical optimization level for the XLA compiler backend.
        --xla_disable_hlo_passes=""             string  Comma-separated list of hlo passes to be disabled. These names must exactly match the passes' names; no whitespace around commas.
        --xla_embed_ir_in_executable=false      bool    Embed the compiler IR as a string in the executable.
        --xla_dump_ir_to=""                     string  Dump the compiler IR into this directory as individual files.
        --xla_eliminate_hlo_implicit_broadcast=true     bool    Eliminate implicit broadcasts when lowering user computations to HLO instructions; use explicit broadcast instead.
        --xla_cpu_multi_thread_eigen=true       bool    When generating calls to Eigen in the CPU backend, use multi-threaded Eigen mode.
        --xla_gpu_cuda_data_dir="./cuda_sdk_lib"        string  If non-empty, speficies a local directory containing ptxas and nvvm libdevice files; otherwise we use those from runfile directories.
        --xla_gpu_ftz=false                     bool    If true, flush-to-zero semantics are enabled in the code generated for GPUs.
        --xla_gpu_disable_multi_streaming=false bool    If true, multi-streaming in the GPU backend is disabled.
        --xla_gpu_max_kernel_unroll_factor=4    int32   Specify the maximum kernel unroll factor for the GPU backend.
        --xla_dump_optimized_hlo_proto_to=""    string  Dump Hlo after all hlo passes are executed as proto binary into this directory.
        --xla_dump_unoptimized_hlo_proto_to=""  string  Dump HLO before any hlo passes are executed as proto binary into this directory.
        --xla_dump_per_pass_hlo_proto_to=""     string  Dump HLO after each pass as an HloProto in binary file format into this directory.
        --xla_test_all_output_layouts=false     bool    Let ClientLibraryTestBase::ComputeAndCompare* test all permutations of output layouts. For example, with a 3D shape, all permutations of the set {0, 1, 2} are tried.
        --xla_test_all_input_layouts=false      bool    Let ClientLibraryTestBase::ComputeAndCompare* test all permutations of *input* layouts. For example, for 2 input arguments with 2D shape and 4D shape, the computation will run 2! * 4! times for every possible layouts
        --xla_hlo_profile=false                 bool    Instrument the computation to collect per-HLO cycle counts
        --xla_dump_computations_to=""           string  Dump computations that XLA executes into the provided directory path
        --xla_dump_executions_to=""             string  Dump parameters and results of computations that XLA executes into the provided directory path
        --xla_backend_extra_options=""          string  Extra options to pass to a backend; comma-separated list of 'key=val' strings (=val may be omitted); no whitespace around commas.
        --xla_reduce_precision=""               string  Directions for adding reduce-precision operations. Format is 'LOCATION=E,M:OPS;NAMES' where LOCATION is the class of locations in which to insert the operations (e.g., 'OP_OUTPUTS'), E and M are the exponent and matissa bit counts respectively, and OPS and NAMES are comma-separated (no spaces) lists of the operation types and names to which to attach the reduce-precision operations.  The NAMES string and its preceding ';' may be omitted.  This option may be repeated to define multiple sets of added reduce-precision operations.
        --xla_gpu_use_cudnn_batchnorm=false     bool    Allows the GPU backend to implement batchnorm HLOs using cudnn, rather than expanding them to a soup of HLOs.
        --xla_cpu_use_mkl_dnn=false             bool    Generate calls to MKL-DNN in the CPU backend.
```

ARM版モジュールを出力する場合

```
--target_triple="armv7-linux-gnueabihf" 
--target_triple="aarch64-linux-gnu"
```

# 補足
## tfcompileのビルド
tfcompileは2018年5月現在ではソースコード提供となっており、利用するにはコンパイルが必要です。
tfcompileをビルドする際にはgoogleのビルドツールbazelを使って以下のコマンドでビルドします。

```bash
$bazel build --config=opt --config=//tensorflow/compiler/aot:tfcompile
```

## tfcompileの生成場所
tfcompileのビルドに成功すると以下のディレクトリに生成されます。
(bazel-binはシンボリックリンクになっており、実態はホームディレクトリのキャッシュフォルダ(./cache)内に生成されます。)

```bash
$(build_directory)/bazel-bin/tensorflow/compiler/aot/tfcompile
```