Cross compile tensorflow for armv7l targets via bazel

Refer:
https://github.com/bazelbuild/bazel/wiki/Building-with-a-custom-toolchain
https://www.tensorflow.org/tutorials/image_recognition



First  you should  have a cross tool chain like:
ls /home/lenger/toolchains/cross_toolchain_target_armv7l_host_x86-64
armv7l-linux-gnueabi  bin  include  lib  libexec  share


Second, please followed the instructions to download the source installation of TensorFlow for your platform.

1. Modify WORKSPACE for tensorflow:


~/tensorflow$ git diff WORKSPACE  
diff --git a/WORKSPACE b/WORKSPACE
index e4f4c80..a0e2c06 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -516,3 +516,10 @@ http_file(
   name = "weblas_weblas_js",
   url = "https://raw.githubusercontent.com/waylonflinn/weblas/v0.9.0/dist/weblas.js",
 )
+
+
+new_local_repository(
+    name = 'toolchain_target_armv7l',
+    path = '/home/lenger/toolchains/cross_toolchain_target_armv7l_host_x86-64',
+    build_file = 'armv7l-compiler/cross_toolchain_target_armv7l_host_x86-64.BUILD'
+)

2. Prepare cross tool configurations:
 
~/tensorflow$ ls armv7l-compiler
BUILD  CROSSTOOL  cross_toolchain_armv7l_host_x86-64.BUILD

~/tensorflow/armv7l-compiler$ cat cross_toolchain_armv7l_host_x86-64.BUILD    
package(default_visibility = ['//visibility:public'])

filegroup(
  name = 'gcc',
  srcs = [
    'bin/armv7l-tizen-linux-gnueabi-gcc',
  ],
)

filegroup(
  name = 'ar',
  srcs = [
    'bin/armv7l-tizen-linux-gnueabi-ar',
  ],
)

filegroup(
  name = 'ld',
  srcs = [
    'bin/armv7l-tizen-linux-gnueabi-ld',
  ],
)

filegroup(
  name = 'nm',
  srcs = [
    'bin/armv7l-tizen-linux-gnueabi-nm',
  ],
)

filegroup(
  name = 'objcopy',
  srcs = [
    'bin/armv7l-tizen-linux-gnueabi-objcopy',
  ],
)

filegroup(
  name = 'objdump',
  srcs = [
    'bin/armv7l-tizen-linux-gnueabi-objdump',
  ],
)

filegroup(
  name = 'strip',
  srcs = [
    'bin/armv7l-tizen-linux-gnueabi-strip',
  ],
)

filegroup(
  name = 'as',
  srcs = [
    'bin/armv7l-tizen-linux-gnueabi-as',
  ],
)

filegroup(
  name = 'compiler_pieces',
  srcs = glob([
    'armv7l-tizen-linux-gnueabi/**',
    'libexec/**',
    'lib/gcc/armv7l-tizen-linux-gnueabi/**',
    'include/**',
  ]),
)

filegroup(
  name = 'compiler_components',
  srcs = [
    ':gcc',
    ':ar',
    ':ld',
    ':nm',
    ':objcopy',
    ':objdump',
    ':strip',
    ':as',
  ],
) 

~/tensorflow/armv7l-compiler$ cat CROSSTOOL 
major_version: "local"
minor_version: ""
default_target_cpu: "armv7l"

default_toolchain {
  cpu: "armv7l"
  toolchain_identifier: "armv7l"
}

default_toolchain {
  cpu: "k8"
  toolchain_identifier: "local"
}

toolchain {
  abi_version: "armv7l"
  abi_libc_version: "armv7l"
  builtin_sysroot: ""
  compiler: "compiler"
  host_system_name: "armv7l"
  needsPic: true
  supports_gold_linker: false
  supports_incremental_linker: false
  supports_fission: false
  supports_interface_shared_objects: false
  supports_normalizing_ar: true
  supports_start_end_lib: false
  supports_thin_archives: true
  target_libc: "armv7l"
  target_cpu: "armv7l"
  target_system_name: "armv7l"
  toolchain_identifier: "armv7l"

  tool_path { name: "ar" path: "/home/lenger/toolchains/cross_toolchain_target_armv7l_host_x86-64/bin/armv7l-tizen-linux-gnueabi-ar" }
  tool_path { name: "compat-ld" path: "/home/lenger/toolchains/cross_toolchain_target_armv7l_host_x86-64/bin/armv7l-tizen-linux-gnueabi-ld" }
  tool_path { name: "cpp" path: "/home/lenger/toolchains/cross_toolchain_target_armv7l_host_x86-64/bin/armv7l-tizen-linux-gnueabi-cpp" }
  tool_path { name: "dwp" path: "/home/lenger/toolchains/cross_toolchain_target_armv7l_host_x86-64/bin/armv7l-tizen-linux-gnueabi-dwp" }
  tool_path { name: "gcc" path: "/home/lenger/toolchains/cross_toolchain_target_armv7l_host_x86-64/bin/armv7l-tizen-linux-gnueabi-gcc" }
  tool_path { name: "gcov" path: "/home/lenger/toolchains/cross_toolchain_target_armv7l_host_x86-64/bin/armv7l-tizen-linux-gnueabi-gcov" }
  tool_path { name: "ld" path: "/home/lenger/toolchains/cross_toolchain_target_armv7l_host_x86-64/bin/armv7l-tizen-linux-gnueabi-ld" }
  tool_path { name: "nm" path: "/home/lenger/toolchains/cross_toolchain_target_armv7l_host_x86-64/bin/armv7l-tizen-linux-gnueabi-nm" }
  tool_path { name: "objcopy" path: "/home/lenger/toolchains/cross_toolchain_target_armv7l_host_x86-64/bin/armv7l-tizen-linux-gnueabi-objcopy" }
  objcopy_embed_flag: "-I"
  objcopy_embed_flag: "binary"
  tool_path { name: "objdump" path: "/home/lenger/toolchains/cross_toolchain_target_armv7l_host_x86-64/bin/armv7l-tizen-linux-gnueabi-objdump" }
  tool_path { name: "strip" path: "/home/lenger/toolchains/cross_toolchain_target_armv7l_host_x86-64/bin/armv7l-tizen-linux-gnueabi-strip" }

  cxx_builtin_include_directory: "/home/lenger/toolchains/cross_toolchain_target_armv7l_host_x86-64/include"
  cxx_builtin_include_directory: "/home/lenger/toolchains/cross_toolchain_target_armv7l_host_x86-64/lib/gcc/armv7l-tizen-linux-gnueabi/6.2.1/include"
  cxx_builtin_include_directory: "/home/lenger/toolchains/cross_toolchain_target_armv7l_host_x86-64/armv7l-tizen-linux-gnueabi/sys-root/usr/include"
  cxx_builtin_include_directory: "/home/lenger/toolchains/cross_toolchain_target_armv7l_host_x86-64/armv7l-tizen-linux-gnueabi/include"
  cxx_builtin_include_directory: "/home/lenger/toolchains/cross_toolchain_target_armv7l_host_x86-64/lib/gcc/armv7l-tizen-linux-gnueabi/6.2.1/include-fixed"

  linker_flag: "--sysroot=/home/lenger/toolchains/cross_toolchain_target_armv7l_host_x86-64/armv7l-tizen-linux-gnueabi/sys-root"
  linker_flag: "-lstdc++"
  linker_flag: "-L/home/lenger/toolchains/cross_toolchain_target_armv7l_host_x86-64/lib"
  linker_flag: "-L/home/lenger/toolchains/cross_toolchain_target_armv7l_host_x86-64/armv7l-tizen-linux-gnueabi/sys-root/lib"
  linker_flag: "-L/home/lenger/toolchains/cross_toolchain_target_armv7l_host_x86-64/armv7l-tizen-linux-gnueabi/sys-root/usr/lib"
  linker_flag: "-L/home/lenger/toolchains/cross_toolchain_target_armv7l_host_x86-64/armv7l-tizen-linux-gnueabi/lib"

  # Anticipated future default.
  # This makes GCC and Clang do what we want when called through symlinks.
  unfiltered_cxx_flag: "-no-canonical-prefixes"
  linker_flag: "-no-canonical-prefixes"

  # Make C++ compilation deterministic. Use linkstamping instead of these
  # compiler symbols.
  unfiltered_cxx_flag: "-Wno-builtin-macro-redefined"
  unfiltered_cxx_flag: "-D__DATE__=\"redacted\""
  unfiltered_cxx_flag: "-D__TIMESTAMP__=\"redacted\""
  unfiltered_cxx_flag: "-D__TIME__=\"redacted\""

  # Security hardening on by default.
  # Conservative choice; -D_FORTIFY_SOURCE=2 may be unsafe in some cases.
  # We need to undef it before redefining it as some distributions now have
  # it enabled by default.
  compiler_flag: "-U_FORTIFY_SOURCE"
  compiler_flag: "-fstack-protector"
  compiler_flag: "-fPIE"
  linker_flag: "-pie"
  linker_flag: "-Wl,-z,relro,-z,now"

  # Enable coloring even if there's no attached terminal. Bazel removes the
  # escape sequences if --nocolor is specified.
  compiler_flag: "-fdiagnostics-color=always"

    # All warnings are enabled. Maybe enable -Werror as well?
  compiler_flag: "-Wall"
  # Enable a few more warnings that aren't part of -Wall.
  compiler_flag: "-Wunused-but-set-parameter"
  # But disable some that are problematic.
  compiler_flag: "-Wno-free-nonheap-object" # has false positives

  # Keep stack frames for debugging, even in opt mode.
  compiler_flag: "-fno-omit-frame-pointer"

  # Stamp the binary with a unique identifier.
  linker_flag: "-Wl,--build-id=md5"
  linker_flag: "-Wl,--hash-style=gnu"

  compilation_mode_flags {
    mode: DBG
    # Enable debug symbols.
    compiler_flag: "-g"
  }
  compilation_mode_flags {
    mode: OPT

    # No debug symbols.
    # Maybe we should enable https://gcc.gnu.org/wiki/DebugFission for opt or
    # even generally? However, that can't happen here, as it requires special
    # handling in Bazel.
    compiler_flag: "-g0"

    # Conservative choice for -O
    # -O3 can increase binary size and even slow down the resulting binaries.
    # Profile first and / or use FDO if you need better performance than this.
    compiler_flag: "-O2"

    # Disable assertions
    compiler_flag: "-DNDEBUG"

    # Removal of unused code and data at link time (can this increase binary size in some cases?).
    compiler_flag: "-ffunction-sections"
    compiler_flag: "-fdata-sections"
    linker_flag: "-Wl,--gc-sections"
  }
}

toolchain {
  toolchain_identifier: "local"
  abi_libc_version: "local"
  abi_version: "local"
  builtin_sysroot: ""
  compiler: "compiler"
  compiler_flag: "-U_FORTIFY_SOURCE"
  compiler_flag: "-D_FORTIFY_SOURCE=2"
  compiler_flag: "-fstack-protector"
  compiler_flag: "-Wall"
  compiler_flag: "-Wl,-z,-relro,-z,now"
  compiler_flag: "-B/usr/bin"
  compiler_flag: "-B/usr/bin"
  compiler_flag: "-Wunused-but-set-parameter"
  compiler_flag: "-Wno-free-nonheap-object"
  compiler_flag: "-fno-omit-frame-pointer"
  cxx_builtin_include_directory: "/usr/include/c++/4.8"
  cxx_builtin_include_directory: "/usr/include/x86_64-linux-gnu/c++/4.8"
  cxx_builtin_include_directory: "/usr/include/c++/4.8/backward"
  cxx_builtin_include_directory: "/usr/lib/gcc/x86_64-linux-gnu/4.8/include"
  cxx_builtin_include_directory: "/usr/local/include"
  cxx_builtin_include_directory: "/usr/lib/gcc/x86_64-linux-gnu/4.8/include-fixed"
  cxx_builtin_include_directory: "/usr/include/x86_64-linux-gnu"
  cxx_builtin_include_directory: "/usr/include"
  cxx_flag: "-std=c++0x"
  host_system_name: "local"
  linker_flag: "-lstdc++"
  linker_flag: "-lm"
  linker_flag: "-Wl,-no-as-needed"
  linker_flag: "-B/usr/bin"
  linker_flag: "-B/usr/bin"
  linker_flag: "-pass-exit-codes"
  needsPic: true
  objcopy_embed_flag: "-I"
  objcopy_embed_flag: "binary"
  supports_fission: false
  supports_gold_linker: false
  supports_incremental_linker: false
  supports_interface_shared_objects: false
  supports_normalizing_ar: false
  supports_start_end_lib: false
  supports_thin_archives: false
  target_cpu: "k8"
  target_libc: "local"
  target_system_name: "local"
  unfiltered_cxx_flag: "-fno-canonical-system-headers"
  unfiltered_cxx_flag: "-Wno-builtin-macro-redefined"
  unfiltered_cxx_flag: "-D__DATE__=\"redacted\""
  unfiltered_cxx_flag: "-D__TIMESTAMP__=\"redacted\""
  unfiltered_cxx_flag: "-D__TIME__=\"redacted\""
  tool_path {name: "ar" path: "/usr/bin/ar" }
  tool_path {name: "cpp" path: "/usr/bin/cpp" }
  tool_path {name: "dwp" path: "/usr/bin/dwp" }
  tool_path {name: "gcc" path: "/usr/bin/gcc" }
  tool_path {name: "gcov" path: "/usr/bin/gcov" }
  tool_path {name: "ld" path: "/usr/bin/ld" }
  tool_path {name: "nm" path: "/usr/bin/nm" }
  tool_path {name: "objcopy" path: "/usr/bin/objcopy" }
  tool_path {name: "objdump" path: "/usr/bin/objdump" }
  tool_path {name: "strip" path: "/usr/bin/strip" }

  compilation_mode_flags {
    mode: DBG
    compiler_flag: "-g"
  }
  compilation_mode_flags {
    mode: OPT
    compiler_flag: "-g0"
    compiler_flag: "-O2"
    compiler_flag: "-DNDEBUG"
    compiler_flag: "-ffunction-sections"
    compiler_flag: "-fdata-sections"
    linker_flag: "-Wl,--gc-sections"
  }
  linking_mode_flags { mode: DYNAMIC }
}

~/tensorflow/armv7l-compiler$ cat BUILD
package(default_visibility = ["//visibility:public"])

cc_toolchain_suite(
    name = "toolchain",
    toolchains = {
        "armv7l|compiler": ":cc-compiler-armv7l",
        "k8|compiler": ":cc-compiler-local",
    },
)

filegroup(
    name = "empty",
    srcs = [],
)

filegroup(
    name = "arm_linux_all_files",
    srcs = [
        "@toolchain_target_armv7l//:compiler_pieces",
    ],
)

cc_toolchain(
    name = "cc-compiler-local",
    all_files = ":empty",
    compiler_files = ":empty",
    cpu = "local",
    dwp_files = ":empty",
    dynamic_runtime_libs = [":empty"],
    linker_files = ":empty",
    objcopy_files = ":empty",
    static_runtime_libs = [":empty"],
    strip_files = ":empty",
    supports_param_files = 1,
)

cc_toolchain(
    name = "cc-compiler-armv7l",
    all_files = ":arm_linux_all_files",
    compiler_files = ":arm_linux_all_files",
    cpu = "armv7l",
    dwp_files = ":empty",
    dynamic_runtime_libs = [":empty"],
    linker_files = ":arm_linux_all_files",
    objcopy_files = "arm_linux_all_files",
    static_runtime_libs = [":empty"],
    strip_files = "arm_linux_all_files",
    supports_param_files = 1,
)

3. Configure tensorflow:

~/tensorflow$ ./configure
Please specify the location of python. [Default is /usr/bin/python]:
Please specify optimization flags to use during compilation when bazel option "--config=opt" is specified [Default is -march=native]: -march=armv7-a
Do you wish to use jemalloc as the malloc implementation? [Y/n]
jemalloc enabled
Do you wish to build TensorFlow with Google Cloud Platform support? [y/N]
No Google Cloud Platform support will be enabled for TensorFlow
Do you wish to build TensorFlow with Hadoop File System support? [y/N]
No Hadoop File System support will be enabled for TensorFlow
Do you wish to build TensorFlow with the XLA just-in-time compiler (experimental)? [y/N]
No XLA support will be enabled for TensorFlow
Found possible Python library paths:
  /usr/local/lib/python2.7/dist-packages
  /usr/lib/python2.7/dist-packages
Please input the desired Python library path to use.  Default is [/usr/local/lib/python2.7/dist-packages]

Using python library path: /usr/local/lib/python2.7/dist-packages
Do you wish to build TensorFlow with OpenCL support? [y/N]
No OpenCL support will be enabled for TensorFlow
Do you wish to build TensorFlow with CUDA support? [y/N]
No CUDA support will be enabled for TensorFlow
Configuration finished
......................
INFO: All external dependencies fetched successfully.


armv7l-tizen-linux-gnueabi-gcc: error: unrecognized argument in option '-march=native'
armv7l-tizen-linux-gnueabi-gcc: note: valid arguments to '-march=' are: armv2 armv2a armv3 armv3m armv4 armv4t armv5 armv5e armv5t armv5te armv6 armv6-m armv6j armv6k armv6kz armv6s-m armv6t2 armv6z armv6zk armv7 armv7-a armv7-m armv7-r armv7e-m armv7ve armv8-a armv8-a+crc armv8-m.base armv8-m.main armv8-m.main+dsp armv8.1-a armv8.1-a+crc armv8.2-a armv8.2-a+fp16 iwmmxt iwmmxt2 native

Please remember specify -march=armv7-a here.

It rely on your target's march. 

4. Let's do cross build:
bazel build --crosstool_top=//armv7l-compiler:toolchain --cpu=armv7l --config=opt tensorflow/examples/label_image/...
INFO: Found 2 targets...
INFO: Elapsed time: 231.657s, Critical Path: 77.85s

specify crosstool via --crosstool_top
--crosstool_top=//armv7l-compiler:toolchain 

specify cpu
--cpu=armv7l  

build target here is label_image
tensorflow/examples/label_image/... 

You may need to change some codes if you encountered compile errors related: 
diff --git a/tensorflow/core/platform/platform.h b/tensorflow/core/platform/platform.h
index 7c44b22..cb720eb 100644
--- a/tensorflow/core/platform/platform.h
+++ b/tensorflow/core/platform/platform.h
@@ -45,7 +45,7 @@ limitations under the License.

 // Since there's no macro for the Raspberry Pi, assume we're on a mobile
 // platform if we're compiling for the ARM CPU.
-#define IS_MOBILE_PLATFORM
+// #define IS_MOBILE_PLATFORM

diff --git a/tensorflow/stream_executor/platform/default/mutex.h b/tensorflow/stream_executor/platform/default/mutex.h
index f28a2c9..d72fe42 100644
--- a/tensorflow/stream_executor/platform/default/mutex.h
+++ b/tensorflow/stream_executor/platform/default/mutex.h
@@ -23,7 +23,7 @@ limitations under the License.

 // std::shared_timed_mutex is a C++14 feature.
 #if (__cplusplus >= 201402L)
-#define STREAM_EXECUTOR_USE_SHARED_MUTEX
+//#define STREAM_EXECUTOR_USE_SHARED_MUTEX
 #endif  // __cplusplus >= 201402L

 #ifdef STREAM_EXECUTOR_USE_SHARED_MUTEX

Once build done, your arm target will be here:


~/tensorflow$ ls bazel-bin/tensorflow/examples/label_image/label_image
bazel-bin/tensorflow/examples/label_image/label_image


5.  Try label_image on your real target:


sh-3.2# ./label_image  --image=/lion.jpg
2017-09-18 18:26:43.869688: I tensorflow/examples/label_image/main.cc:206] lion (292): 0.976528
2017-09-18 18:26:43.869866: I tensorflow/examples/label_image/main.cc:206] stopwatch (827): 0.00053628
2017-09-18 18:26:43.869933: I tensorflow/examples/label_image/main.cc:206] leopard (289): 0.000467814
2017-09-18 18:26:43.869995: I tensorflow/examples/label_image/main.cc:206] cougar (287): 0.000456517
2017-09-18 18:26:43.870055: I tensorflow/examples/label_image/main.cc:206] tiger (293): 0.000347707

Well done,  it recognize a lion with 97% accuracy.