Guard `find_library(tensorflow_c_api ...)` by checking for TENSORFLOW_C_LIB_PATH to be set by the user

[mehdi_amini]

Summary

Also have CMake fails if the user provides a TENSORFLOW_C_LIB_PATH but
we can't find TensorFlow at this path.

At the moment the CMake script tries to figure if TensorFlow is
available on the system and enables support for it. This is in general
not desirable to customize build features this way and instead it is
preferable to let the user opt-in explicitly into the features they want
to enable. This is in line with other optional external dependencies
like Z3.
There are a few reasons to this but amongst others:

• reproducibility: making features "magically" enabled based on whether we find a package on the system or not makes it harder to handle bug reports from users.
• user control: they can't have TensorFlow on the system and build LLVM without TensorFlow right now. They also would suddenly distribute LLVM with a different set of features unknowingly just because their build machine environment would change subtly.

Right now this is motivated by a user reporting build failures on their system:

.../mesa-git/llvm-git/src/llvm-project/llvm/lib/Analysis/TFUtils.cpp:23:10: fatal error: tensorflow/c/c_api.h: No such file or directory

23 | #include "tensorflow/c/c_api.h"
   |          ^~~~~~

It looks like we detected TensorFlow at configure time but couldn't set all the paths correctly.

[mtrofin]

There are 2 types of TF dependencies, orthogonal from each other, controlled by TENSORFLOW_AOT_PATH and TENSORFLOW_C_LIB_PATH, respectively.

The former should already provide the desired behavior.

The latter is the problem, due to find_library behavior (line 840 old code): I think it's missing a NO_DEFAULT_PATH, which should then result in the desired behavior.

[mehdi_amini]

In D88371#2298235, @mtrofin wrote:

There are 2 types of TF dependencies, orthogonal from each other, controlled by TENSORFLOW_AOT_PATH and TENSORFLOW_C_LIB_PATH, respectively.

The former should already provide the desired behavior.

Right!

The latter is the problem, due to find_library behavior (line 840 old code): I think it's missing a NO_DEFAULT_PATH, which should then result in the desired behavior.

I don't think we should invoke find_library without an opt-in from the user, this is what this patch is addressing here (see how Z3 is setup for example)

[mehdi_amini]

Remove the new option and rely on TENSORFLOW_C_LIB_PATH to be set instead

[mtrofin]

lgtm, thanks for doing this!

llvm/CMakeLists.txt

configure_file(
   ${LLVM_MAIN_INCLUDE_DIR}/llvm/Config/Disassemblers.def.in
   ${LLVM_INCLUDE_DIR}/llvm/Config/Disassemblers.def
   )
 configure_file(
   ${LLVM_MAIN_INCLUDE_DIR}/llvm/Config/Targets.def.in
   ${LLVM_INCLUDE_DIR}/llvm/Config/Targets.def
   )
 
+# They are not referenced. See set_output_directory().
+set( CMAKE_RUNTIME_OUTPUT_DIRECTORY ${LLVM_BINARY_DIR}/bin )
+set( CMAKE_LIBRARY_OUTPUT_DIRECTORY ${LLVM_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX} )
+set( CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${LLVM_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX} )
+
 # For up-to-date instructions for installing the Tensorflow dependency, refer to
 # the bot setup script: https://github.com/google/ml-compiler-opt/blob/master/buildbot/buildbot_init.sh
 # In this case, the latest C API library is available for download from
 # https://www.tensorflow.org/install/lang_c.
 # We will expose the conditional compilation variable,
 # LLVM_HAVE_TF_API, through llvm-config.h, so that a user of the LLVM library may
 # also leverage the dependency.
 set(TENSORFLOW_C_LIB_PATH "" CACHE PATH "Path to TensorFlow C library install")
-find_library(tensorflow_c_api tensorflow PATHS ${TENSORFLOW_C_LIB_PATH}/lib)
-
-if (tensorflow_c_api)
+if (TENSORFLOW_C_LIB_PATH)
+  find_library(tensorflow_c_api tensorflow PATHS ${TENSORFLOW_C_LIB_PATH}/lib NO_DEFAULT_PATH REQUIRED)
   set(LLVM_HAVE_TF_API "ON" CACHE BOOL "Full Tensorflow API available")
   include_directories(${TENSORFLOW_C_LIB_PATH}/include)
 endif()
 
-# They are not referenced. See set_output_directory().
-set( CMAKE_RUNTIME_OUTPUT_DIRECTORY ${LLVM_BINARY_DIR}/bin )
-set( CMAKE_LIBRARY_OUTPUT_DIRECTORY ${LLVM_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX} )
-set( CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${LLVM_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX} )
-
 # For up-to-date instructions for installing the Tensorflow dependency, refer to
 # the bot setup script: https://github.com/google/ml-compiler-opt/blob/master/buildbot/buildbot_init.sh
 # Specifically, assuming python3 is installed:
 # python3 -m pip install --upgrade pip && python3 -m pip install --user tf_nightly==2.3.0.dev20200528
 # Then set TENSORFLOW_AOT_PATH to the package install - usually it's ~/.local/lib/python3.7/site-packages/tensorflow
 #
 set(TENSORFLOW_AOT_PATH "" CACHE PATH "Path to TensorFlow pip install dir")