commit 20d746a7877f45c1060a54096875fda7cb06c9ab
author Scott Todd <scotttodd@google.com> Thu Jan 12 09:11:59 2023 -0800
committer GitHub <noreply@github.com> Thu Jan 12 09:11:59 2023 -0800
tree 1e425bfacbda099a9bde7bd80393bc22f7e18046
parent 0d33d961508306969ed9ef6ebda63538750b4e12

Replace CMake `iree_get_executable_path()` with `IMPORTED` targets. (#11764)

This changes how to access host tools during CMake's configure step from
using a custom `iree_get_executable_path()` function to using `IMPORTED`
targets
(https://cmake.org/cmake/help/latest/guide/importing-exporting/index.html).
A new helper `iree_import_binary` is added to keep
`IREE_HOST_BINARY_ROOT` use centralized.

## Details

Previously, each CMake file that wanted to use a tool that might be
provided by either the current build or a "host install" would use the
`iree_get_executable_path` function, e.g.
```cmake
# tools/CMakeLists.txt
if(IREE_BUILD_COMPILER)
  iree_cc_binary(NAME iree-compile ...)
endif()

# some other file
iree_get_executable_path(_COMPILE_TOOL_EXECUTABLE "iree-compile")
add_custom_command(
  OUTPUT ${_OUTPUT_FILES}
  COMMAND ${_COMPILE_TOOL_EXECUTABLE} ${_ARGS}
  DEPENDS ${_COMPILE_TOOL_EXECUTABLE} ${_SRC_PATH}
)
```
The `iree_get_executable_path` function attempted to find an executable
with that name and would add an `IMPORTED` target if one did not already
exist.

We can simplify that to just defining `IMPORTED` targets directly, right
where the targets are normally defined when `IREE_BUILD_COMPILER` is
enabled (or `CMAKE_CROSSCOMPILING` is disabled). Importing targets where
they are defined (rather than where they are used) will also let us more
cleanly solve issues like https://github.com/iree-org/iree/issues/11331.

Now this looks more like
```cmake
# tools/CMakeLists.txt
if(IREE_HOST_BINARY_ROOT AND NOT IREE_BUILD_COMPILER)
  iree_import_binary(NAME iree-compile)
elseif(IREE_BUILD_COMPILER)
  iree_cc_binary(NAME iree-compile ...)
else()
  # warn about not building or importing
endif()

# some other file - use the target without caring if it was imported or not
add_custom_command(
  OUTPUT ${_OUTPUT_FILES}
  COMMAND iree-compile ${_ARGS}
  DEPENDS iree-compile ${_SRC_PATH}
)
```

## Other context

This is an alternate take on https://github.com/iree-org/iree/pull/11441
and was discussed a bit here:
https://github.com/iree-org/iree/pull/11402#discussion_r1041471357.

Future PRs may
* Remove the implicit `bin/` suffix from `IREE_HOST_BINARY_ROOT` to
decouple it from the CMake `install` process
* Add `SHARED_LIBRARY_DEPS` to `iree_import_binary`, along with
`iree_import_library`, for proper dependency modeling of
IREECompiler.dll (assuming that `IMPORTED_RUNTIME_ARTIFACTS` doesn't
already do what I want)