[PATCH] D40947: [cmake] Make setting of CMAKE_C(XX)_COMPILER flags overridable for cross-builds

[Pavel Labath via Phabricator via llvm-commits]

labath added a comment.

In https://reviews.llvm.org/D40947#951479, @hintonda wrote:

> Did this fix an existing problem?


Yes. It allowed us to cross-compile with a command line like:
`-DCMAKE_TOOLCHAIN_FILE=/android/toolchain/file.cmake -DCROSS_TOOLCHAIN_FLAGS_NATIVE=-DCMAKE_TOOLCHAIN_FILE=/host/toolchain/file.cmake`

Without this we would have had to append additional -DCMAKE_C??_COMPILER to the command line (duplicating the logic already present in the toolchain file). The other thing that seemed to work is to use `set(CMAKE_CXX_COMPILER ... FORCE)` in the toolchain file, but that also seemed like an ugly workaround.

However, I agree with the rest of your analysis.

> It looks like some of this logic is OBE.
> 
> Here's my reasoning.
> 
> As far as I can tell, this code is only run when either `-DLLVM_OPTIMIZED_TABLEGEN=ON` or `CMAKE_SYSTEM_NAME` variable is passed or set in the appropriate cache or toolchain file -- which cmake ultimately uses to set `CMAKE_CROSSCOMPILING=ON` internally.
> 
> In both cases, these variables are used to set `LLVM_USE_HOST_TOOLS`, which, if true, includes CrossCompile.cmake in llvm/CMakeLists.txt.  CrossCompile.cmake defines two functions, `llvm_create_cross_target` and `llvm_create_cross_target_internal`, and ultimately calls `llvm_create_cross_target_internal(NATIVE "" Release)` -- I can't find any other places where either of these two functions are called.
> 
> If this is in fact the only call to `llvm_create_cross_target_internal`, in this case with toolchain = "", the toolchain file will be `${LLVM_MAIN_SRC_DIR}/cmake/platforms/${toolchain}.cmake` which doesn't match any existing file, and is therefore never passed via `CROSS_TOOLCHAIN_FLAGS_INIT` to the custom cmake command.
> 
> Are there any calls to `llvm_create_cross_target` or `llvm_create_cross_target_internal` that I've missed where a valid toolchain is actually passed?  If not, is there a reason to maintain the toolchain logic in `llvm_create_cross_target`?

The analysis seems correct, although I can't claim I am sufficiently familiar with all llvm sub-projects to be certain one of them isn't using this. I preserved the `${LLVM_MAIN_SRC_DIR}/cmake/platforms/${toolchain}.cmake` logic because I wanted to be conservative. What I am certain about is that we aren't using that logic, and wouldn't be sorry to see it go (if that's what you're proposing -- it's not really clear to me where you're heading with this).


Repository:
  rL LLVM

https://reviews.llvm.org/D40947