[libc] Install GPU headers to `gpu-none-llvm/` subfolder

[jhuber6]

Summary

The GPU support for the libc generates all its own headers. Since
these headers use the same names as the system headers we need to make
sure that they are separate. Currently, we either use the system headers
on the GPU or the GPU headers on the system. This patch makes them
explicitly separate. A follow-up patch will then make clang look in
this folder by default.

[sivachandra]

I prefer a name which is more in line with the triple formats listed here: https://clang.llvm.org/docs/CrossCompilation.html#target-triple

1. gpu-none-llvm
2. gpu-amd-none-llvm or gpu-nvidia-none-llvm

[jhuber6]

In D146970#4225116, @sivachandra wrote:

I prefer a name which is more in line with the triple formats listed here: https://clang.llvm.org/docs/CrossCompilation.html#target-triple

1. gpu-none-llvm
2. gpu-amd-none-llvm or gpu-nvidia-none-llvm

These should be common between the gpu architecture, any target specific stuff should be hidden behind the __NVPTX__ and __AMDGPU__ macros. I'm in favor of gpu-llvm or llvm-gpu however. Since it makes it clear that these are GPU specific headers, separate from the system headers, and they were provided by and used in the LLVM toolchain.

[sivachandra]

In D146970#4225119, @jhuber6 wrote:

These should be common between the gpu architecture, any target specific stuff should be hidden behind the __NVPTX__ and __AMDGPU__ macros. I'm in favor of gpu-llvm or llvm-gpu however. Since it makes it clear that these are GPU specific headers, separate from the system headers, and they were provided by and used in the LLVM toolchain.

I think gpu-llvm is almost ideal: It is for the GPU target and contains LLVM related/specific bits. Since target triples normally contains three components, I am tempted to say this should be gpu-none-llvm. The none kind of also indicates it is vendor agnostic.

[jhuber6]

In D146970#4225136, @sivachandra wrote:

I think gpu-llvm is almost ideal: It is for the GPU target and contains LLVM related/specific bits. Since target triples normally contains three components, I am tempted to say this should be gpu-none-llvm. The none kind of also indicates it is vendor agnostic.

Works for me.

[sivachandra]

In D146970#4225153, @jhuber6 wrote:

In D146970#4225136, @sivachandra wrote:

I think gpu-llvm is almost ideal: It is for the GPU target and contains LLVM related/specific bits. Since target triples normally contains three components, I am tempted to say this should be gpu-none-llvm. The none kind of also indicates it is vendor agnostic.

Works for me.

OK with this change.

[jhuber6]

Changing to gpu-none-llvm.

[sivachandra]

Update the commit message.

libc/include/CMakeLists.txt

 foreach(target IN LISTS all_install_header_targets)
   get_target_property(header_file ${target} HEADER_FILE_PATH)
   if(NOT header_file)
     message(FATAL_ERROR "Installable header file '${target}' does not have the "
                         "HEADER_FILE_PATH property set.")
   endif()
   file(RELATIVE_PATH relative_path ${LIBC_INCLUDE_BINARY_DIR} ${header_file})
   get_filename_component(nested_dir ${relative_path} DIRECTORY)
+  # The GPU headers are installed to 'gpu-none-llvm/' to prevent shadowing the
+  # system headers.
+  if(LIBC_TARGET_ARCHITECTURE_IS_GPU)
+    set(nested_dir gpu-none-llvm/${nested_dir})
+  endif()
   install(FILES ${header_file}
           DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/${nested_dir}
           COMPONENT libc-headers)
 endforeach()