[runtimes] Improve the documentation for LIBCXX_ADDITIONAL_COMPILE_FLAGS (#112733)

This clarifies how that option is meant to be used to avoid confusion.
As a drive-by, also fix an incorrect usage in the recently-added GPU
caches.

Author: ldionne

libcxx/CMakeLists.txt

@@ -456,7 +456,9 @@ set(LIBCXX_COMPILE_FLAGS "")
 set(LIBCXX_LINK_FLAGS "")
 set(LIBCXX_LIBRARIES "")
 set(LIBCXX_ADDITIONAL_COMPILE_FLAGS "" CACHE STRING
-    "Additional Compile only flags which can be provided in cache")
+    "Additional compile flags to use when building libc++. This should be a CMake ;-delimited list of individual
+     compiler options to use. For options that must be passed as-is to the compiler without deduplication (e.g.
+     `-Xclang -foo` option groups), consider using `SHELL:` (https://cmake.org/cmake/help/latest/command/add_compile_options.html#option-de-duplication).")
 set(LIBCXX_ADDITIONAL_LIBRARIES "" CACHE STRING
     "Additional libraries libc++ is linked to which can be provided in cache")
 

libcxx/cmake/caches/AMDGPU.cmake

@@ -29,7 +29,7 @@ set(LIBCXXABI_USE_LLVM_UNWINDER OFF CACHE BOOL "")
 
 # Necessary compile flags for AMDGPU.
 set(LIBCXX_ADDITIONAL_COMPILE_FLAGS
-    "-nogpulib;-flto;-fconvergent-functions;-Xclang;-mcode-object-version=none" CACHE STRING "")
+    "-nogpulib;-flto;-fconvergent-functions;SHELL:-Xclang -mcode-object-version=none" CACHE STRING "")
 set(LIBCXXABI_ADDITIONAL_COMPILE_FLAGS
-    "-nogpulib;-flto;-fconvergent-functions;-Xclang;-mcode-object-version=none" CACHE STRING "")
+    "-nogpulib;-flto;-fconvergent-functions;SHELL:-Xclang -mcode-object-version=none" CACHE STRING "")
 set(CMAKE_REQUIRED_FLAGS "-nogpulib" CACHE STRING "")

libcxx/docs/VendorDocumentation.rst

@@ -213,11 +213,13 @@ General purpose options
 
   Output name for the shared libc++ runtime library.
 
-.. option:: LIBCXX_ADDITIONAL_COMPILE_FLAGS:STRING
+.. option:: {LIBCXX,LIBCXXABI,LIBUNWIND}_ADDITIONAL_COMPILE_FLAGS:STRING
 
   **Default**: ``""``
 
-  Additional Compile only flags which can be provided in cache.
+  Additional compile flags to use when building the runtimes. This should be a CMake ``;``-delimited list of individual
+  compiler options to use. For options that must be passed as-is to the compiler without deduplication (e.g.
+  ``-Xclang -foo`` option groups), consider using ``SHELL:`` as `documented here <https://cmake.org/cmake/help/latest/command/add_compile_options.html#option-de-duplication>`_.
 
 .. option:: LIBCXX_ADDITIONAL_LIBRARIES:STRING
 
@@ -346,12 +348,6 @@ The following options allow building libc++ for a different ABI version.
   Build and use the LLVM unwinder. Note: This option can only be used when
   libc++abi is the C++ ABI library used.
 
-.. option:: LIBCXXABI_ADDITIONAL_COMPILE_FLAGS:STRING
-
-  **Default**: ``""``
-
-  Additional Compile only flags which can be provided in cache.
-
 .. option:: LIBCXXABI_ADDITIONAL_LIBRARIES:STRING
 
   **Default**: ``""``

libcxxabi/CMakeLists.txt

@@ -222,8 +222,7 @@ set(LIBCXXABI_CXX_FLAGS "")
 set(LIBCXXABI_COMPILE_FLAGS "")
 set(LIBCXXABI_LINK_FLAGS "")
 set(LIBCXXABI_LIBRARIES "")
-set(LIBCXXABI_ADDITIONAL_COMPILE_FLAGS "" CACHE STRING
-    "Additional Compile only flags which can be provided in cache")
+set(LIBCXXABI_ADDITIONAL_COMPILE_FLAGS "" CACHE STRING "See documentation LIBCXX_ADDITIONAL_COMPILE_FLAGS")
 set(LIBCXXABI_ADDITIONAL_LIBRARIES "" CACHE STRING
     "Additional libraries libc++abi is linked to which can be provided in cache")
 

libunwind/CMakeLists.txt

@@ -162,8 +162,7 @@ set(LIBUNWIND_C_FLAGS "")
 set(LIBUNWIND_CXX_FLAGS "")
 set(LIBUNWIND_COMPILE_FLAGS "")
 set(LIBUNWIND_LINK_FLAGS "")
-set(LIBUNWIND_ADDITIONAL_COMPILE_FLAGS "" CACHE STRING
-    "Additional Compile only flags which can be provided in cache")
+set(LIBUNWIND_ADDITIONAL_COMPILE_FLAGS "" CACHE STRING "See documentation for LIBCXX_ADDITIONAL_COMPILE_FLAGS")
 set(LIBUNWIND_ADDITIONAL_LIBRARIES "" CACHE STRING
     "Additional libraries libunwind is linked to which can be provided in cache")