llvm dev - Dec 2019 - LLVM Runtimes: Multilib support and variable overrides

Hi Petr, I've added you to this question directly as it seems you're the
one that last touched it.

I'm currently having trouble working with llvm/runtimes with regards to
LLVM_RUNTIME_MULTILIBS where I have some pseudo-cmake in my cache that looks
like:

foreach (target in ${RUNTIME_TARGETS})
    set(RUNTIMES_${target}_LIBCXX_ENABLE_EXCEPTIONS OFF CACHE BOOL "")
    set(RUNTIMES_${target}+except_LIBCXX_ENABLE_EXCEPTIONS ON CACHE BOOL
"")
endforeach()
set(LLVM_RUNTIME_MULTILIBS "except" CACHE STRING "")
set(LLVM_RUNTIME_MULTILIB_except_TARGETS "${RUNTIME_TARGETS}" CACHE
STRING "")
set(LLVM_RUNTIME_TARGETS "${RUNTIME_TARGETS}" CACHE STRING
"")

I'm finding that the '+except' case has LIBCXX_ENABLE_EXCEPTIONS
OFF. I've tracked it down to how variables are passed to the subproject:

In llvm/runtimes/CMakeLists.txt, there's a loop:

    get_cmake_property(variableNames VARIABLES)
    foreach(variableName ${variableNames})
      string(FIND "${variableName}" "RUNTIMES_${name}_" out)
      if("${out}" EQUAL 0)
        string(REPLACE "RUNTIMES_${name}_" "" new_name
${variableName})
        list(APPEND ${name}_extra_args
"-D${new_name}=${${variableName}}")
      endif()
      string(FIND "${variableName}" "RUNTIMES_${target}_"
out)
      if("${out}" EQUAL 0)
        string(REPLACE "RUNTIMES_${target}_" "" new_name
${variableName})
        list(APPEND ${name}_extra_args
"-D${new_name}=${${variableName}}")
      endif()
    endforeach()

This loop preferences variable order in the cmake cache. In my case, the
RUNTIMES_${target}_LIBCXX_ENABLE_EXCEPTIONS variables all appear in the cache
after the RUNTIMES_${target}+except_LIBCXX_ENABLE_EXCEPTIONS variables, and the
former takes precedence. Is this expected behavior and I'm doing something
incorrectly, or should this loop preference specific overrides (+except) over
less specific overrides?

Regards,
JB
Code Generation
Texas Instruments
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