[SE] Stop using llvm-config --cxxflags

[jhen]

Summary

Build configuration was adding $(llvm-config --cxxflags) to the
StreamExecutor CXXFLAGS, but this was causing "-O3" to be passed even
for debug builds, and was making debugging difficult.

The llvm-config call was originally introduced to handle the -fno-rtti
flag because an RTTI StreamExecutor could not link with a no-RTTI LLVM.
This patch converts to using the --has-rtti flag to llvm-config and
only adding the -fno-rtti flag if needed, not all the rest of the LLVM
CXXFLAGS.

I have tested this with clang-4.0 and gcc-4.8 on Ubuntu. Some work will
probably have to be done to support MSVC.

[jprice]

I believe LLVMConfig.cmake defines an LLVM_ENABLE_RTTI variable which can give you this information directly, without the need for the execute_process().

[jhen]

• LLVM_ENABLE_RTTI

[jhen]

In D24474#540362, @jprice wrote:

I believe LLVMConfig.cmake defines an LLVM_ENABLE_RTTI variable which can give you this information directly, without the need for the execute_process().

Oh cool! That's much better. Thanks!

streamexecutor/CMakeLists.txt

if(STREAM_EXECUTOR_STANDALONE)
 
     find_package(LLVM REQUIRED CONFIG)
     message(STATUS "Found LLVM ${LLVM_PACKAGE_VERSION}")
     message(STATUS "Using LLVMConfig.cmake in: ${LLVM_DIR}")
 
     include_directories(${LLVM_INCLUDE_DIRS})
     add_definitions(${LLVM_DEFINITIONS})
 
-    # Get the LLVM cxxflags by using llvm-config.
-    #
-    # This is necessary to get -fno-rtti if LLVM is compiled that way.
+    # If LLVM does not have RTTI, don't use it here either.
     execute_process(
         COMMAND
         "${LLVM_BINARY_DIR}/bin/llvm-config"
-        --cxxflags
+        --has-rtti
         OUTPUT_VARIABLE
-        LLVM_CXXFLAGS
+        STREAM_EXECUTOR_LLVM_HAS_RTTI
         OUTPUT_STRIP_TRAILING_WHITESPACE)
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${LLVM_CXXFLAGS}")
+    if (NOT STREAM_EXECUTOR_LLVM_HAS_RTTI)
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-rtti")
+    endif()
 
     set(LLVM_CMAKE_PATH "${LLVM_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm")
     list(APPEND CMAKE_MODULE_PATH "${LLVM_CMAKE_PATH}")
     include(AddLLVM)
 
     if(STREAM_EXECUTOR_UNIT_TESTS)
         enable_testing()
         find_package(GTest REQUIRED)