Don't add unnecessary compiler flags to llvm-config output

[tstellar]

Summary

llvm-config --cxxflags --cflags, should only output the minimal flags
required to link against the llvm libraries. They currently contain
all flags used to compile llvm including flags like -g, -pedantic,
-Wall, etc, which users may not always want.

This changes the llvm-config output to only include flags that have been
explictly added to the COMPILE_FLAGS property of the llvm-config target
by the llvm build system.

llvm.org/PR8220

Output from llvm-config when running cmake with:
cmake -G Ninja .. -DCMAKE_CXX_FLAGS=-funroll-loops

Before:

--cppflags: -I$HEADERS_DIR/llvm/include -I$HEADERS_DIR/llvm/build/include

-D_GNU_SOURCE -D_DEBUG -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS

--cflags: -I$HEADERS_DIR/llvm/include -I$HEADERS_DIR/llvm/build/include

-fPIC -Werror=date-time -Wall -Wextra -Wno-unused-parameter -Wwrite-strings \
-Wno-missing-field-initializers -pedantic -Wno-long-long -Wimplicit-fallthrough \
-Wno-comment -fdiagnostics-color -g -D_GNU_SOURCE -D_DEBUG -D__STDC_CONSTANT_MACROS \
-D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS

--cxxflags: -I$HEADERS_DIR/llvm/include -I$HEADERS_DIR/llvm/build/include\

-funroll-loops -fPIC -fvisibility-inlines-hidden -Werror=date-time -std=c++11 -Wall \
-Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wno-missing-field-initializers \
-pedantic -Wno-long-long -Wimplicit-fallthrough -Wno-maybe-uninitialized \
-Wno-class-memaccess -Wno-noexcept-type -Wdelete-non-virtual-dtor -Wno-comment \
-fdiagnostics-color -g  -fno-exceptions -fno-rtti -D_GNU_SOURCE -D_DEBUG \
-D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS"

After:

--cppflags: -I$HEADERS_DIR/llvm/include -I$HEADERS_DIR/llvm/build/include \

-D_GNU_SOURCE -D_DEBUG -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS

--cflags: -I$HEADERS_DIR/llvm/include -I$HEADERS_DIR/llvm/build/include \

-D_GNU_SOURCE -D_DEBUG -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS

--cxxflags: -I$HEADERS_DIR/llvm/include -I$HEADERS_DIR/llvm/build/include \

-std=c++11   -fno-exceptions -fno-rtti \
-D_GNU_SOURCE -D_DEBUG -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS

[sylvestre.ledru]

Much better. thanks!

[mgorny]

Thanks, this is something we've been hacking out in Gentoo for a long time. One nit though.

tools/llvm-config/CMakeLists.txt

37 ↗

(On Diff #177067)

Shouldn't cxxflags also include -std?

[tstellar]

tools/llvm-config/CMakeLists.txt

37 ↗

(On Diff #177067)

Yes, I think we need to keep -std=c++11.

[tstellar]

Make sure llvm-config reports the c/c++ language standard used.

[mgorny]

Thanks. Works fine for me.

[nik]

llvm/trunk/tools/llvm-config/CMakeLists.txt

35

I guess -D_GLIBCXX_USE_CXX11_ABI=0 would be such a flag that shouldn't be handled here?

Is there a way to have this shown up in "llvm-config --cxxflags" as in previous versions by modifying the cmake invocation? I've tried several things to get it "into" LLVM_DEFINITIONS or COMPILE_FLAGS, but my cmake foo is too limited as of now :/

[tstellar]

llvm/trunk/tools/llvm-config/CMakeLists.txt

35

Does the LLVM build system add this flag somewhere, or does the user need to add it manually when configuring?

llvm/trunk/tools/llvm-config/CMakeLists.txt

else()
     endif()
   endif()
 endforeach()
 string(REPLACE ";" " " SYSTEM_LIBS "${SYSTEM_LIBS}")
 
 # Fetch target specific compile options, e.g. RTTI option
 get_property(COMPILE_FLAGS TARGET llvm-config PROPERTY COMPILE_FLAGS)
 
+# The language standard potentially affects the ABI/API of LLVM, so we want
+# to make sure it is reported by llvm-config.
+# NOTE: We don't want to start extracting any random C/CXX flags that the
+# user may add that could affect the ABI.  We only want to extract flags
+# that have been added by the LLVM build system.
+string(REGEX MATCH "-std=[^ ]\+" LLVM_CXX_STD_FLAG ${CMAKE_CXX_FLAGS})
+string(REGEX MATCH "-std=[^ ]\+" LLVM_C_STD_FLAG ${CMAKE_C_FLAGS})
+
 # Use configure_file to create BuildVariables.inc.
 set(LLVM_SRC_ROOT ${LLVM_MAIN_SRC_DIR})
 set(LLVM_OBJ_ROOT ${LLVM_BINARY_DIR})
-set(LLVM_CPPFLAGS "${CMAKE_CPP_FLAGS} ${CMAKE_CPP_FLAGS_${uppercase_CMAKE_BUILD_TYPE}} ${LLVM_DEFINITIONS}")
-set(LLVM_CFLAGS "${CMAKE_C_FLAGS} ${CMAKE_C_FLAGS_${uppercase_CMAKE_BUILD_TYPE}} ${LLVM_DEFINITIONS}")
-set(LLVM_CXXFLAGS "${CMAKE_CXX_FLAGS} ${CMAKE_CXX_FLAGS_${uppercase_CMAKE_BUILD_TYPE}} ${COMPILE_FLAGS} ${LLVM_DEFINITIONS}")
+set(LLVM_CPPFLAGS "${LLVM_DEFINITIONS}")
+set(LLVM_CFLAGS "${LLVM_C_STD_FLAG} ${LLVM_DEFINITIONS}")
+set(LLVM_CXXFLAGS "${LLVM_CXX_STD_FLAG} ${COMPILE_FLAGS} ${LLVM_DEFINITIONS}")
 set(LLVM_BUILD_SYSTEM cmake)
 set(LLVM_HAS_RTTI ${LLVM_CONFIG_HAS_RTTI})
 set(LLVM_DYLIB_VERSION "${LLVM_VERSION_MAJOR}${LLVM_VERSION_SUFFIX}")
 set(LLVM_HAS_GLOBAL_ISEL "ON")
 
 # Use the C++ link flags, since they should be a superset of C link flags.
 set(LLVM_LDFLAGS "${CMAKE_CXX_LINK_FLAGS}")
 set(LLVM_BUILDMODE ${CMAKE_BUILD_TYPE})