BUG: build_native_tool does not pass on CMAKE_CXX_STANDARD

[h-vetinari]

Trying to build mlir 16.0.0-rc1 in conda-forge, I'm running into the issue that MLIR now requires C++17 to build, but specifying CMAKE_CXX_STANDARD to the build does not get respected in cross-compilation.

From what I can tell, this happens when invoking build_native_tool, which then fails on any compiler that doesn't yet default to C++17.

[49/1416] Building native mlir-tblgen...
[1/47] Building CXX object tools/mlir-tblgen/CMakeFiles/mlir-tblgen.dir/DirectiveCommonGen.cpp.o
FAILED: tools/mlir-tblgen/CMakeFiles/mlir-tblgen.dir/DirectiveCommonGen.cpp.o 
$BUILD_PREFIX/bin/x86_64-apple-darwin13.4.0-clang++ -DMLIR_CUDA_CONVERSIONS_ENABLED=1 -DMLIR_ROCM_CONVERSIONS_ENABLED=1 -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I$SRC_DIR/build/NATIVE/tools/mlir-tblgen -I$SRC_DIR/mlir/tools/mlir-tblgen -I$SRC_DIR/mlir/include -I$SRC_DIR/build/NATIVE/include -isystem $BUILD_PREFIX/include -O2 -fPIC -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -Werror=mismatched-tags -O3 -DNDEBUG -isysroot /Applications/Xcode_13.2.1.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.0.sdk -mmacosx-version-min=11.0  -fno-exceptions -MD -MT tools/mlir-tblgen/CMakeFiles/mlir-tblgen.dir/DirectiveCommonGen.cpp.o -MF tools/mlir-tblgen/CMakeFiles/mlir-tblgen.dir/DirectiveCommonGen.cpp.o.d -o tools/mlir-tblgen/CMakeFiles/mlir-tblgen.dir/DirectiveCommonGen.cpp.o -c $SRC_DIR/mlir/tools/mlir-tblgen/DirectiveCommonGen.cpp
In file included from $SRC_DIR/mlir/tools/mlir-tblgen/DirectiveCommonGen.cpp:14:
In file included from $SRC_DIR/mlir/include/mlir/TableGen/GenInfo.h:12:
In file included from $SRC_DIR/mlir/include/mlir/Support/LLVM.h:24:
$BUILD_PREFIX/include/llvm/Support/Casting.h:266:32: error: no member named 'optional' in namespace 'std'

[llvmbot]

@llvm/issue-subscribers-mlir

[joker-eph]

I don't quite get how this happens, the CMakeLists.txt will by default do set(CMAKE_CXX_STANDARD 17 ), why isn't this enough to get the right -std=c++17 used here?

[h-vetinari]

why isn't this enough to get the right -std=c++17 used here?

TBH, I don't know myself, but I have the suspicion that it was masked by the fact that on new enough clang (and GCC) the default is already C++17. For various unhappy reasons we're still on clang 14 on osx.

[h-vetinari]

I don't quite get how this happens, the CMakeLists.txt will by default do set(CMAKE_CXX_STANDARD 17 )

Ah, from what I can tell, this isn't actually happening in mlir/ itself (except in an example), which is the only folder that we're building (llvm is built separately before).

[joker-eph]

You're invoking CMake pointing directly at MLIR?
I forgot we can do that, but now that you say I kind of remember that you can build against an already built LLVM right?
We should just add the directive in the MLIR CMake then, can you try and let me know if it is enough?

[h-vetinari]

You're invoking CMake pointing directly at MLIR?

Yes.

We should just add the directive in the MLIR CMake then, can you try and let me know if it is enough?

Can confirm it works with the following patch:

diff --git a/mlir/CMakeLists.txt b/mlir/CMakeLists.txt
index f00c819b25..4594b8830e 100644
--- a/mlir/CMakeLists.txt
+++ b/mlir/CMakeLists.txt
@@ -22,6 +22,7 @@ endif()

 # Must go below project(..)
 include(GNUInstallDirs)
+set(CMAKE_CXX_STANDARD 17)

 if(MLIR_STANDALONE_BUILD)
   find_package(LLVM CONFIG REQUIRED)

[h-vetinari]

Thanks for the quick handling!

[h-vetinari]

Could we get a backport to release/16.x?

/cherry-pick 0096d17

[llvmbot]

Failed to cherry-pick: 0096d17

https://github.com/llvm/llvm-project/actions/runs/4120730621

Please manually backport the fix and push it to your github fork. Once this is done, please add a comment like this:

/branch <user>/<repo>/<branch>

[h-vetinari]

/branch h-vetinari/llvm-project/mlir_cxx

[llvmbot]

/pull-request llvm/llvm-project-release-prs#284

[h-vetinari]

Could we reopen this please until the backport is merged?