fix gcc11 compatibility

[aleqs]

Hey,

I recently switched from gcc 10 to gcc 11 (in the project using the c++20 standard) and discovered lightgbm sources no longer compiled for me. This PR fixes the issue for me. No logic is affected, and I checked build compaitbility against gcc 10/11 and clang 12 before submitting.

Alex

[ghost]

All CLA requirements met.

[jameslamb]

Thanks for your interesting in LightGBM!

Can you clarify what you mean by "gcc11 using the c++20 standard"? Did you modify this line that declares that LightGBM follows c++11?

LightGBM/CMakeLists.txt

Line 295 in bfb346c

"${CMAKE_CXX_FLAGS} -std=c++11 -pthread -Wextra -Wall -Wno-ignored-attributes -Wno-unknown-pragmas -Wno-return-type"

And if so, could you help us understanding how C++20 compatibility would help you?

My concern is that right now this project doesn't have tests compiling LightGBM following C++20, so there's even if your PR provides that compatibility today, there's no guarantee that a future PR won't break it. I'd like to understand why supporting a different standard is important to you.

[aleqs]

Hey James,

Sorry I think I was being a bit unclear. I made no other changes but the ones I submitted. I referred to the project using lightgbm being under c++20, which as you pointed out should've made no difference. I can see how this can be confusing. I'll edit the issue now to make it clearer.

Alex

[aleqs]

I ran a series of tests against the same set of compilers (gcc 10/11, clang 12) in a separate, lightgbm-only environment. All the results are the same: gcc11 is the only one failing before my change, and none of them fail after my change

[StrikerRUS]

We are compiling LightGBM with gcc-11 at our CI and have no issues:

LightGBM/.ci/test.sh

Lines 4 to 5 in bfb346c

	export CXX=g++-11
	export CC=gcc-11

-- The C compiler identification is GNU 11.2.0
-- The CXX compiler identification is GNU 11.2.0
-- Checking whether C compiler has -isysroot
-- Checking whether C compiler has -isysroot - yes
-- Checking whether C compiler supports OSX deployment target flag
-- Checking whether C compiler supports OSX deployment target flag - yes
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/local/bin/gcc-11 - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Checking whether CXX compiler has -isysroot
-- Checking whether CXX compiler has -isysroot - yes
-- Checking whether CXX compiler supports OSX deployment target flag
-- Checking whether CXX compiler supports OSX deployment target flag - yes
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/local/bin/g++-11 - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Found OpenMP_C: -fopenmp (found version "4.5") 
-- Found OpenMP_CXX: -fopenmp (found version "4.5") 
-- Found OpenMP: TRUE (found version "4.5")  
-- Performing Test MM_PREFETCH
-- Performing Test MM_PREFETCH - Success
-- Using _mm_prefetch
-- Performing Test MM_MALLOC
-- Performing Test MM_MALLOC - Success
-- Using _mm_malloc
-- Configuring done
-- Generating done
-- Build files have been written to: /Users/runner/work/LightGBM/LightGBM/build
[  8%] Building CXX object CMakeFiles/lightgbm_objs.dir/src/boosting/boosting.cpp.o
[ 11%] Building CXX object CMakeFiles/lightgbm_capi_objs.dir/src/c_api.cpp.o
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[ 17%] Building CXX object CMakeFiles/lightgbm_objs.dir/src/boosting/prediction_early_stop.cpp.o
[ 20%] Building CXX object CMakeFiles/lightgbm_objs.dir/src/io/bin.cpp.o
[ 23%] Building CXX object CMakeFiles/lightgbm_objs.dir/src/io/config.cpp.o
[ 26%] Building CXX object CMakeFiles/lightgbm_objs.dir/src/io/config_auto.cpp.o
[ 29%] Building CXX object CMakeFiles/lightgbm_objs.dir/src/io/dataset.cpp.o
[ 29%] Built target lightgbm_capi_objs
[ 32%] Building CXX object CMakeFiles/lightgbm_objs.dir/src/io/dataset_loader.cpp.o
[ 35%] Building CXX object CMakeFiles/lightgbm_objs.dir/src/io/file_io.cpp.o
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[ 70%] Building CXX object CMakeFiles/lightgbm_objs.dir/src/network/network.cpp.o
[ 73%] Building CXX object CMakeFiles/lightgbm_objs.dir/src/objective/objective_function.cpp.o
[ 76%] Building CXX object CMakeFiles/lightgbm_objs.dir/src/treelearner/cuda_tree_learner.cpp.o
[ 79%] Building CXX object CMakeFiles/lightgbm_objs.dir/src/treelearner/data_parallel_tree_learner.cpp.o
[ 82%] Building CXX object CMakeFiles/lightgbm_objs.dir/src/treelearner/feature_parallel_tree_learner.cpp.o
[ 85%] Building CXX object CMakeFiles/lightgbm_objs.dir/src/treelearner/gpu_tree_learner.cpp.o
[ 88%] Building CXX object CMakeFiles/lightgbm_objs.dir/src/treelearner/linear_tree_learner.cpp.o
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[ 97%] Building CXX object CMakeFiles/lightgbm_objs.dir/src/treelearner/voting_parallel_tree_learner.cpp.o
[ 97%] Built target lightgbm_objs
[100%] Linking CXX shared library ../lib_lightgbm.so
[100%] Built target _lightgbm

https://github.com/microsoft/LightGBM/runs/4212251986?check_suite_focus=true#step:3:601

Please at least provide the error logs you've observed in your environment.

[aleqs]

Sure, please see the log below:

In file included from /home/aleqs/dev/test_project/cmake-build-release-gcc/lightgbm-src/src/io/bin.cpp:16:
/home/aleqs/dev/test_project/cmake-build-release-gcc/lightgbm-src/src/io/dense_bin.hpp:473:28: error: expected unqualified-id before ‘const’
  473 |   DenseBin<VAL_T, IS_4BIT>(const DenseBin<VAL_T, IS_4BIT>& other)
      |                            ^~~~~
/home/aleqs/dev/test_project/cmake-build-release-gcc/lightgbm-src/src/io/dense_bin.hpp:473:28: error: expected ‘)’ before ‘const’
  473 |   DenseBin<VAL_T, IS_4BIT>(const DenseBin<VAL_T, IS_4BIT>& other)
      |                           ~^~~~~
      |                            )
In file included from /home/aleqs/dev/test_project/cmake-build-release-gcc/lightgbm-src/src/io/bin.cpp:17:
/home/aleqs/dev/test_project/cmake-build-release-gcc/lightgbm-src/src/io/multi_val_dense_bin.hpp:220:27: error: expected unqualified-id before ‘const’
  220 |   MultiValDenseBin<VAL_T>(const MultiValDenseBin<VAL_T>& other)
      |                           ^~~~~
/home/aleqs/dev/test_project/cmake-build-release-gcc/lightgbm-src/src/io/multi_val_dense_bin.hpp:220:27: error: expected ‘)’ before ‘const’
  220 |   MultiValDenseBin<VAL_T>(const MultiValDenseBin<VAL_T>& other)
      |                          ~^~~~~
      |                           )
In file included from /home/aleqs/dev/test_project/cmake-build-release-gcc/lightgbm-src/src/io/bin.cpp:18:
/home/aleqs/dev/test_project/cmake-build-release-gcc/lightgbm-src/src/io/multi_val_sparse_bin.hpp:306:7: error: expected unqualified-id before ‘const’
  306 |       const MultiValSparseBin<INDEX_T, VAL_T>& other)
      |       ^~~~~
/home/aleqs/dev/test_project/cmake-build-release-gcc/lightgbm-src/src/io/multi_val_sparse_bin.hpp:305:37: error: expected ‘)’ before ‘const’
  305 |   MultiValSparseBin<INDEX_T, VAL_T>(
      |                                    ~^
      |                                     )
  306 |       const MultiValSparseBin<INDEX_T, VAL_T>& other)
      |       ~~~~~                          
In file included from /home/aleqs/dev/test_project/cmake-build-release-gcc/lightgbm-src/src/io/bin.cpp:19:
/home/aleqs/dev/test_project/cmake-build-release-gcc/lightgbm-src/src/io/sparse_bin.hpp:601:20: error: expected unqualified-id before ‘const’
  601 |   SparseBin<VAL_T>(const SparseBin<VAL_T>& other)
      |                    ^~~~~
/home/aleqs/dev/test_project/cmake-build-release-gcc/lightgbm-src/src/io/sparse_bin.hpp:601:20: error: expected ‘)’ before ‘const’
  601 |   SparseBin<VAL_T>(const SparseBin<VAL_T>& other)
      |                   ~^~~~~
      |                    )

[jameslamb]

@aleqs thanks, I think we need more information to understand what's different between your environment and this project's existing CI jobs using gcc 11 (as @StrikerRUS mentioned).

Could you please provide the following?

• operating system
• exact commands you're using to compile LightGBM
• output of cmake --version
• output of gcc --version
• output of git log -n 1 in your clone of LightGBM`
• (if possible) full logs from compilation, not just errors, similar to what @StrikerRUS shared in fix gcc11 compatibility #4803 (comment)

We really appreciate you taking the time to report this and propose this PR, but need to fully understand the issue to be more confident in the fix.

[aleqs]

Alright, I'll get back to you with this a bit later, this will take some time on my side

[aleqs]

Hey, sorry about the delay. It looks like I won't have time to look into this properly in the foreseeable future, so probably best to mark this issue accordingly and/or close it.

[jameslamb]

Thanks @aleqs , we'll close this for now. If you come back to it in the future and are able to provide a reproducible example, we'll be happy to help.

[kdarby-cqg]

HI, I am getting the same error using conan (v1) package on v 3.3.5

cmake version 3.26.1

lightgbm/3.3.5: Calling build()
-- The C compiler identification is GNU 12.1.1
-- The CXX compiler identification is GNU 12.1.1
It seems this is a c++ 20 error

https://gcc.gnu.org/pipermail/gcc-patches/2020-May/546183.html

[ 21%] Building CXX object source_subfolder/CMakeFiles/lightgbm.dir/src/io/bin.cpp.o
[ 23%] Building CXX object source_subfolder/CMakeFiles/lightgbm.dir/src/io/config_auto.cpp.o
[ 24%] Building CXX object source_subfolder/CMakeFiles/lightgbm.dir/src/io/config.cpp.o
[ 26%] Building CXX object source_subfolder/CMakeFiles/lightgbm.dir/src/io/dataset.cpp.o
[ 27%] Building CXX object source_subfolder/CMakeFiles/lightgbm.dir/src/io/config_auto.cpp.o
In file included from /home/algo/.conan/data/lightgbm/3.3.5///build/3402b31e727359cf1945dc386665e8a8c324a0f7/source_subfolder/src/io/bin.cpp:16:
/home/algo/.conan/data/lightgbm/3.3.5///build/3402b31e727359cf1945dc386665e8a8c324a0f7/source_subfolder/src/io/dense_bin.hpp:473:28: error: expected unqualified-id before ‘const’
473 | DenseBin<VAL_T, IS_4BIT>(const DenseBin<VAL_T, IS_4BIT>& other)

[jameslamb]

Thanks for that! But let's move the discussion off of this old closed PR and to a new issue. I just opened #6033. I'm also working (indirectly) on adding C++20 support in #5981.

If you have any additional details to share that might help with that, please comment on #6033. I'm going to lock discussion here.