Compiler warnings with NVCC 11.2

[alliepiper]

What is the issue you have?

Instantiating the json::basic_json template and compiling with the CUDA NVCC compiler emits the warnings below.

Please describe the steps to reproduce the issue.

Compile the following with nvcc (observed on 11.2 with MSVC 2019 host):

#include <nlohmann/json.hpp>
int main()
{
  nlohmann::ordered_json json = {"Test"};
  json.dump();
}

The following warnings are emitted:

_deps/nlohmann_json-src/include\nlohmann/detail/hash.hpp(114): warning: missing return statement at end of non-void function "nlohmann::detail::hash(const BasicJsonType &) [with BasicJsonType=nlohmann::json]"
          detected during instantiation of "size_t nlohmann::detail::hash(const BasicJsonType &) [with BasicJsonType=nlohmann::json]" 
_deps/nlohmann_json-src/include\nlohmann/json.hpp(8709): here

_deps/nlohmann_json-src/include\nlohmann/detail/output/serializer.hpp(704): warning: pointless comparison of unsigned integer with zero
          detected during:
            instantiation of "void nlohmann::detail::serializer<BasicJsonType>::dump_integer(NumberType) [with BasicJsonType=nlohmann::basic_json<nlohmann::ordered_map, std::vector, std::string, __nv_bool, int64_t, uint64_t, double, std::allocator, nlohmann::adl_serializer, std::vector<uint8_t, std::allocator<uint8_t>>>, NumberType=uint8_t, <unnamed>=0]" 
(266): here
            instantiation of "void nlohmann::detail::serializer<BasicJsonType>::dump(const BasicJsonType &, __nv_bool, __nv_bool, unsigned int, unsigned int) [with BasicJsonType=nlohmann::basic_json<nlohmann::ordered_map, std::vector, std::string, __nv_bool, int64_t, uint64_t, double, std::allocator, nlohmann::adl_serializer, std::vector<uint8_t, std::allocator<uint8_t>>>]" 
_deps/nlohmann_json-src/include\nlohmann/json.hpp(2281): here
            instantiation of "nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::string_t nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::dump(int, char, __nv_bool, nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::error_handler_t) const [with ObjectType=nlohmann::ordered_map, ArrayType=std::vector, StringType=std::string, BooleanType=__nv_bool, NumberIntegerType=int64_t, NumberUnsignedType=uint64_t, NumberFloatType=double, AllocatorType=std::allocator, JSONSerializer=nlohmann::adl_serializer, BinaryType=std::vector<uint8_t, std::allocator<uint8_t>>]" 
test.cu(2): here

_deps/nlohmann_json-src/include\nlohmann/detail/output/serializer.hpp(704): warning: pointless comparison of unsigned integer with zero
          detected during:
            instantiation of "void nlohmann::detail::serializer<BasicJsonType>::dump_integer(NumberType) [with BasicJsonType=nlohmann::basic_json<nlohmann::ordered_map, std::vector, std::string, __nv_bool, int64_t, uint64_t, double, std::allocator, nlohmann::adl_serializer, std::vector<uint8_t, std::allocator<uint8_t>>>, NumberType=uint64_t, <unnamed>=0]" 
(338): here
            instantiation of "void nlohmann::detail::serializer<BasicJsonType>::dump(const BasicJsonType &, __nv_bool, __nv_bool, unsigned int, unsigned int) [with BasicJsonType=nlohmann::basic_json<nlohmann::ordered_map, std::vector, std::string, __nv_bool, int64_t, uint64_t, double, std::allocator, nlohmann::adl_serializer, std::vector<uint8_t, std::allocator<uint8_t>>>]" 
_deps/nlohmann_json-src/include\nlohmann/json.hpp(2281): here
            instantiation of "nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::string_t nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::dump(int, char, __nv_bool, nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::error_handler_t) const [with ObjectType=nlohmann::ordered_map, ArrayType=std::vector, StringType=std::string, BooleanType=__nv_bool, NumberIntegerType=int64_t, NumberUnsignedType=uint64_t, NumberFloatType=double, AllocatorType=std::allocator, JSONSerializer=nlohmann::adl_serializer, BinaryType=std::vector<uint8_t, std::allocator<uint8_t>>]" 
test.cu(2): here

Can you provide a small but working code example?

See above.

What is the expected behavior?

No warnings.

And what is the actual behavior instead?

Warnings.

Which compiler and operating system are you using?

• Compiler: NVCC 11.2 + MSVC 2019
• Operating system: Windows 10

Which version of the library did you use?

• latest release version 3.9.1
• other release - please state the version: ___
• the develop branch

[nlohmann]

I recently fixed a lot of warnings with #2561. Can you please re-try with the latest develop branch?

[alliepiper]

Thanks for the heads up!

It's better -- The missing return statement warning is gone.

But I still see a warning about comparing an unsigned int with zero on this line.

[nlohmann]

Thanks for checking! The comparison in that line has a history, and I have not found a way to get rid of it. I'll check if I can add NVCC to the CI.

[nlohmann]

I'm sorry for asking such a basic question, but can you provide me with the CMake call you made?

I tried the following:

• install apt-get install nvidia-cuda-toolkit
• call CXX=nvcc cmake .. for the JSON project

Unfortunately, I get the following error:

-- The CXX compiler identification is GNU 8.4.0
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - failed
-- Check for working CXX compiler: /usr/bin/nvcc
-- Check for working CXX compiler: /usr/bin/nvcc - broken
CMake Error at /usr/local/share/cmake-3.19/Modules/CMakeTestCXXCompiler.cmake:59 (message):
  The C++ compiler

    "/usr/bin/nvcc"

  is not able to compile a simple test program.

  It fails with the following output:

    Change Dir: /root/json/build/CMakeFiles/CMakeTmp
    
    Run Build Command(s):/usr/bin/make cmTC_59cc2/fast && /usr/bin/make  -f CMakeFiles/cmTC_59cc2.dir/build.make CMakeFiles/cmTC_59cc2.dir/build
    make[1]: Entering directory '/root/json/build/CMakeFiles/CMakeTmp'
    Building CXX object CMakeFiles/cmTC_59cc2.dir/testCXXCompiler.cxx.o
    /usr/bin/nvcc    -o CMakeFiles/cmTC_59cc2.dir/testCXXCompiler.cxx.o -c /root/json/build/CMakeFiles/CMakeTmp/testCXXCompiler.cxx
    Linking CXX executable cmTC_59cc2
    /usr/local/bin/cmake -E cmake_link_script CMakeFiles/cmTC_59cc2.dir/link.txt --verbose=1
    /usr/bin/nvcc -rdynamic CMakeFiles/cmTC_59cc2.dir/testCXXCompiler.cxx.o -o cmTC_59cc2 
    nvcc fatal   : Unknown option 'rdynamic'
    make[1]: *** [CMakeFiles/cmTC_59cc2.dir/build.make:106: cmTC_59cc2] Error 1
    make[1]: Leaving directory '/root/json/build/CMakeFiles/CMakeTmp'
    make: *** [Makefile:140: cmTC_59cc2/fast] Error 2

[t-b]

@nlohmann Why not use the suggestion from #755 (comment)?

This can be also simplified to not have enable_if with something like

template <typename T>
bool IsNegative(T);

template <>
bool IsNegative<number_integer_t>(number_integer_t x)
{
  return x < 0;
}

template <>
bool IsNegative<number_unsigned_t>(number_unsigned_t)
{
  return false;
}

of course this is overkill. But a good compiler can already figure out that is not worthy a warning.

[alliepiper]

Thanks for taking a look!

It doesn't look like building json with nvcc would replicate this, as nvcc will not invoke the device compilers for pure-C++ targets, and CMake won't easily support using it as a C++ compiler.

The easiest way to replicate this by hand would be:

$ cat test.cu
#include <nlohmann/json.hpp>
int main()
{
  nlohmann::ordered_json json = {"Test"};
  json.dump();
}

$ nvcc -I /path/to/json/single-header/ test.cu
<warnings>

Something like this could be added to CI by using a CUDA-language CMake target.

[nlohmann]

That works, thanks. I will add a CI job.

[nlohmann]

It seems I don't get warnings, but errors.

$ nvcc main.cu
/root/example/json.hpp:3564:156: error: expansion pattern 'CompatibleObjectType' contains no argument packs
 struct is_compatible_object_type_impl <
                                                                                                                                                            ^  
/root/example/json.hpp:3564:159: error: template argument 4 is invalid
 struct is_compatible_object_type_impl <
                                                                                                                                                               ^
/root/example/json.hpp:3564:172: error: 'value' is not a member of 'nlohmann::detail::value_t'
 struct is_compatible_object_type_impl <
                                                                                                                                                                            ^    
/root/example/json.hpp:3564:172: error: 'value' is not a member of 'nlohmann::detail::value_t'
/root/example/json.hpp:3564:239: error: expansion pattern 'CompatibleObjectType' contains no argument packs
 struct is_compatible_object_type_impl <
                                                                                                                                                                                                                                               ^  
/root/example/json.hpp:3564:242: error: template argument 4 is invalid
 struct is_compatible_object_type_impl <
                                                                                                                                                                                                                                                  ^
/root/example/json.hpp:3564:266: error: template argument 1 is invalid
 struct is_compatible_object_type_impl <
                                                                                                                                                                                                                                                                          ^
/root/example/json.hpp:3564:268: error: template argument 3 is invalid
 struct is_compatible_object_type_impl <
                                                                                                                                                                                                                                                                            ^
/root/example/json.hpp:3588:165: error: expansion pattern 'ConstructibleObjectType' contains no argument packs
 struct is_constructible_object_type_impl <
                                                                                                                                                                     ^  
/root/example/json.hpp:3588:168: error: template argument 4 is invalid
 struct is_constructible_object_type_impl <
                                                                                                                                                                        ^
/root/example/json.hpp:3588:181: error: 'value' is not a member of 'nlohmann::detail::value_t'
 struct is_constructible_object_type_impl <
                                                                                                                                                                                     ^    
/root/example/json.hpp:3588:181: error: 'value' is not a member of 'nlohmann::detail::value_t'
/root/example/json.hpp:3588:251: error: expansion pattern 'ConstructibleObjectType' contains no argument packs
 struct is_constructible_object_type_impl <
                                                                                                                                                                                                                                                           ^  
/root/example/json.hpp:3588:254: error: template argument 4 is invalid
 struct is_constructible_object_type_impl <
                                                                                                                                                                                                                                                              ^
/root/example/json.hpp:3588:278: error: template argument 1 is invalid
 struct is_constructible_object_type_impl <
                                                                                                                                                                                                                                                                                      ^
/root/example/json.hpp:3588:280: error: template argument 3 is invalid
 struct is_constructible_object_type_impl <
                                                                                                                                                                                                                                                                                        ^
/root/example/json.hpp:3657:152: error: expansion pattern 'CompatibleArrayType' contains no argument packs
 struct is_compatible_array_type_impl <
                                                                                                                                                        ^  
/root/example/json.hpp:3657:155: error: template argument 4 is invalid
 struct is_compatible_array_type_impl <
                                                                                                                                                           ^
/root/example/json.hpp:3657:168: error: 'value' is not a member of 'nlohmann::detail::value_t'
 struct is_compatible_array_type_impl <
                                                                                                                                                                        ^    
/root/example/json.hpp:3657:168: error: 'value' is not a member of 'nlohmann::detail::value_t'
/root/example/json.hpp:3657:234: error: expansion pattern 'CompatibleArrayType' contains no argument packs
 struct is_compatible_array_type_impl <
                                                                                                                                                                                                                                          ^  
/root/example/json.hpp:3657:237: error: template argument 4 is invalid
 struct is_compatible_array_type_impl <
                                                                                                                                                                                                                                             ^
/root/example/json.hpp:3657:335: error: template argument 1 is invalid
 struct is_compatible_array_type_impl <
                                                                                                                                                                                                                                                                                                                                               ^
/root/example/json.hpp:3657:337: error: template argument 3 is invalid
 struct is_compatible_array_type_impl <
                                                                                                                                                                                                                                                                                                                                                 ^
/root/example/json.hpp:3687:437: error: expansion pattern 'ConstructibleArrayType' contains no argument packs
 struct is_constructible_array_type_impl <
                                                                                                                                                                                                                                                                                                                                                                                                                                                     ^  
/root/example/json.hpp:3687:440: error: template argument 4 is invalid
 struct is_constructible_array_type_impl <
                                                                                                                                                                                                                                                                                                                                                                                                                                                        ^
/root/example/json.hpp:3687:86: error: parse error in template argument list
 struct is_constructible_array_type_impl <
                                                                                      ^                                                   

// main.cpp
#include "json.hpp"

int main()
{}

-- The CXX compiler identification is GNU 11.0.1
-- The CUDA compiler identification is NVIDIA 10.1.243

Running on Ubuntu. Any ideas?

[alliepiper]

Strange, I can't get the same result here. Does it work if you change the extension to .cu (or pass nvcc -x cu main.cpp)?

If it helps, the warning can be repro'd by copying the current single-header json.hpp into godbolt's CUDA backend: https://www.godbolt.org/z/WWM1hW4v9

[nlohmann]

I re-tried in the Docker container I used for the CI (https://hub.docker.com/repository/docker/nlohmann/json-ci) that has nvcc 10.1 installed.

• Checked out develop branch.
• Created main.cpp in json/single_include/nlohmann as below.
• Called nvcc -x cu main.cpp.

Output:

json.hpp:3564:156: error: expansion pattern 'CompatibleObjectType' contains no argument packs
 struct is_compatible_object_type_impl <
                                                                                                                                                            ^  
json.hpp:3564:159: error: template argument 4 is invalid
 struct is_compatible_object_type_impl <
                                                                                                                                                               
json.hpp:3564:172: error: 'value' is not a member of 'nlohmann::detail::value_t'
 struct is_compatible_object_type_impl <
                                                                                                                                                                            ^    
json.hpp:3564:172: error: 'value' is not a member of 'nlohmann::detail::value_t'
json.hpp:3564:239: error: expansion pattern 'CompatibleObjectType' contains no argument packs
 struct is_compatible_object_type_impl <
                                                                                                                                                                                                                                                 
json.hpp:3564:242: error: template argument 4 is invalid
 struct is_compatible_object_type_impl <
                                                                                                                                                                                                                                                  ^
json.hpp:3564:266: error: template argument 1 is invalid
 struct is_compatible_object_type_impl <
                                                                                                                                                                                                                                                                          ^
json.hpp:3564:268: error: template argument 3 is invalid
 struct is_compatible_object_type_impl <
                                                                                                                                                                                                                                                                            ^
json.hpp:3588:165: error: expansion pattern 'ConstructibleObjectType' contains no argument packs
 struct is_constructible_object_type_impl <
                                                                                                                                                                     ^  
json.hpp:3588:168: error: template argument 4 is invalid
 struct is_constructible_object_type_impl <
                                                                                                                                                                        ^
json.hpp:3588:181: error: 'value' is not a member of 'nlohmann::detail::value_t'
 struct is_constructible_object_type_impl <
                                                                                                                                                                                     ^    
json.hpp:3588:181: error: 'value' is not a member of 'nlohmann::detail::value_t'
json.hpp:3588:251: error: expansion pattern 'ConstructibleObjectType' contains no argument packs
 struct is_constructible_object_type_impl <
                                                                                                                                                                                                                                                           ^  
json.hpp:3588:254: error: template argument 4 is invalid
 struct is_constructible_object_type_impl <
                                                                                                                                                                                                                                                              ^
json.hpp:3588:278: error: template argument 1 is invalid
 struct is_constructible_object_type_impl <
                                                                                                                                                                                                                                                                                      ^
json.hpp:3588:280: error: template argument 3 is invalid
 struct is_constructible_object_type_impl <
                                                                                                                                                                                                                                                                                        ^
json.hpp:3657:152: error: expansion pattern 'CompatibleArrayType' contains no argument packs
 struct is_compatible_array_type_impl <
                                                                                                                                                        ^  
json.hpp:3657:155: error: template argument 4 is invalid
 struct is_compatible_array_type_impl <
                                                                                                                                                           ^
json.hpp:3657:168: error: 'value' is not a member of 'nlohmann::detail::value_t'
 struct is_compatible_array_type_impl <
                                                                                                                                                                        ^    
json.hpp:3657:168: error: 'value' is not a member of 'nlohmann::detail::value_t'
json.hpp:3657:234: error: expansion pattern 'CompatibleArrayType' contains no argument packs
 struct is_compatible_array_type_impl <
                                                                                                                                                                                                                                          ^  
json.hpp:3657:237: error: template argument 4 is invalid
 struct is_compatible_array_type_impl <
                                                                                                                                                                                                                                             ^
json.hpp:3657:335: error: template argument 1 is invalid
 struct is_compatible_array_type_impl <
                                                                                                                                                                                                                                                                                                                                               ^
json.hpp:3657:337: error: template argument 3 is invalid
 struct is_compatible_array_type_impl <
                                                                                                                                                                                                                                                                                                                                                 ^
json.hpp:3687:437: error: expansion pattern 'ConstructibleArrayType' contains no argument packs
 struct is_constructible_array_type_impl <
                                                                                                                                                                                                                                                                                                                                                                                                                                                     ^  
json.hpp:3687:440: error: template argument 4 is invalid
 struct is_constructible_array_type_impl <
                                                                                                                                                                                                                                                                                                                                                                                                                                                        ^
json.hpp:3687:86: error: parse error in template argument list
 struct is_constructible_array_type_impl <
                                                                                      ^                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              

// main.cpp
#include "json.hpp"

int main()
{}

$ nvcc --version
nvcc: NVIDIA (R) Cuda compiler driver
Copyright (c) 2005-2019 NVIDIA Corporation
Built on Sun_Jul_28_19:07:16_PDT_2019
Cuda compilation tools, release 10.1, V10.1.243

[nickaein]

@allisonvacanti

Hi Allison,

I'm not familiar with nvcc, but it seems that there are cases in which nvcc uses an external C++ compiler. For instance, this page says:

A general purpose C++ host compiler is needed by nvcc in the following situations:

• During non-CUDA phases (except the run phase), because these phases will be forwarded by nvcc to this compiler.
• During CUDA phases, for several preprocessing stages and host code compilation (see also The CUDA Compilation Trajectory).

So there is the possibility that warnings are being emitted from this host compiler instead of nvcc itself.

You mentioned here that there is a "device compiler" in play, and an exact Google search of emitted error message:

"pointless comparison of unsigned integer with zero"

hints that the compiler could be Intel compiler or a variant of arm/avr compiler (which AFAIK are usually derived from gcc). Can you check on what compiler is being used in this case?

[alliepiper]

These do appear to be coming from NVCC itself, and not from the underlying host compiler (which was MSVC 2019 in this case).