Work around spurious "missing return statement" warnings for NVCC and Intel compilers

[JaredCrean2]

This is a small change to avoid a compiler warning on both Intel and Nvidia compilers. The warning is incorrect, but it is muddying up compile output and the dashboard results for the Sierra Toolkit.

The warning is that (sometimes) the compiler thinks:

double foo()
{
  if constexpr (...)
  {
    return 42;
  } else
  {
    return 0;
  }
}  // warning here about lack of return statement in non-void function

is missing a return statement.

[aprokop]

@JaredCrean2 Thank you for looking into it! Sorry about the issue, we will fix the nvcc warnings in the near future (#272).

In the mean time, I tested the compiler version on godbolt, and every NVCC version prior to 11.5 and Intel (icc) prior to 2021.7 are susceptible.

To be a bit kinder to ourselves in the future, could you please try out the following patch:

--- a/src/details/ArborX_DetailsLegacy.hpp
+++ b/src/details/ArborX_DetailsLegacy.hpp
@@ -52,6 +52,12 @@ public:
       expand(bounding_volume, Access::get(_primitives, i));
       return value_type{bounding_volume, (index_type)i};
     }
+#if (defined(KOKKOS_COMPILER_NVCC) && (KOKKOS_COMPILER_NVCC < 1150)) || \
+    (defined(KOKKOS_COMPILER_INTEL) && (KOKKOS_COMPILER_INTEL <= 2021))
+    // FIXME_NVCC, FIXME_INTEL: workaround for spurios "missing return
+    // statement at end of non-void function" warning
+    return value_type{};
+#endif
   }

   KOKKOS_FUNCTION

and see if it fixes the issue? I would prefer this way to clearly indicate when we can clean this up in the future, as well as to not affect the code for any other compilers.

Note: all Intel compilers 2021.* have 2021 as version.

[aprokop]

Kokkos does it slightly different: https://github.com/kokkos/kokkos/blob/63f05204d0eaeec83e25f87eeb8025a188683f92/algorithms/src/std_algorithms/impl/Kokkos_UniqueCopy.hpp#L178-L181, by relying on __builtin_unreachable(). However, that doesn't work with NVCC prior to 11.3.

[JaredCrean2]

The patch does fix the warnings. For the intel compiler, the details are:

[jccrean@cee-build032 code]$ mpicxx --version
icpc (ICC) 2021.3.0 20210609

And the warning is:

/ceeGPFS/fgs/jccrean/code/TPLs_src/ArborX/include/details/ArborX_DetailsLegacy.hpp(58): warning #1011: missing return statement at end of non-void function "ArborX::Details::LegacyValues<Primitives, BoundingVolume>::operator() [with Primitives=stk::search::impl::ViewWrapperForArborXTraits<Kokkos::View<const std::pair<stk::search::Point<double>, <unnamed>::Id> *, Kokkos::HostSpace, Kokkos::MemoryTraits<1U>>>, BoundingVolume=ArborX::Box]"
    }
    ^
         

Although I can't get godbolt to reproduce it with a minimal example. The Intel classic compiler is deprecated in favor of the new Clang-based compiler, so may ifdefing on __INTEL_COMPILER is good enough (the Clang-base compiler has __INTEL_LLVM_COMPILER defined to distinguish itself from the classic compiler)

[aprokop]

Although I can't get godbolt to reproduce it with a minimal example. The Intel classic compiler is deprecated in favor of the new Clang-based compiler, so may ifdefing on __INTEL_COMPILER is good enough (the Clang-base compiler has __INTEL_LLVM_COMPILER defined to distinguish itself from the classic compiler)

Here's the reproducer for Intel (classic, icc). The newer versions of Intel (icx LLVM-based) seem to be ok. Kokkos provides KOKKOS_COMPILER_INTEL for classic, and KOKKOS_COMPILER_INTEL_LLVM for legacy.

I'm not sure if you noticed I edited my patch to include Intel.

[JaredCrean2]

Ah, I was missing the -Wall. I did check your patch against the Intel compiler in the Sierra Toolkit (STK) code and it works.

Do you want me to update this PR with the new patch, or do you want to make a new PR?

[aprokop]

Do you want me to update this PR with the new patch, or do you want to make a new PR?

Please update this PR.

[JaredCrean2]

Done. Thanks for looking at this so quickly @aprokop

[aprokop]

Done. Thanks for looking at this so quickly @aprokop

Needs formatting fix. I can do that if you prefer.

[JaredCrean2]

I added a commit after running clang-format. Feel free to make any additional changes if needed.

[aprokop]

GCC failed for unrelated reason (cc1plus killed). Merging.