SUSY HRDCOD=1 builds fail because sin/cos/atan are not constexpr (only CUDA fails, while C++ succeeds)

[valassi]

SUSY HRDCOD=1 builds fail because sin/cos/atan are not constexpr

See for instance susy gg to tt in PR #625, and in particular 586195d

The log says

The HRDCOD=1 build now fails with

ccache /usr/local/cuda-12.0/bin/nvcc  -O3  -lineinfo -I. -I../../src -I../../../../../tools -I/usr/local/cuda-12.0/include/ -DUSE_NVTX -gencode arch=compute_70,code=compute_70 -gencode arch=compute_70,code=sm_70 -use_fast_math -std=c++17  -ccbin /usr/lib64/ccache/g++ -DMGONGPU_FPTYPE_DOUBLE -DMGONGPU_FPTYPE2_DOUBLE -DMGONGPU_HARDCODE_PARAM -Xcompiler -fPIC -c gcheck_sa.cu -o gcheck_sa.o
../../src/Parameters_MSSM_SLHA2.h(310): error: expression must have a constant value
../../src/Parameters_MSSM_SLHA2.h(310): note #2703-D: cannot call non-constexpr function "atan(double) noexcept(true)"
/usr/local/cuda-12.0/include/crt/math_functions.h(4137): here
../../src/Parameters_MSSM_SLHA2.h(725): error: expression must have a constant value
../../src/Parameters_MSSM_SLHA2.h(725): note #2703-D: cannot call non-constexpr function "cos(double) noexcept(true)"
/usr/local/cuda-12.0/include/crt/math_functions.h(553): here
../../src/Parameters_MSSM_SLHA2.h(726): error: expression must have a constant value
../../src/Parameters_MSSM_SLHA2.h(726): note #2703-D: cannot call non-constexpr function "sin(double) noexcept(true)"
/usr/local/cuda-12.0/include/crt/math_functions.h(520): here
3 errors detected in the compilation of "gcheck_sa.cu".

[valassi]

Update Feb 2024. Actually, it is only nvcc that complains that sin/cos/atan are not constexpr. The C++ only build (CUDA_HOME=none using the old setup) succeeds, and the runtime tests are ok.

Maybe we can try to hardcode the gcc implementation of constexpr sin/cos/atan into our nvcc code?

See #625 (comment)

[valassi]

I have made a first prototype in PR #625 here 46694af

Still need some fixes (extend the range beyond 0 to pi/2, add arctan).

The implementation is very simply based on a Taylor series expansion, which is easily done in constexpr.

[valassi]

The fix for HRDCOD=1 builds in SUSY is now complete

• I implemented the relevant trig functions as constexpr using Taylor series
• I added device to the relevant constexpr variables
  I close this issue. The fix will be committed in PR fix susy gg_tt builds and add it to the repo #625.

(SUSY itself is almost completed, only some minor pieces missing to CODEGEN)

cc @oliviermattelaer @roiser