[Libomptarget] Build the device library even if the sm list is empty

[jhuber6]

Summary

We previously had some logic that stopped us from building the device runtime if
there were no NVPTX architectures provided. This is incorrect because we could
have AMDGPU libraries. Even if the lists are empty we should be able to attempt
to build these and get dummy output. THis wilil make it much easier for our
tooling which expects certain libraries. If the user wishes to disable the
library entirely they should use `-DLIBOMPTARGET_BUILD_DEVICERTL_BCLIB=OFF"

[tianshilei1992]

Does it affect the option that we can build bc lib for one arch but not for another, like I just would like to build AMD bc lib? We can't assume the LLVM/Clang used for the compilation supports all arch.

[jhuber6]

In D130266#3668795, @tianshilei1992 wrote:

Does it affect the option that we can build bc lib for one arch but not for another, like I just would like to build AMD bc lib? We can't assume the LLVM/Clang used for the compilation supports all arch.

If the lists are empty, the for loops setting the architecture won't generate any code right. For the static library it won't get embedded in the binary either.

[tianshilei1992]

In D130266#3668798, @jhuber6 wrote:

In D130266#3668795, @tianshilei1992 wrote:

Does it affect the option that we can build bc lib for one arch but not for another, like I just would like to build AMD bc lib? We can't assume the LLVM/Clang used for the compilation supports all arch.

If the lists are empty, the for loops setting the architecture won't generate any code right. For the static library it won't get embedded in the binary either.

Right. The logic here is a little messy because contains some inconsistency when we implemented DeviceRTL first for NVPTX and later for AMDGPU.

openmp/libomptarget/DeviceRTL/CMakeLists.txt

elseif(LIBOMPTARGET_NVPTX_COMPUTE_CAPABILITIES STREQUAL "auto")
   if (NOT LIBOMPTARGET_DEP_CUDA_FOUND)
     libomptarget_error_say("[NVPTX] Cannot auto detect compute capability as CUDA not found.")
   endif()
   set(nvptx_sm_list ${LIBOMPTARGET_DEP_CUDA_ARCH})
 else()
   string(REPLACE "," ";" nvptx_sm_list "${LIBOMPTARGET_NVPTX_COMPUTE_CAPABILITIES}")
 endif()
 
-# If user set LIBOMPTARGET_NVPTX_COMPUTE_CAPABILITIES to empty, we disable the
-# build.
-if (NOT nvptx_sm_list)
-  libomptarget_say("Not building CUDA offloading DeviceRTL: empty compute capability list")
-  return()
-endif()
-
 # Check all SM values
 foreach(sm ${nvptx_sm_list})
   if (NOT ${sm} IN_LIST all_capabilities)
     libomptarget_warning_say("[NVPTX] Compute capability ${sm} is not supported. Make sure clang can work with it.")
   endif()
 endforeach()
 
 set(amdgpu_mcpus gfx700 gfx701 gfx801 gfx803 gfx900 gfx902 gfx906 gfx908 gfx90a gfx90c gfx940 gfx1010 gfx1030 gfx1031 gfx1032 gfx1033 gfx1034 gfx1035 gfx1036)