[OpenMP] Disable early vectorization of loads/stores in the runtime

[jdoerfert]

Summary

We are having a hard time optimizing some vectorized loads/stores later
on which causes this optimization to degrade performance.

[jhuber6]

This pass will get run eventually when it's linked into the user code, so this should be fine.

openmp/libomptarget/DeviceRTL/CMakeLists.txt

112–119

This new flag's presence is a little esoteric, maybe add a comment saying why it's there.

[jdoerfert]

openmp/libomptarget/DeviceRTL/CMakeLists.txt

112–119

Will add:

# We disable the slp vectorizer during the runtime optimization to avoid
# vectorized accesses to the shared state. Generally, those are "good" but
# the optimizer pipeline (esp. Attributor) does not fully support vectorized
# instructions yet and we end up missing out on way more important constant
# propagation. That said, we will run the vectorizer again during LTO.

openmp/libomptarget/DeviceRTL/CMakeLists.txt

set(src_files
   ${source_directory}/Reduction.cpp
   ${source_directory}/State.cpp
   ${source_directory}/Synchronization.cpp
   ${source_directory}/Tasking.cpp
   ${source_directory}/Utils.cpp
   ${source_directory}/Workshare.cpp
 )
 
-set(clang_opt_flags -O3 -mllvm -openmp-opt-disable -DSHARED_SCRATCHPAD_SIZE=512)
-set(link_opt_flags  -O3        -openmp-opt-disable -attributor-enable=module)
+# We disable the slp vectorizer during the runtime optimization to avoid
+# vectorized accesses to the shared state. Generally, those are "good" but
+# the optimizer pipeline (esp. Attributor) does not fully support vectorized
+# instructions yet and we end up missing out on way more important constant
+# propagation. That said, we will run the vectorizer again after the runtime 
+# has been linked into the user program.
+set(clang_opt_flags -O3 -mllvm -openmp-opt-disable -DSHARED_SCRATCHPAD_SIZE=512 -mllvm -vectorize-slp=false )
+set(link_opt_flags  -O3        -openmp-opt-disable -attributor-enable=module -vectorize-slp=false )
 set(link_export_flag -passes=internalize -internalize-public-api-file=${source_directory}/exports)
 
 # Prepend -I to each list element
 set (LIBOMPTARGET_LLVM_INCLUDE_DIRS_DEVICERTL "${LIBOMPTARGET_LLVM_INCLUDE_DIRS}")
 list(TRANSFORM LIBOMPTARGET_LLVM_INCLUDE_DIRS_DEVICERTL PREPEND "-I")
 
 # Set flags for LLVM Bitcode compilation.
 set(bc_flags -c -foffload-lto -std=c++17 -fvisibility=hidden