[clang] Adding Separate OpenMP Offloading Backend to `libcxx/include/__algorithm/pstl_backends` (PR #66968)

[Louis Dionne via cfe-commits]

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@@ -772,15 +774,19 @@ elseif (LIBCXX_HARDENING_MODE STREQUAL "unchecked")
   config_define(0 _LIBCPP_ENABLE_DEBUG_MODE_DEFAULT)
 endif()
 
-if (LIBCXX_PSTL_CPU_BACKEND STREQUAL "serial")
+if (LIBCXX_PSTL_BACKEND STREQUAL "serial")
   config_define(1 _LIBCPP_PSTL_CPU_BACKEND_SERIAL)
-elseif(LIBCXX_PSTL_CPU_BACKEND STREQUAL "std_thread")
+elseif (LIBCXX_PSTL_BACKEND STREQUAL "std-thread")
   config_define(1 _LIBCPP_PSTL_CPU_BACKEND_THREAD)
-elseif(LIBCXX_PSTL_CPU_BACKEND STREQUAL "libdispatch")
+elseif (LIBCXX_PSTL_BACKEND STREQUAL "libdispatch")
   config_define(1 _LIBCPP_PSTL_CPU_BACKEND_LIBDISPATCH)
+elseif (LIBCXX_PSTL_BACKEND STREQUAL "openmp")
+  config_define(1 _LIBCPP_PSTL_BACKEND_OPENMP)
+  # Making sure that OpenMP is enabled during build
+  add_compile_options(-fopenmp)
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ldionne wrote:

I think I would move this to `cxx_add_basic_build_flags` instead to keep these flags grouped. You'll also want to move to `target_add_compile_flags_if_supported` instead of `add_compile_options`.

https://github.com/llvm/llvm-project/pull/66968