MSVC OpenMP: switch to the /openmp:llvm runtime

CMake/OpenMP.cmake

@@ -1,21 +1,27 @@
 message(STATUS "Checking for OpenMP ... ")
 
-# MSVC's ordinary /openmp mode does not support the omp simd directive used by
-# the sparse kernels. Recent MSVC versions provide it through
-# /openmp:experimental. Allow callers to override this setting explicitly.
+# MSVC's classic /openmp mode implements OpenMP 2.0 only: it rejects the
+# omp simd directive used by the sparse kernels, 64-bit loop indices, and
+# the collapse clause used by downstream consumers such as RandLAPACK. The
+# /openmp:llvm runtime supports all of these (and subsumes what
+# /openmp:experimental offered). Callers can still override the mode
+# explicitly with -DOpenMP_CXX_FLAGS=... at configure time.
 if (MSVC AND NOT DEFINED OpenMP_CXX_FLAGS)
-    set(OpenMP_CXX_FLAGS "/openmp:experimental" CACHE STRING
+    set(OpenMP_CXX_FLAGS "/openmp:llvm" CACHE STRING
         "OpenMP compiler flags for C++")
 endif()
+if (MSVC)
+    set(RandBLAS_OpenMP_MSVC_FLAGS "${OpenMP_CXX_FLAGS}")
+endif()
 
 find_package(OpenMP COMPONENTS CXX)
 
 # FindOpenMP may replace OpenMP_CXX_FLAGS while probing the compiler. Ensure
-# the imported target used by RandBLAS carries the SIMD-capable MSVC option.
+# the imported target used by RandBLAS carries the requested MSVC mode.
 if (MSVC AND OpenMP_CXX_FOUND AND TARGET OpenMP::OpenMP_CXX)
     set_property(TARGET OpenMP::OpenMP_CXX PROPERTY
-        INTERFACE_COMPILE_OPTIONS "/openmp:experimental")
-    set(OpenMP_CXX_FLAGS "/openmp:experimental")
+        INTERFACE_COMPILE_OPTIONS "${RandBLAS_OpenMP_MSVC_FLAGS}")
+    set(OpenMP_CXX_FLAGS "${RandBLAS_OpenMP_MSVC_FLAGS}")
 endif()
 
 set(tmp FALSE)

CMake/RandBLASConfig.cmake.in

@@ -23,21 +23,27 @@ find_dependency(Random123)
 # OpenMP
 set(RandBLAS_HAS_OpenMP @RandBLAS_HAS_OpenMP@)
 if (RandBLAS_HAS_OpenMP)
-    # MSVC's standard /openmp mode does not support the omp simd directive
-    # used by RandBLAS's sparse kernels.
+    # MSVC's classic /openmp mode implements OpenMP 2.0 only: it rejects the
+    # omp simd directive used by RandBLAS's sparse kernels, 64-bit loop
+    # indices, and the collapse clause used by downstream consumers. The
+    # /openmp:llvm runtime supports all of these. Consumers can still
+    # override the mode explicitly with -DOpenMP_CXX_FLAGS=...
     if (MSVC AND NOT DEFINED OpenMP_CXX_FLAGS)
-        set(OpenMP_CXX_FLAGS "/openmp:experimental" CACHE STRING
+        set(OpenMP_CXX_FLAGS "/openmp:llvm" CACHE STRING
             "OpenMP compiler flags for C++")
     endif()
+    if (MSVC)
+        set(RandBLAS_OpenMP_MSVC_FLAGS "${OpenMP_CXX_FLAGS}")
+    endif()
 
     find_dependency(OpenMP COMPONENTS CXX)
 
     # FindOpenMP may retain a previously detected /openmp setting. Ensure that
-    # consumers of the installed package receive the SIMD-capable MSVC mode.
+    # consumers of the installed package receive the requested MSVC mode.
     if (MSVC AND OpenMP_CXX_FOUND AND TARGET OpenMP::OpenMP_CXX)
         set_property(TARGET OpenMP::OpenMP_CXX PROPERTY
-            INTERFACE_COMPILE_OPTIONS "/openmp:experimental")
-        set(OpenMP_CXX_FLAGS "/openmp:experimental")
+            INTERFACE_COMPILE_OPTIONS "${RandBLAS_OpenMP_MSVC_FLAGS}")
+        set(OpenMP_CXX_FLAGS "${RandBLAS_OpenMP_MSVC_FLAGS}")
     endif()
 endif()
 

RandBLAS/sparse_data/csr_spmm_impl.hh

@@ -65,7 +65,12 @@ static void apply_csr_to_vector_ik_impl(
     // ^ silence compiler complaints if UnitStride == true.
     for (int64_t i = 0; i < len_Av; ++i) {
         T Av_i_diff = 0.0;
+        // MSVC's /openmp:llvm mode rejects the simd directive (C7660), and its
+        // /openmp:experimental mode ignores the reduction clause without
+        // vectorizing -- so on MSVC this pragma buys nothing either way.
+        #if !defined(_MSC_VER)
         #pragma omp simd reduction(+:Av_i_diff)
+        #endif
         for (int64_t ell = rowptr[i]; ell < rowptr[i+1]; ++ell) {
             int64_t j = colidxs[ell];
             if constexpr (UnitStride) {