add explanatory comments

RecoTracker/MkFitCore/src/MkFinder.cc

@@ -299,7 +299,11 @@ namespace mkfit {
     };
 
     if (L.is_barrel()) {
-      // Pull out the part of the loop that vectorizes
+      // Pull out the part of the loop that vectorizes with icc and gcc
+      // In llvm16 clang issues a warning that it can't vectorize
+      // the loop.  Unfortunately, there doesn't seem to be a
+      // pragma to suppress the warning, so we ifdef it out.  This
+      // should be rechecked if llvm vectorization improves.
 #if !defined(__clang__)
 #pragma omp simd
 #endif
@@ -341,7 +345,7 @@ namespace mkfit {
       //layer half-thikness for dphi spread calculation; only for very restrictive iters
       const float layerD = std::abs(L.layer_info()->zmax() - L.layer_info()->zmin()) * 0.5f *
                            (m_iteration_params->maxConsecHoles == 0 || m_iteration_params->maxHolesPerCand == 0);
-      // Pull out the part of the loop that vectorizes
+      // Pull out the part of the loop that vectorizes with icc and gcc
 #if !defined(__clang__)
 #pragma omp simd
 #endif
@@ -1210,7 +1214,7 @@ namespace mkfit {
                                      m_prop_config->finding_intra_layer_pflags,
                                      m_prop_config->finding_requires_propagation_to_hit_pos);
 
-       //#pragma omp simd  // DOES NOT VECTORIZE AS IT IS NOW
+      //#pragma omp simd  // DOES NOT VECTORIZE AS IT IS NOW
       for (int itrack = 0; itrack < N_proc; ++itrack) {
         // We can be in failed state from the initial propagation before selectHitIndices
         // and there hit_count for track is set to -1 and WSR state to Failed, handled below.

RecoTracker/MkFitCore/src/PropagationMPlex.cc

@@ -297,6 +297,9 @@ namespace mkfit {
     MPlexLL errorPropTmp(0.f);   //initialize to zero
     MPlexLL errorPropSwap(0.f);  //initialize to zero
 
+    // loop does not vectorize with llvm16, and it issues a warning
+    // that apparently can't be suppressed with a pragma.  Needs to
+    // be rechecked if future llvm versions improve vectorization.
 #if !defined(__clang__)
 #pragma omp simd
 #endif