[Openmp-commits] [PATCH] D130114: [OpenMP] Use Undef instead of null as pointer for inactive lanes

Johannes Doerfert via Phabricator via Openmp-commits openmp-commits at lists.llvm.org
Tue Jul 19 12:37:29 PDT 2022

jdoerfert created this revision.
jdoerfert added reviewers: tianshilei1992, jhuber6.
Herald added subscribers: guansong, bollu, yaxunl.
Herald added a project: All.
jdoerfert requested review of this revision.
Herald added a subscriber: sstefan1.
Herald added a project: OpenMP.

Our conditional writes in the runtime look like this:

  if (active)
    *ptr = value;

In the RAII we need to assign `ptr` which comes from a lookup call.
If a thread that is not the main thread calls lookup with the intention
to write the pointer, we'll create a new thread state. As such, we need
to avoid calling lookup for inactive threads. We used to use `nullptr`
as their `ptr` value but that can cause pessimistic reasoning. We now
use `undef` instead.

  rG LLVM Github Monorepo



Index: openmp/libomptarget/DeviceRTL/include/Utils.h
--- openmp/libomptarget/DeviceRTL/include/Utils.h
+++ openmp/libomptarget/DeviceRTL/include/Utils.h
@@ -14,6 +14,8 @@
 #include "Types.h"
+#pragma omp begin declare target device_type(nohost)
 namespace _OMP {
 namespace utils {
@@ -72,10 +74,15 @@
   return V - V % Align;
+/// A  pointer variable that has by design an `undef` value. Use with care.
+__attribute__((loader_uninitialized)) static void *const UndefPtr;
 #define OMP_LIKELY(EXPR) __builtin_expect((bool)(EXPR), true)
 #define OMP_UNLIKELY(EXPR) __builtin_expect((bool)(EXPR), false)
 } // namespace utils
 } // namespace _OMP
+#pragma omp end declare target
Index: openmp/libomptarget/DeviceRTL/include/State.h
--- openmp/libomptarget/DeviceRTL/include/State.h
+++ openmp/libomptarget/DeviceRTL/include/State.h
@@ -277,7 +277,8 @@
 template <typename VTy, typename Ty> struct ValueRAII {
   ValueRAII(VTy &V, Ty NewValue, Ty OldValue, bool Active, IdentTy *Ident)
-      : Ptr(Active ? &V.lookup(/* IsReadonly */ false, Ident) : nullptr),
+      : Ptr(Active ? &V.lookup(/* IsReadonly */ false, Ident)
+                   : (Ty *)utils::UndefPtr),
         Val(OldValue), Active(Active) {
     if (!Active)

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