[coro][pgo] Don't promote pgo counters in the suspend basic block (#71263)

If a suspend happens in the resume part (this can happen in the case of chained coroutines), and that's part of a loop, the pre-split CFG has the suspend block as an exit of that loop. PGO Counter Promotion will then try to commit the temporary counter to the global in that "exit" block (it also does that in the other loop exit BBs, which also includes
the "destroy" case). This interferes with symmetric transfer.

We don't need to commit the counter in the suspend case - it's not a loop exit from the perspective of the behavior of the program. The regular loop exit, together with the "destroy" case, completely cover any updates that may need to happen to the global counter.

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
index 168998f..ccee970 100644
--- a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
+++ b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
@@ -2149,3 +2149,15 @@ bool llvm::hasOnlySimpleTerminator(const Function &F) {
   }
   return true;
 }
+
+bool llvm::isPresplitCoroSuspendExitEdge(const BasicBlock &Src,
+                                         const BasicBlock &Dest) {
+  assert(Src.getParent() == Dest.getParent());
+  if (!Src.getParent()->isPresplitCoroutine())
+    return false;
+  if (auto *SW = dyn_cast<SwitchInst>(Src.getTerminator()))
+    if (auto *Intr = dyn_cast<IntrinsicInst>(SW->getCondition()))
+      return Intr->getIntrinsicID() == Intrinsic::coro_suspend &&
+             SW->getDefaultDest() == &Dest;
+  return false;
+}