[llvm] [coro][pgo] Do not insert counters in the `suspend` block (PR #71262)

[Mircea Trofin via llvm-commits]

================
@@ -121,31 +123,70 @@ template <class Edge, class BBInfo> class CFGMST {
 
     static const uint32_t CriticalEdgeMultiplier = 1000;
 
+    auto GetCoroSuspendSwitch =
+        [&](const Instruction *TI) -> const SwitchInst * {
+      if (!F.isPresplitCoroutine())
+        return nullptr;
+      if (auto *SWInst = dyn_cast<SwitchInst>(TI))
+        if (auto *Intrinsic = dyn_cast<IntrinsicInst>(SWInst->getCondition()))
+          if (Intrinsic->getIntrinsicID() == Intrinsic::coro_suspend)
+            return SWInst;
+      return nullptr;
+    };
+
     for (BasicBlock &BB : F) {
       Instruction *TI = BB.getTerminator();
+      const SwitchInst *CoroSuspendSwitch = GetCoroSuspendSwitch(TI);
       uint64_t BBWeight =
           (BFI != nullptr ? BFI->getBlockFreq(&BB).getFrequency() : 2);
       uint64_t Weight = 2;
       if (int successors = TI->getNumSuccessors()) {
         for (int i = 0; i != successors; ++i) {
           BasicBlock *TargetBB = TI->getSuccessor(i);
-          bool Critical = isCriticalEdge(TI, i);
-          uint64_t scaleFactor = BBWeight;
-          if (Critical) {
-            if (scaleFactor < UINT64_MAX / CriticalEdgeMultiplier)
-              scaleFactor *= CriticalEdgeMultiplier;
-            else
-              scaleFactor = UINT64_MAX;
+          const bool Critical = isCriticalEdge(TI, i);
+          const bool IsCoroSuspendTarget =
+              CoroSuspendSwitch &&
+              CoroSuspendSwitch->getDefaultDest() == TargetBB;
+          // We must not add instrumentation to the BB representing the
+          // "suspend" path, else CoroSplit won't be able to lower
+          // llvm.coro.suspend to a tail call. We do want profiling info for
+          // the other branches (resume/destroy). So we do 2 things:
+          // 1. we prefer instrumenting those other edges by setting the weight
+          //    of the "suspend" edge to max, and
+          // 2. we mark the edge as "Removed" to guarantee it is not considered
+          //    for instrumentation. That could technically happen:
+          //    (from test/Transforms/Coroutines/coro-split-musttail.ll)
+          //
+          // %suspend = call i8 @llvm.coro.suspend(token %save, i1 false)
+          // switch i8 %suspend, label %exit [
+          //   i8 0, label %await.ready
+          //   i8 1, label %exit
+          // ]
+          if (IsCoroSuspendTarget) {
+            Weight = UINT64_MAX;
+          } else {
+            bool Critical = isCriticalEdge(TI, i);
+            uint64_t scaleFactor = BBWeight;
+            if (Critical) {
+              if (scaleFactor < UINT64_MAX / CriticalEdgeMultiplier)
+                scaleFactor *= CriticalEdgeMultiplier;
+              else
+                scaleFactor = UINT64_MAX;
+            }
+            if (BPI != nullptr)
+              Weight =
+                  BPI->getEdgeProbability(&BB, TargetBB).scale(scaleFactor);
+            if (Weight == 0)
+              Weight++;
           }
-          if (BPI != nullptr)
-            Weight = BPI->getEdgeProbability(&BB, TargetBB).scale(scaleFactor);
-          if (Weight == 0)
-            Weight++;
           auto *E = &addEdge(&BB, TargetBB, Weight);
           E->IsCritical = Critical;
+          // See comment above - we must guarantee the coro suspend BB isn't
+          // instrumented.
+          if (IsCoroSuspendTarget)
+            E->Removed = true;
----------------
mtrofin wrote:

I take that back. Fixing.

https://github.com/llvm/llvm-project/pull/71262