[llvm] [MacroFusion] Early return when insts already clustered (PR #191710)

[Tomer Shafir via llvm-commits]

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@@ -52,22 +55,15 @@ bool llvm::hasLessThanNumFused(const SUnit &SU, unsigned FuseLimit) {
 
 bool llvm::fuseInstructionPair(ScheduleDAGInstrs &DAG, SUnit &FirstSU,
                                SUnit &SecondSU) {
-  // Check that neither instr is already paired with another along the edge
-  // between them.
-  for (SDep &SI : FirstSU.Succs)
-    if (SI.isCluster())
-      return false;
-
-  for (SDep &SI : SecondSU.Preds)
-    if (SI.isCluster())
-      return false;
-
-  assert(FirstSU.ParentClusterIdx == InvalidClusterId &&
-         SecondSU.ParentClusterIdx == InvalidClusterId);
-
-  // Though the reachability checks above could be made more generic,
-  // perhaps as part of ScheduleDAGInstrs::addEdge(), since such edges are valid,
-  // the extra computation cost makes it less interesting in general cases.
+  // Check that neither instr is already associated with a cluster (either
+  // fusion or non-fusion)
+  if (FirstSU.isClustered() || SecondSU.isClustered()) {
+    ++NumFusionConflicts;
+    LLVM_DEBUG(dbgs() << "Fusion conflict: cannot fuse SU(" << FirstSU.NodeNum
+                      << ") and SU(" << SecondSU.NodeNum
+                      << ") - already clustered\n");
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tomershafir wrote:

So I think printing the `IsClustered` status for both in any case would be a decent solution, keeping the code simple and getting the visibility

https://github.com/llvm/llvm-project/pull/191710