[NFC][MacroFusion] Adding the assertion if someone want to fuse more than 2 instructions.

[steven.zhang]

Summary

As discussed in https://reviews.llvm.org/D69998, we miss to create some dependency edges if chained more than 2 instructions. Adding an assertion here if someone want to chain more than 2 instructions.

[fhahn]

LGTM, thanks!

llvm/lib/CodeGen/MacroFusion.cpp

80

nit: maybe it would be slightly better to put a whole sentence here, e.g. something like 'Currently we only support chaining together 2 instructions.'

[steven.zhang]

llvm/lib/CodeGen/MacroFusion.cpp

80

ok, I will update it when commit it.

llvm/lib/CodeGen/MacroFusion.cpp

static bool fuseInstructionPair(ScheduleDAGInstrs &DAG, SUnit &FirstSU,
   // perhaps as part of ScheduleDAGInstrs::addEdge(), since such edges are valid,
   // the extra computation cost makes it less interesting in general cases.
 
   // Create a single weak edge between the adjacent instrs. The only effect is
   // to cause bottom-up scheduling to heavily prioritize the clustered instrs.
   if (!DAG.addEdge(&SecondSU, SDep(&FirstSU, SDep::Cluster)))
     return false;
 
+  // TODO - If we want to chain more than two instructions, we need to create
+  // artifical edges to make data dependencies from the FirstSU and dependencies
+  // of the SecondSU both dependent on other chained instructions, to prevent
+  // them from being scheduled into these chained instructions.
+  assert(hasLessThanNumFused(FirstSU, 2) &&
+         "Chain two instructions together at most");
+
   // Adjust the latency between both instrs.
   for (SDep &SI : FirstSU.Succs)
     if (SI.getSUnit() == &SecondSU)
       SI.setLatency(0);
 
   for (SDep &SI : SecondSU.Preds)
     if (SI.getSUnit() == &FirstSU)
       SI.setLatency(0);