[MachineTraceMetrics] Add local strategy

[asi-sc]

Summary

This strategy makes each trace local to the basic block. For in-order cores some
heuristics work better when we do local decisions. For example, MachineCombiner
may expect that instructions outside the current basic block do not lengthen
the critical path when we execute instructions in order or the core has a
small re-order buffer.

This patch only introduce the strategy, real use-case is added in the further
pathes.

Depends on D140539

[asi-sc]

Rebase

[asi-sc]

Ping

[spatel]

llvm/lib/CodeGen/MachineTraceMetrics.cpp

321–322

I see that this is replicating the previous code comment, but we shouldn't repeat the class/variable/function name in the block comment, and it should probably use "///" to produce the expected doxygen output.

[spatel]

LGTM - see inline comment for adjustment to code comment.

llvm/include/llvm/CodeGen/MachineTraceMetrics.h

struct LiveRegUnit {
 
   LiveRegUnit(unsigned RU) : RegUnit(RU) {}
 };
 
 /// Strategies for selecting traces.
 enum class MachineTraceStrategy {
   /// Select the trace through a block that has the fewest instructions.
   TS_MinInstrCount,
+  /// Select the trace that contains only the current basic block. For instance,
+  /// this strategy can be used by MachineCombiner to make better decisions when
+  /// we estimate critical path for in-order cores.
+  TS_Local,
   TS_NumStrategies
 };
 
 class MachineTraceMetrics : public MachineFunctionPass {
   const MachineFunction *MF = nullptr;
   const TargetInstrInfo *TII = nullptr;
   const TargetRegisterInfo *TRI = nullptr;
   const MachineRegisterInfo *MRI = nullptr;

llvm/lib/CodeGen/MachineTraceMetrics.cpp

class MinInstrCountEnsemble : public MachineTraceMetrics::Ensemble {
   const MachineBasicBlock *pickTracePred(const MachineBasicBlock*) override;
   const MachineBasicBlock *pickTraceSucc(const MachineBasicBlock*) override;
 
 public:
   MinInstrCountEnsemble(MachineTraceMetrics *mtm)
     : MachineTraceMetrics::Ensemble(mtm) {}
 };
 
+/// Pick only the current basic block for the trace and do not choose any
+/// predecessors/successors.
+class LocalEnsemble : public MachineTraceMetrics::Ensemble {
+  const char *getName() const override { return "Local"; }
+  const MachineBasicBlock *pickTracePred(const MachineBasicBlock *) override {
+    return nullptr;
+  };
+  const MachineBasicBlock *pickTraceSucc(const MachineBasicBlock *) override {
+    return nullptr;
+  };
+
+public:
+  LocalEnsemble(MachineTraceMetrics *MTM)
+      : MachineTraceMetrics::Ensemble(MTM) {}
+};
 } // end anonymous namespace
 
 // Select the preferred predecessor for MBB.
 const MachineBasicBlock*
 MinInstrCountEnsemble::pickTracePred(const MachineBasicBlock *MBB) {
   if (MBB->pred_empty())
     return nullptr;
   const MachineLoop *CurLoop = getLoopFor(MBB);
MachineTraceMetrics::getEnsemble(MachineTraceStrategy strategy) {
   Ensemble *&E = Ensembles[static_cast<size_t>(strategy)];
   if (E)
     return E;
 
   // Allocate new Ensemble on demand.
   switch (strategy) {
   case MachineTraceStrategy::TS_MinInstrCount:
     return (E = new MinInstrCountEnsemble(this));
+  case MachineTraceStrategy::TS_Local:
+    return (E = new LocalEnsemble(this));
   default: llvm_unreachable("Invalid trace strategy enum");
   }
 }
 
 void MachineTraceMetrics::invalidate(const MachineBasicBlock *MBB) {
   LLVM_DEBUG(dbgs() << "Invalidate traces through " << printMBBReference(*MBB)
                     << '\n');
   BlockInfo[MBB->getNumber()].invalidate();