Add trace accessor methods, implement primitive if-conversion heuristic.

Compare the critical paths of the two traces through an if-conversion
candidate. If the difference is larger than the branch brediction
penalty, reject the if-conversion. If would never pay.

llvm-svn: 161433

lib/CodeGen/EarlyIfConversion.cpp

@@ -601,10 +601,24 @@ void EarlyIfConverter::invalidateTraces() {
 bool EarlyIfConverter::shouldConvertIf() {
   if (!MinInstr)
     MinInstr = Traces->getEnsemble(MachineTraceMetrics::TS_MinInstrCount);
-  DEBUG({
-    dbgs() << MinInstr->getTrace(IfConv.Head);
-    MinInstr->print(dbgs());
-  });
+
+  // MCSchedModel doesn't yet provide a misprediction penalty.
+  unsigned MispredictPenalty = 10;
+
+  // Compare the critical path through TBB and FBB. If the difference is
+  // greater than the branch misprediction penalty, it would never pay to
+  // if-convert. The triangle/diamond topology guarantees that these traces
+  // have the same head and tail, so they can be compared.
+  MachineTraceMetrics::Trace TBBTrace = MinInstr->getTrace(IfConv.TBB);
+  MachineTraceMetrics::Trace FBBTrace = MinInstr->getTrace(IfConv.FBB);
+  DEBUG(dbgs() << "TBB: " << TBBTrace << "FBB: " << FBBTrace);
+  unsigned TBBCrit = TBBTrace.getCriticalPath();
+  unsigned FBBCrit = FBBTrace.getCriticalPath();
+  unsigned ExtraCrit = TBBCrit > FBBCrit ? TBBCrit-FBBCrit : FBBCrit-TBBCrit;
+  if (ExtraCrit >= MispredictPenalty) {
+    DEBUG(dbgs() << "Critical path difference too large.\n");
+    return false;
+  }
   return true;
 }
 

lib/CodeGen/MachineTraceMetrics.cpp

@@ -14,6 +14,7 @@
 #include "llvm/CodeGen/MachineLoopInfo.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/CodeGen/Passes.h"
+#include "llvm/MC/MCInstrItineraries.h"
 #include "llvm/Target/TargetInstrInfo.h"
 #include "llvm/Target/TargetRegisterInfo.h"
 #include "llvm/Support/Debug.h"
@@ -1017,6 +1018,29 @@ MachineTraceMetrics::Ensemble::getTrace(const MachineBasicBlock *MBB) {
   return Trace(*this, BlockInfo[MBB->getNumber()]);
 }
 
+unsigned
+MachineTraceMetrics::Trace::getInstrSlack(const MachineInstr *MI) const {
+  assert(MI && "Not an instruction.");
+  assert(getBlockNum() == unsigned(MI->getParent()->getNumber()) &&
+         "MI must be in the trace center block");
+  InstrCycles Cyc = getInstrCycles(MI);
+  return getCriticalPath() - (Cyc.Depth + Cyc.Height);
+}
+
+unsigned MachineTraceMetrics::Trace::getResourceDepth(bool Bottom) const {
+  // For now, we compute the resource depth from instruction count / issue
+  // width. Eventually, we should compute resource depth per functional unit
+  // and return the max.
+  unsigned Instrs = TBI.InstrDepth;
+  if (Bottom)
+    Instrs += TE.MTM.BlockInfo[getBlockNum()].InstrCount;
+  if (const MCSchedModel *Model = TE.MTM.ItinData->SchedModel)
+    if (Model->IssueWidth != 0)
+      return Instrs / Model->IssueWidth;
+  // Assume issue width 1 without a schedule model.
+  return Instrs;
+}
+
 void MachineTraceMetrics::Ensemble::print(raw_ostream &OS) const {
   OS << getName() << " ensemble:\n";
   for (unsigned i = 0, e = BlockInfo.size(); i != e; ++i) {

lib/CodeGen/MachineTraceMetrics.h

@@ -188,23 +188,6 @@ public:
     void print(raw_ostream&) const;
   };
 
-  /// A trace represents a plausible sequence of executed basic blocks that
-  /// passes through the current basic block one. The Trace class serves as a
-  /// handle to internal cached data structures.
-  class Trace {
-    Ensemble &TE;
-    TraceBlockInfo &TBI;
-
-  public:
-    explicit Trace(Ensemble &te, TraceBlockInfo &tbi) : TE(te), TBI(tbi) {}
-    void print(raw_ostream&) const;
-
-    /// Compute the total number of instructions in the trace.
-    unsigned getInstrCount() const {
-      return TBI.InstrDepth + TBI.InstrHeight;
-    }
-  };
-
   /// InstrCycles represents the cycle height and depth of an instruction in a
   /// trace.
   struct InstrCycles {
@@ -218,6 +201,48 @@ public:
     unsigned Height;
   };
 
+  /// A trace represents a plausible sequence of executed basic blocks that
+  /// passes through the current basic block one. The Trace class serves as a
+  /// handle to internal cached data structures.
+  class Trace {
+    Ensemble &TE;
+    TraceBlockInfo &TBI;
+
+    unsigned getBlockNum() const { return &TBI - &TE.BlockInfo[0]; }
+
+  public:
+    explicit Trace(Ensemble &te, TraceBlockInfo &tbi) : TE(te), TBI(tbi) {}
+    void print(raw_ostream&) const;
+
+    /// Compute the total number of instructions in the trace.
+    unsigned getInstrCount() const {
+      return TBI.InstrDepth + TBI.InstrHeight;
+    }
+
+    /// Return the resource dpeth of the top/bottom of the trace center block.
+    /// This is the number of cycles required to execute all instructions from
+    /// the trace head to the trace center block. The resource depth only
+    /// considers execution resources, it ignores data dependencies.
+    /// When Bottom is set, instructions in the trace center block are included.
+    unsigned getResourceDepth(bool Bottom) const;
+
+    /// Return the length of the (data dependency) critical path through the
+    /// trace.
+    unsigned getCriticalPath() const { return TBI.CriticalPath; }
+
+    /// Return the depth and height of MI. The depth is only valid for
+    /// instructions in or above the trace center block. The height is only
+    /// valid for instructions in or below the trace center block.
+    InstrCycles getInstrCycles(const MachineInstr *MI) const {
+      return TE.Cycles.lookup(MI);
+    }
+
+    /// Return the slack of MI. This is the number of cycles MI can be delayed
+    /// before the critical path becomes longer.
+    /// MI must be an instruction in the trace center block.
+    unsigned getInstrSlack(const MachineInstr *MI) const;
+  };
+
   /// A trace ensemble is a collection of traces selected using the same
   /// strategy, for example 'minimum resource height'. There is one trace for
   /// every block in the function.