Start scaffolding for a MachineTraceMetrics analysis pass.

This is still a work in progress.

Out-of-order CPUs usually execute instructions from multiple basic
blocks simultaneously, so it is necessary to look at longer traces when
estimating the performance effects of code transformations.

The MachineTraceMetrics analysis will pick a typical trace through a
given basic block and provide performance metrics for the trace. Metrics
will include:

- Instruction count through the trace.
- Issue count per functional unit.
- Critical path length, and per-instruction 'slack'.

These metrics can be used to determine the performance limiting factor
when executing the trace, and how it will be affected by a code
transformation.

Initially, this will be used by the early if-conversion pass.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@160796 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/Passes.h

@@ -392,6 +392,10 @@ namespace llvm {
   /// into tails of their predecessors.
   extern char &TailDuplicateID;
 
+  /// MachineTraceMetrics - This pass computes critical path and CPU resource
+  /// usage in an ensemble of traces.
+  extern char &MachineTraceMetricsID;
+
   /// EarlyIfConverter - This pass performs if-conversion on SSA form by
   /// inserting cmov instructions.
   extern char &EarlyIfConverterID;

include/llvm/InitializePasses.h

@@ -172,6 +172,7 @@ void initializeMachineLoopRangesPass(PassRegistry&);
 void initializeMachineModuleInfoPass(PassRegistry&);
 void initializeMachineSchedulerPass(PassRegistry&);
 void initializeMachineSinkingPass(PassRegistry&);
+void initializeMachineTraceMetricsPass(PassRegistry&);
 void initializeMachineVerifierPassPass(PassRegistry&);
 void initializeMemCpyOptPass(PassRegistry&);
 void initializeMemDepPrinterPass(PassRegistry&);

lib/CodeGen/CMakeLists.txt

@@ -61,6 +61,7 @@ add_llvm_library(LLVMCodeGen
   MachineSSAUpdater.cpp
   MachineScheduler.cpp
   MachineSink.cpp
+  MachineTraceMetrics.cpp
   MachineVerifier.cpp
   OcamlGC.cpp
   OptimizePHIs.cpp

lib/CodeGen/EarlyIfConversion.cpp

@@ -17,6 +17,7 @@
 //===----------------------------------------------------------------------===//
 
 #define DEBUG_TYPE "early-ifcvt"
+#include "MachineTraceMetrics.h"
 #include "llvm/Function.h"
 #include "llvm/ADT/BitVector.h"
 #include "llvm/ADT/PostOrderIterator.h"
@@ -515,6 +516,8 @@ class EarlyIfConverter : public MachineFunctionPass {
   MachineRegisterInfo *MRI;
   MachineDominatorTree *DomTree;
   MachineLoopInfo *Loops;
+  MachineTraceMetrics *Traces;
+  MachineTraceMetrics::Ensemble *MinInstr;
   SSAIfConv IfConv;
 
 public:
@@ -527,6 +530,8 @@ private:
   bool tryConvertIf(MachineBasicBlock*);
   void updateDomTree(ArrayRef<MachineBasicBlock*> Removed);
   void updateLoops(ArrayRef<MachineBasicBlock*> Removed);
+  void invalidateTraces();
+  bool shouldConvertIf();
 };
 } // end anonymous namespace
 
@@ -537,6 +542,7 @@ INITIALIZE_PASS_BEGIN(EarlyIfConverter,
                       "early-ifcvt", "Early If Converter", false, false)
 INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
 INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
+INITIALIZE_PASS_DEPENDENCY(MachineTraceMetrics)
 INITIALIZE_PASS_END(EarlyIfConverter,
                       "early-ifcvt", "Early If Converter", false, false)
 
@@ -546,6 +552,8 @@ void EarlyIfConverter::getAnalysisUsage(AnalysisUsage &AU) const {
   AU.addPreserved<MachineDominatorTree>();
   AU.addRequired<MachineLoopInfo>();
   AU.addPreserved<MachineLoopInfo>();
+  AU.addRequired<MachineTraceMetrics>();
+  AU.addPreserved<MachineTraceMetrics>();
   MachineFunctionPass::getAnalysisUsage(AU);
 }
 
@@ -576,12 +584,31 @@ void EarlyIfConverter::updateLoops(ArrayRef<MachineBasicBlock*> Removed) {
     Loops->removeBlock(Removed[i]);
 }
 
+/// Invalidate MachineTraceMetrics before if-conversion.
+void EarlyIfConverter::invalidateTraces() {
+  Traces->invalidate(IfConv.Head);
+  Traces->invalidate(IfConv.Tail);
+  Traces->invalidate(IfConv.TBB);
+  Traces->invalidate(IfConv.FBB);
+}
+
+/// Apply cost model and heuristics to the if-conversion in IfConv.
+/// Return true if the conversion is a good idea.
+///
+bool EarlyIfConverter::shouldConvertIf() {
+  if (!MinInstr)
+    MinInstr = Traces->getEnsemble(MachineTraceMetrics::TS_MinInstrCount);
+  DEBUG(dbgs() << MinInstr->getTrace(IfConv.Head));
+  return true;
+}
+
 /// Attempt repeated if-conversion on MBB, return true if successful.
 ///
 bool EarlyIfConverter::tryConvertIf(MachineBasicBlock *MBB) {
   bool Changed = false;
-  while (IfConv.canConvertIf(MBB)) {
+  while (IfConv.canConvertIf(MBB) && shouldConvertIf()) {
     // If-convert MBB and update analyses.
+    invalidateTraces();
     SmallVector<MachineBasicBlock*, 4> RemovedBlocks;
     IfConv.convertIf(RemovedBlocks);
     Changed = true;
@@ -600,6 +627,8 @@ bool EarlyIfConverter::runOnMachineFunction(MachineFunction &MF) {
   MRI = &MF.getRegInfo();
   DomTree = &getAnalysis<MachineDominatorTree>();
   Loops = getAnalysisIfAvailable<MachineLoopInfo>();
+  Traces = &getAnalysis<MachineTraceMetrics>();
+  MinInstr = 0;
 
   bool Changed = false;
   IfConv.runOnMachineFunction(MF);

lib/CodeGen/MachineTraceMetrics.cpp

@@ -0,0 +1,477 @@
+//===- lib/CodeGen/MachineTraceMetrics.cpp ----------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "early-ifcvt"
+#include "MachineTraceMetrics.h"
+#include "llvm/CodeGen/MachineBasicBlock.h"
+#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
+#include "llvm/CodeGen/MachineLoopInfo.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/Target/TargetInstrInfo.h"
+#include "llvm/Target/TargetRegisterInfo.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/raw_ostream.h"
+#include "llvm/ADT/PostOrderIterator.h"
+
+using namespace llvm;
+
+char MachineTraceMetrics::ID = 0;
+char &llvm::MachineTraceMetricsID = MachineTraceMetrics::ID;
+
+INITIALIZE_PASS_BEGIN(MachineTraceMetrics,
+                  "machine-trace-metrics", "Machine Trace Metrics", false, true)
+INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
+INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
+INITIALIZE_PASS_END(MachineTraceMetrics,
+                  "machine-trace-metrics", "Machine Trace Metrics", false, true)
+
+MachineTraceMetrics::MachineTraceMetrics()
+  : MachineFunctionPass(ID), TII(0), TRI(0), MRI(0), Loops(0) {
+  std::fill(Ensembles, array_endof(Ensembles), (Ensemble*)0);
+}
+
+void MachineTraceMetrics::getAnalysisUsage(AnalysisUsage &AU) const {
+  AU.setPreservesAll();
+  AU.addRequired<MachineBranchProbabilityInfo>();
+  AU.addRequired<MachineLoopInfo>();
+  MachineFunctionPass::getAnalysisUsage(AU);
+}
+
+bool MachineTraceMetrics::runOnMachineFunction(MachineFunction &MF) {
+  TII = MF.getTarget().getInstrInfo();
+  TRI = MF.getTarget().getRegisterInfo();
+  MRI = &MF.getRegInfo();
+  Loops = &getAnalysis<MachineLoopInfo>();
+  unsigned NumBlocks = MF.getNumBlockIDs();
+  BlockInfo.resize(NumBlocks);
+  return false;
+}
+
+void MachineTraceMetrics::releaseMemory() {
+  BlockInfo.clear();
+  for (unsigned i = 0; i != TS_NumStrategies; ++i) {
+    delete Ensembles[i];
+    Ensembles[i] = 0;
+  }
+}
+
+//===----------------------------------------------------------------------===//
+//                          Fixed block information
+//===----------------------------------------------------------------------===//
+//
+// The number of instructions in a basic block and the CPU resources used by
+// those instructions don't depend on any given trace strategy.
+
+/// Is MI an instruction that should be considered free because it will likely
+/// be eliminated by later passes?
+static bool isFree(const MachineInstr *MI) {
+  switch(MI->getOpcode()) {
+  default: return false;
+  case TargetOpcode::PHI:
+  case TargetOpcode::PROLOG_LABEL:
+  case TargetOpcode::EH_LABEL:
+  case TargetOpcode::GC_LABEL:
+  case TargetOpcode::KILL:
+  case TargetOpcode::EXTRACT_SUBREG:
+  case TargetOpcode::INSERT_SUBREG:
+  case TargetOpcode::IMPLICIT_DEF:
+  case TargetOpcode::SUBREG_TO_REG:
+  case TargetOpcode::COPY_TO_REGCLASS:
+  case TargetOpcode::DBG_VALUE:
+  case TargetOpcode::REG_SEQUENCE:
+  case TargetOpcode::COPY:
+    return true;
+  }
+}
+
+/// Compute the resource usage in basic block MBB.
+const MachineTraceMetrics::FixedBlockInfo*
+MachineTraceMetrics::getResources(const MachineBasicBlock *MBB) {
+  assert(MBB && "No basic block");
+  FixedBlockInfo *FBI = &BlockInfo[MBB->getNumber()];
+  if (FBI->hasResources())
+    return FBI;
+
+  // Compute resource usage in the block.
+  // FIXME: Compute per-functional unit counts.
+  FBI->HasCalls = false;
+  unsigned InstrCount = 0;
+  for (MachineBasicBlock::const_iterator I = MBB->begin(), E = MBB->end();
+       I != E; ++I) {
+    const MachineInstr *MI = I;
+    if (isFree(MI))
+      continue;
+    ++InstrCount;
+    if (MI->isCall())
+      FBI->HasCalls = true;
+  }
+  FBI->InstrCount = InstrCount;
+  return FBI;
+}
+
+//===----------------------------------------------------------------------===//
+//                         Ensemble utility functions
+//===----------------------------------------------------------------------===//
+
+MachineTraceMetrics::Ensemble::Ensemble(MachineTraceMetrics *ct)
+  : CT(*ct) {
+  BlockInfo.resize(CT.BlockInfo.size());
+}
+
+// Virtual destructor serves as an anchor.
+MachineTraceMetrics::Ensemble::~Ensemble() {}
+
+MachineLoop*
+MachineTraceMetrics::Ensemble::getLoopFor(const MachineBasicBlock *MBB) {
+  return CT.Loops->getLoopFor(MBB);
+}
+
+// Update resource-related information in the TraceBlockInfo for MBB.
+// Only update resources related to the trace above MBB.
+void MachineTraceMetrics::Ensemble::
+computeDepthResources(const MachineBasicBlock *MBB) {
+  TraceBlockInfo *TBI = &BlockInfo[MBB->getNumber()];
+
+  // Compute resources from trace above. The top block is simple.
+  if (!TBI->Pred) {
+    TBI->InstrDepth = 0;
+    return;
+  }
+
+  // Compute from the block above. A post-order traversal ensures the
+  // predecessor is always computed first.
+  TraceBlockInfo *PredTBI = &BlockInfo[TBI->Pred->getNumber()];
+  assert(PredTBI->hasValidDepth() && "Trace above has not been computed yet");
+  const FixedBlockInfo *PredFBI = CT.getResources(TBI->Pred);
+  TBI->InstrDepth = PredTBI->InstrDepth + PredFBI->InstrCount;
+}
+
+// Update resource-related information in the TraceBlockInfo for MBB.
+// Only update resources related to the trace below MBB.
+void MachineTraceMetrics::Ensemble::
+computeHeightResources(const MachineBasicBlock *MBB) {
+  TraceBlockInfo *TBI = &BlockInfo[MBB->getNumber()];
+
+  // Compute resources for the current block.
+  TBI->InstrHeight = CT.getResources(MBB)->InstrCount;
+
+  // The trace tail is done.
+  if (!TBI->Succ)
+    return;
+
+  // Compute from the block below. A post-order traversal ensures the
+  // predecessor is always computed first.
+  TraceBlockInfo *SuccTBI = &BlockInfo[TBI->Succ->getNumber()];
+  assert(SuccTBI->hasValidHeight() && "Trace below has not been computed yet");
+  TBI->InstrHeight += SuccTBI->InstrHeight;
+}
+
+// Check if depth resources for MBB are valid and return the TBI.
+// Return NULL if the resources have been invalidated.
+const MachineTraceMetrics::TraceBlockInfo*
+MachineTraceMetrics::Ensemble::
+getDepthResources(const MachineBasicBlock *MBB) const {
+  const TraceBlockInfo *TBI = &BlockInfo[MBB->getNumber()];
+  return TBI->hasValidDepth() ? TBI : 0;
+}
+
+// Check if height resources for MBB are valid and return the TBI.
+// Return NULL if the resources have been invalidated.
+const MachineTraceMetrics::TraceBlockInfo*
+MachineTraceMetrics::Ensemble::
+getHeightResources(const MachineBasicBlock *MBB) const {
+  const TraceBlockInfo *TBI = &BlockInfo[MBB->getNumber()];
+  return TBI->hasValidHeight() ? TBI : 0;
+}
+
+//===----------------------------------------------------------------------===//
+//                         Trace Selection Strategies
+//===----------------------------------------------------------------------===//
+//
+// A trace selection strategy is implemented as a sub-class of Ensemble. The
+// trace through a block B is computed by two DFS traversals of the CFG
+// starting from B. One upwards, and one downwards. During the upwards DFS,
+// pickTracePred() is called on the post-ordered blocks. During the downwards
+// DFS, pickTraceSucc() is called in a post-order.
+//
+
+// MinInstrCountEnsemble - Pick the trace that executes the least number of
+// instructions.
+namespace {
+class MinInstrCountEnsemble : public MachineTraceMetrics::Ensemble {
+  const char *getName() { return "MinInstr"; }
+  const MachineBasicBlock *pickTracePred(const MachineBasicBlock*);
+  const MachineBasicBlock *pickTraceSucc(const MachineBasicBlock*);
+
+public:
+  MinInstrCountEnsemble(MachineTraceMetrics *ct)
+    : MachineTraceMetrics::Ensemble(ct) {}
+};
+}
+
+// Select the preferred predecessor for MBB.
+const MachineBasicBlock*
+MinInstrCountEnsemble::pickTracePred(const MachineBasicBlock *MBB) {
+  if (MBB->pred_empty())
+    return 0;
+  MachineLoop *CurLoop = getLoopFor(MBB);
+  // Don't leave loops, and never follow back-edges.
+  if (CurLoop && MBB == CurLoop->getHeader())
+    return 0;
+  unsigned CurCount = CT.getResources(MBB)->InstrCount;
+  const MachineBasicBlock *Best = 0;
+  unsigned BestDepth = 0;
+  for (MachineBasicBlock::const_pred_iterator
+       I = MBB->pred_begin(), E = MBB->pred_end(); I != E; ++I) {
+    const MachineBasicBlock *Pred = *I;
+    const MachineTraceMetrics::TraceBlockInfo *PredTBI =
+      getDepthResources(Pred);
+    // Ignore invalidated predecessors. This never happens on the first scan,
+    // but if we rejected this predecessor earlier, it won't be revalidated.
+    if (!PredTBI)
+      continue;
+    // Don't consider predecessors in other loops.
+    if (getLoopFor(Pred) != CurLoop)
+      continue;
+    // Pick the predecessor that would give this block the smallest InstrDepth.
+    unsigned Depth = PredTBI->InstrDepth + CurCount;
+    if (!Best || Depth < BestDepth)
+      Best = Pred, BestDepth = Depth;
+  }
+  return Best;
+}
+
+// Select the preferred successor for MBB.
+const MachineBasicBlock*
+MinInstrCountEnsemble::pickTraceSucc(const MachineBasicBlock *MBB) {
+  if (MBB->pred_empty())
+    return 0;
+  MachineLoop *CurLoop = getLoopFor(MBB);
+  const MachineBasicBlock *Best = 0;
+  unsigned BestHeight = 0;
+  for (MachineBasicBlock::const_succ_iterator
+       I = MBB->succ_begin(), E = MBB->succ_end(); I != E; ++I) {
+    const MachineBasicBlock *Succ = *I;
+    const MachineTraceMetrics::TraceBlockInfo *SuccTBI =
+      getHeightResources(Succ);
+    // Ignore invalidated successors.
+    if (!SuccTBI)
+      continue;
+    // Don't consider back-edges.
+    if (CurLoop && Succ == CurLoop->getHeader())
+      continue;
+    // Don't consider successors in other loops.
+    if (getLoopFor(Succ) != CurLoop)
+      continue;
+    // Pick the successor that would give this block the smallest InstrHeight.
+    unsigned Height = SuccTBI->InstrHeight;
+    if (!Best || Height < BestHeight)
+      Best = Succ, BestHeight = Height;
+  }
+  return Best;
+}
+
+// Get an Ensemble sub-class for the requested trace strategy.
+MachineTraceMetrics::Ensemble *
+MachineTraceMetrics::getEnsemble(MachineTraceMetrics::Strategy strategy) {
+  assert(strategy < TS_NumStrategies && "Invalid trace strategy enum");
+  Ensemble *&E = Ensembles[strategy];
+  if (E)
+    return E;
+
+  // Allocate new Ensemble on demand.
+  switch (strategy) {
+  case TS_MinInstrCount: return (E = new MinInstrCountEnsemble(this));
+  default: llvm_unreachable("Invalid trace strategy enum");
+  }
+}
+
+void MachineTraceMetrics::invalidate(const MachineBasicBlock *MBB) {
+  DEBUG(dbgs() << "Invalidate traces through BB#" << MBB->getNumber() << '\n');
+  BlockInfo[MBB->getNumber()].invalidate();
+  for (unsigned i = 0; i != TS_NumStrategies; ++i)
+    if (Ensembles[i])
+      Ensembles[i]->invalidate(MBB);
+}
+
+//===----------------------------------------------------------------------===//
+//                               Trace building
+//===----------------------------------------------------------------------===//
+//
+// Traces are built by two CFG traversals. To avoid recomputing too much, use a
+// set abstraction that confines the search to the current loop, and doesn't
+// revisit blocks.
+
+namespace {
+struct LoopBounds {
+  MutableArrayRef<MachineTraceMetrics::TraceBlockInfo> Blocks;
+  const MachineLoopInfo *Loops;
+  const MachineLoop *CurLoop;
+  bool Downward;
+  LoopBounds(MutableArrayRef<MachineTraceMetrics::TraceBlockInfo> blocks,
+             const MachineLoopInfo *loops, const MachineLoop *curloop)
+    : Blocks(blocks), Loops(loops), CurLoop(curloop), Downward(false) {}
+};
+}
+
+// Specialize po_iterator_storage in order to prune the post-order traversal so
+// it is limited to the current loop and doesn't traverse the loop back edges.
+namespace llvm {
+template<>
+class po_iterator_storage<LoopBounds, true> {
+  LoopBounds &LB;
+public:
+  po_iterator_storage(LoopBounds &lb) : LB(lb) {}
+  void finishPostorder(const MachineBasicBlock*) {}
+
+  bool insertEdge(const MachineBasicBlock *From, const MachineBasicBlock *To) {
+    // Skip already visited To blocks.
+    MachineTraceMetrics::TraceBlockInfo &TBI = LB.Blocks[To->getNumber()];
+    if (LB.Downward ? TBI.hasValidHeight() : TBI.hasValidDepth())
+      return false;
+    // Don't follow CurLoop backedges.
+    if (LB.CurLoop && (LB.Downward ? To : From) == LB.CurLoop->getHeader())
+      return false;
+    // Don't leave CurLoop.
+    if (LB.Loops->getLoopFor(To) != LB.CurLoop)
+      return false;
+    // This is a new block. The PO traversal will compute height/depth
+    // resources, causing us to reject new edges to To. This only works because
+    // we reject back-edges, so the CFG is cycle-free.
+    return true;
+  }
+};
+}
+
+/// Compute the trace through MBB.
+void MachineTraceMetrics::Ensemble::computeTrace(const MachineBasicBlock *MBB) {
+  DEBUG(dbgs() << "Computing " << getName() << " trace through BB#"
+               << MBB->getNumber() << '\n');
+  // Set up loop bounds for the backwards post-order traversal.
+  LoopBounds Bounds(BlockInfo, CT.Loops, getLoopFor(MBB));
+
+  // Run an upwards post-order search for the trace start.
+  Bounds.Downward = false;
+  typedef ipo_ext_iterator<const MachineBasicBlock*, LoopBounds> UpwardPO;
+  for (UpwardPO I = ipo_ext_begin(MBB, Bounds), E = ipo_ext_end(MBB, Bounds);
+       I != E; ++I) {
+    DEBUG(dbgs() << "  pred for BB#" << I->getNumber() << ": ");
+    TraceBlockInfo &TBI = BlockInfo[I->getNumber()];
+    // All the predecessors have been visited, pick the preferred one.
+    TBI.Pred = pickTracePred(*I);
+    DEBUG({
+      if (TBI.Pred)
+        dbgs() << "BB#" << TBI.Pred->getNumber() << '\n';
+      else
+        dbgs() << "null\n";
+    });
+    // The trace leading to I is now known, compute the depth resources.
+    computeDepthResources(*I);
+  }
+
+  // Run a downwards post-order search for the trace end.
+  Bounds.Downward = true;
+  typedef po_ext_iterator<const MachineBasicBlock*, LoopBounds> DownwardPO;
+  for (DownwardPO I = po_ext_begin(MBB, Bounds), E = po_ext_end(MBB, Bounds);
+       I != E; ++I) {
+    DEBUG(dbgs() << "  succ for BB#" << I->getNumber() << ": ");
+    TraceBlockInfo &TBI = BlockInfo[I->getNumber()];
+    // All the successors have been visited, pick the preferred one.
+    BlockInfo[I->getNumber()].Succ = pickTraceSucc(*I);
+    DEBUG({
+      if (TBI.Pred)
+        dbgs() << "BB#" << TBI.Succ->getNumber() << '\n';
+      else
+        dbgs() << "null\n";
+    });
+    // The trace leaving I is now known, compute the height resources.
+    computeHeightResources(*I);
+  }
+}
+
+/// Invalidate traces through BadMBB.
+void
+MachineTraceMetrics::Ensemble::invalidate(const MachineBasicBlock *BadMBB) {
+  SmallVector<const MachineBasicBlock*, 16> WorkList;
+  TraceBlockInfo &BadTBI = BlockInfo[BadMBB->getNumber()];
+
+  // Invalidate height resources of blocks above MBB.
+  if (BadTBI.hasValidHeight()) {
+    BadTBI.invalidateHeight();
+    WorkList.push_back(BadMBB);
+    do {
+      const MachineBasicBlock *MBB = WorkList.pop_back_val();
+      DEBUG(dbgs() << "Invalidate BB#" << MBB->getNumber() << ' ' << getName()
+            << " height.\n");
+      // Find any MBB predecessors that have MBB as their preferred successor.
+      // They are the only ones that need to be invalidated.
+      for (MachineBasicBlock::const_pred_iterator
+           I = MBB->pred_begin(), E = MBB->pred_end(); I != E; ++I) {
+        TraceBlockInfo &TBI = BlockInfo[(*I)->getNumber()];
+        if (TBI.hasValidHeight() && TBI.Succ == MBB) {
+          TBI.invalidateHeight();
+          WorkList.push_back(*I);
+        }
+      }
+    } while (!WorkList.empty());
+  }
+
+  // Invalidate depth resources of blocks below MBB.
+  if (BadTBI.hasValidDepth()) {
+    BadTBI.invalidateDepth();
+    WorkList.push_back(BadMBB);
+    do {
+      const MachineBasicBlock *MBB = WorkList.pop_back_val();
+      DEBUG(dbgs() << "Invalidate BB#" << MBB->getNumber() << ' ' << getName()
+            << " depth.\n");
+      // Find any MBB successors that have MBB as their preferred predecessor.
+      // They are the only ones that need to be invalidated.
+      for (MachineBasicBlock::const_succ_iterator
+           I = MBB->succ_begin(), E = MBB->succ_end(); I != E; ++I) {
+        TraceBlockInfo &TBI = BlockInfo[(*I)->getNumber()];
+        if (TBI.hasValidDepth() && TBI.Pred == MBB) {
+          TBI.invalidateDepth();
+          WorkList.push_back(*I);
+        }
+      }
+    } while (!WorkList.empty());
+  }
+}
+
+
+MachineTraceMetrics::Trace
+MachineTraceMetrics::Ensemble::getTrace(const MachineBasicBlock *MBB) {
+  // FIXME: Check cache tags, recompute as needed.
+  computeTrace(MBB);
+  return Trace(*this, BlockInfo[MBB->getNumber()]);
+}
+
+void MachineTraceMetrics::Trace::print(raw_ostream &OS) const {
+  OS << TE.getName() << " trace:";
+  if (TBI.hasValidHeight() && TBI.hasValidDepth())
+    OS << ' ' << getInstrCount() << " instrs.";
+
+  const MachineTraceMetrics::TraceBlockInfo *Block = &TBI;
+  OS << "\n *";
+  while (Block->hasValidDepth() && Block->Pred) {
+    unsigned Num = Block->Pred->getNumber();
+    OS << " <- BB#" << Num;
+    Block = &TE.BlockInfo[Num];
+  }
+
+  Block = &TBI;
+  OS << "\n *";
+  while (Block->hasValidHeight() && Block->Succ) {
+    unsigned Num = Block->Succ->getNumber();
+    OS << " -> BB#" << Num;
+    Block = &TE.BlockInfo[Num];
+  }
+  OS << '\n';
+}

lib/CodeGen/MachineTraceMetrics.h

@@ -0,0 +1,218 @@
+//===- lib/CodeGen/MachineTraceMetrics.h - Super-scalar metrics -*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines the interface for the MachineTraceMetrics analysis pass
+// that estimates CPU resource usage and critical data dependency paths through
+// preferred traces. This is useful for super-scalar CPUs where execution speed
+// can be limited both by data dependencies and by limited execution resources.
+//
+// Out-of-order CPUs will often be executing instructions from multiple basic
+// blocks at the same time. This makes it difficult to estimate the resource
+// usage accurately in a single basic block. Resources can be estimated better
+// by looking at a trace through the current basic block.
+//
+// For every block, the MachineTraceMetrics pass will pick a preferred trace
+// that passes through the block. The trace is chosen based on loop structure,
+// branch probabilities, and resource usage. The intention is to pick likely
+// traces that would be the most affected by code transformations.
+//
+// It is expensive to compute a full arbitrary trace for every block, so to
+// save some computations, traces are chosen to be convergent. This means that
+// if the traces through basic blocks A and B ever cross when moving away from
+// A and B, they never diverge again. This applies in both directions - If the
+// traces meet above A and B, they won't diverge when going further back.
+//
+// Traces tend to align with loops. The trace through a block in an inner loop
+// will begin at the loop entry block and end at a back edge. If there are
+// nested loops, the trace may begin and end at those instead.
+//
+// For each trace, we compute the critical path length, which is the number of
+// cycles required to execute the trace when execution is limited by data
+// dependencies only. We also compute the resource height, which is the number
+// of cycles required to execute all instructions in the trace when ignoring
+// data dependencies.
+//
+// Every instruction in the current block has a slack - the number of cycles
+// execution of the instruction can be delayed without extending the critical
+// path.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CODEGEN_MACHINE_TRACE_METRICS_H
+#define LLVM_CODEGEN_MACHINE_TRACE_METRICS_H
+
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+
+namespace llvm {
+
+class TargetInstrInfo;
+class TargetRegisterInfo;
+class MachineBasicBlock;
+class MachineRegisterInfo;
+class MachineLoopInfo;
+class MachineLoop;
+class raw_ostream;
+
+class MachineTraceMetrics : public MachineFunctionPass {
+  const TargetInstrInfo *TII;
+  const TargetRegisterInfo *TRI;
+  const MachineRegisterInfo *MRI;
+  const MachineLoopInfo *Loops;
+
+public:
+  class Ensemble;
+  class Trace;
+  static char ID;
+  MachineTraceMetrics();
+  void getAnalysisUsage(AnalysisUsage&) const;
+  bool runOnMachineFunction(MachineFunction&);
+  void releaseMemory();
+
+  friend class Ensemble;
+  friend class Trace;
+
+  /// Per-basic block information that doesn't depend on the trace through the
+  /// block.
+  struct FixedBlockInfo {
+    /// The number of non-trivial instructions in the block.
+    /// Doesn't count PHI and COPY instructions that are likely to be removed.
+    unsigned InstrCount;
+
+    /// True when the block contains calls.
+    bool HasCalls;
+
+    FixedBlockInfo() : InstrCount(~0u), HasCalls(false) {}
+
+    /// Returns true when resource information for this block has been computed.
+    bool hasResources() const { return InstrCount != ~0u; }
+
+    /// Invalidate resource information.
+    void invalidate() { InstrCount = ~0u; }
+  };
+
+  /// Get the fixed resource information about MBB. Compute it on demand.
+  const FixedBlockInfo *getResources(const MachineBasicBlock*);
+
+  /// Per-basic block information that relates to a specific trace through the
+  /// block. Convergent traces means that only one of these is required per
+  /// block in a trace ensemble.
+  struct TraceBlockInfo {
+    /// Trace predecessor, or NULL for the first block in the trace.
+    const MachineBasicBlock *Pred;
+
+    /// Trace successor, or NULL for the last block in the trace.
+    const MachineBasicBlock *Succ;
+
+    /// Accumulated number of instructions in the trace above this block.
+    /// Does not include instructions in this block.
+    unsigned InstrDepth;
+
+    /// Accumulated number of instructions in the trace below this block.
+    /// Includes instructions in this block.
+    unsigned InstrHeight;
+
+    TraceBlockInfo() : Pred(0), Succ(0), InstrDepth(~0u), InstrHeight(~0u) {}
+
+    /// Returns true if the depth resources have been computed from the trace
+    /// above this block.
+    bool hasValidDepth() const { return InstrDepth != ~0u; }
+
+    /// Returns true if the height resources have been computed from the trace
+    /// below this block.
+    bool hasValidHeight() const { return InstrHeight != ~0u; }
+
+    /// Invalidate depth resources when some block above this one has changed.
+    void invalidateDepth() { InstrDepth = ~0u; }
+
+    /// Invalidate height resources when a block below this one has changed.
+    void invalidateHeight() { InstrHeight = ~0u; }
+  };
+
+  /// 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;
+    }
+  };
+
+  /// 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.
+  class Ensemble {
+    SmallVector<TraceBlockInfo, 4> BlockInfo;
+    friend class Trace;
+
+    void computeTrace(const MachineBasicBlock*);
+    void computeDepthResources(const MachineBasicBlock*);
+    void computeHeightResources(const MachineBasicBlock*);
+
+  protected:
+    MachineTraceMetrics &CT;
+    virtual const MachineBasicBlock *pickTracePred(const MachineBasicBlock*) =0;
+    virtual const MachineBasicBlock *pickTraceSucc(const MachineBasicBlock*) =0;
+    explicit Ensemble(MachineTraceMetrics*);
+    MachineLoop *getLoopFor(const MachineBasicBlock*);
+    const TraceBlockInfo *getDepthResources(const MachineBasicBlock*) const;
+    const TraceBlockInfo *getHeightResources(const MachineBasicBlock*) const;
+
+  public:
+    virtual ~Ensemble();
+    virtual const char *getName() =0;
+    void invalidate(const MachineBasicBlock *MBB);
+
+    /// Get the trace that passes through MBB.
+    /// The trace is computed on demand.
+    Trace getTrace(const MachineBasicBlock *MBB);
+  };
+
+  /// Strategies for selecting traces.
+  enum Strategy {
+    /// Select the trace through a block that has the fewest instructions.
+    TS_MinInstrCount,
+
+    TS_NumStrategies
+  };
+
+  /// Get the trace ensemble representing the given trace selection strategy.
+  /// The returned Ensemble object is owned by the MachineTraceMetrics analysis,
+  /// and valid for the lifetime of the analysis pass.
+  Ensemble *getEnsemble(Strategy);
+
+  /// Invalidate cached information about MBB. This must be called *before* MBB
+  /// is erased, or the CFG is otherwise changed.
+  void invalidate(const MachineBasicBlock *MBB);
+
+private:
+  // One entry per basic block, indexed by block number.
+  SmallVector<FixedBlockInfo, 4> BlockInfo;
+
+  // One ensemble per strategy.
+  Ensemble* Ensembles[TS_NumStrategies];
+};
+
+inline raw_ostream &operator<<(raw_ostream &OS,
+                               const MachineTraceMetrics::Trace &Tr) {
+  Tr.print(OS);
+  return OS;
+}
+
+} // end namespace llvm
+
+#endif