Compute instruction heights through a trace.

The height on an instruction is the minimum number of cycles from the
instruction is issued to the end of the trace. Heights are computed for
all instructions in and below the trace center block.

The method for computing heights is different from the depth
computation. As we visit instructions in the trace bottom-up, heights of
used instructions are pushed upwards. This way, we avoid scanning long
use lists, looking for uses in the current trace.

At each basic block boundary, a list of live-in registers and their
minimum heights is saved in the trace block info. These live-in lists
are used when restarting depth computations on a trace that
converges with an already computed trace. They will also be used to
accurately compute the critical path length.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161138 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 246 insertions, 10 deletions

diff --git a/lib/CodeGen/MachineTraceMetrics.cpp b/lib/CodeGen/MachineTraceMetrics.cpp
index 0711614966..272a55fb94 100644
--- a/lib/CodeGen/MachineTraceMetrics.cpp
+++ b/lib/CodeGen/MachineTraceMetrics.cpp
@@ -576,16 +576,19 @@ static void getPHIDeps(const MachineInstr *UseMI,
 }
 
 // Keep track of physreg data dependencies by recording each live register unit.
+// Associate each regunit with an instruction operand. Depending on the
+// direction instructions are scanned, it could be the operand that defined the
+// regunit, or the highest operand to read the regunit.
 namespace {
 struct LiveRegUnit {
   unsigned RegUnit;
-  unsigned DefOp;
-  const MachineInstr *DefMI;
+  unsigned Cycle;
+  const MachineInstr *MI;
+  unsigned Op;
 
   unsigned getSparseSetIndex() const { return RegUnit; }
 
-  LiveRegUnit(unsigned RU, const MachineInstr *MI = 0, unsigned OpNo = 0)
-    : RegUnit(RU), DefOp(OpNo), DefMI(MI) {}
+  LiveRegUnit(unsigned RU) : RegUnit(RU), Cycle(0), MI(0), Op(0) {}
 };
 }
 
@@ -619,7 +622,7 @@ static void updatePhysDepsDownwards(const MachineInstr *UseMI,
       SparseSet<LiveRegUnit>::iterator I = RegUnits.find(*Units);
       if (I == RegUnits.end())
         continue;
-      Deps.push_back(DataDep(I->DefMI, I->DefOp, MO.getOperandNo()));
+      Deps.push_back(DataDep(I->MI, I->Op, MO.getOperandNo()));
       break;
     }
   }
@@ -636,15 +639,12 @@ static void updatePhysDepsDownwards(const MachineInstr *UseMI,
     for (MCRegUnitIterator Units(UseMI->getOperand(DefOp).getReg(), TRI);
          Units.isValid(); ++Units) {
       LiveRegUnit &LRU = RegUnits[*Units];
-      LRU.DefMI = UseMI;
-      LRU.DefOp = DefOp;
+      LRU.MI = UseMI;
+      LRU.Op = DefOp;
     }
   }
 }
 
-
-
-
 /// Compute instruction depths for all instructions above or in MBB in its
 /// trace. This assumes that the trace through MBB has already been computed.
 void MachineTraceMetrics::Ensemble::
@@ -713,11 +713,247 @@ computeInstrDepths(const MachineBasicBlock *MBB) {
   }
 }
 
+// Identify physreg dependencies for MI when scanning instructions upwards.
+// Return the issue height of MI after considering any live regunits.
+// Height is the issue height computed from virtual register dependencies alone.
+static unsigned updatePhysDepsUpwards(const MachineInstr *MI, unsigned Height,
+                                      SparseSet<LiveRegUnit> &RegUnits,
+                                      const InstrItineraryData *ItinData,
+                                      const TargetInstrInfo *TII,
+                                      const TargetRegisterInfo *TRI) {
+  SmallVector<unsigned, 8> ReadOps;
+  for (ConstMIOperands MO(MI); MO.isValid(); ++MO) {
+    if (!MO->isReg())
+      continue;
+    unsigned Reg = MO->getReg();
+    if (!TargetRegisterInfo::isPhysicalRegister(Reg))
+      continue;
+    if (MO->readsReg())
+      ReadOps.push_back(MO.getOperandNo());
+    if (!MO->isDef())
+      continue;
+    // This is a def of Reg. Remove corresponding entries from RegUnits, and
+    // update MI Height to consider the physreg dependencies.
+    for (MCRegUnitIterator Units(Reg, TRI); Units.isValid(); ++Units) {
+      SparseSet<LiveRegUnit>::iterator I = RegUnits.find(*Units);
+      if (I == RegUnits.end())
+        continue;
+      unsigned DepHeight = I->Cycle;
+      if (!MI->isTransient()) {
+        // We may not know the UseMI of this dependency, if it came from the
+        // live-in list.
+        if (I->MI)
+          DepHeight += TII->computeOperandLatency(ItinData,
+                                                  MI, MO.getOperandNo(),
+                                                  I->MI, I->Op);
+        else
+          // No UseMI. Just use the MI latency instead.
+          DepHeight += TII->getInstrLatency(ItinData, MI);
+      }
+      Height = std::max(Height, DepHeight);
+      // This regunit is dead above MI.
+      RegUnits.erase(I);
+    }
+  }
+
+  // Now we know the height of MI. Update any regunits read.
+  for (unsigned i = 0, e = ReadOps.size(); i != e; ++i) {
+    unsigned Reg = MI->getOperand(ReadOps[i]).getReg();
+    for (MCRegUnitIterator Units(Reg, TRI); Units.isValid(); ++Units) {
+      LiveRegUnit &LRU = RegUnits[*Units];
+      // Set the height to the highest reader of the unit.
+      if (LRU.Cycle <= Height && LRU.MI != MI) {
+        LRU.Cycle = Height;
+        LRU.MI = MI;
+        LRU.Op = ReadOps[i];
+      }
+    }
+  }
+
+  return Height;
+}
+
+
+typedef DenseMap<const MachineInstr *, unsigned> MIHeightMap;
+
+// Push the height of DefMI upwards if required to match UseMI.
+// Return true if this is the first time DefMI was seen.
+static bool pushDepHeight(const DataDep &Dep,
+                          const MachineInstr *UseMI, unsigned UseHeight,
+                          MIHeightMap &Heights,
+                          const InstrItineraryData *ItinData,
+                          const TargetInstrInfo *TII) {
+  // Adjust height by Dep.DefMI latency.
+  if (!Dep.DefMI->isTransient())
+    UseHeight += TII->computeOperandLatency(ItinData, Dep.DefMI, Dep.DefOp,
+                                            UseMI, Dep.UseOp);
+
+  // Update Heights[DefMI] to be the maximum height seen.
+  MIHeightMap::iterator I;
+  bool New;
+  tie(I, New) = Heights.insert(std::make_pair(Dep.DefMI, UseHeight));
+  if (New)
+    return true;
+
+  // DefMI has been pushed before. Give it the max height.
+  if (I->second < UseHeight)
+    I->second = UseHeight;
+  return false;
+}
+
+/// Assuming that DefMI was used by Trace.back(), add it to the live-in lists
+/// of all the blocks in Trace. Stop when reaching the block that contains
+/// DefMI.
+void MachineTraceMetrics::Ensemble::
+addLiveIns(const MachineInstr *DefMI,
+           ArrayRef<const MachineBasicBlock*> Trace) {
+  assert(!Trace.empty() && "Trace should contain at least one block");
+  unsigned Reg = DefMI->getOperand(0).getReg();
+  assert(TargetRegisterInfo::isVirtualRegister(Reg));
+  const MachineBasicBlock *DefMBB = DefMI->getParent();
+
+  // Reg is live-in to all blocks in Trace that follow DefMBB.
+  for (unsigned i = Trace.size(); i; --i) {
+    const MachineBasicBlock *MBB = Trace[i-1];
+    if (MBB == DefMBB)
+      return;
+    TraceBlockInfo &TBI = BlockInfo[MBB->getNumber()];
+    // Just add the register. The height will be updated later.
+    TBI.LiveIns.push_back(Reg);
+  }
+}
+
+/// Compute instruction heights in the trace through MBB. This updates MBB and
+/// the blocks below it in the trace. It is assumed that the trace has already
+/// been computed.
+void MachineTraceMetrics::Ensemble::
+computeInstrHeights(const MachineBasicBlock *MBB) {
+  // The bottom of the trace may already be computed.
+  // Find the blocks that need updating.
+  SmallVector<const MachineBasicBlock*, 8> Stack;
+  do {
+    TraceBlockInfo &TBI = BlockInfo[MBB->getNumber()];
+    assert(TBI.hasValidHeight() && "Incomplete trace");
+    if (TBI.HasValidInstrHeights)
+      break;
+    Stack.push_back(MBB);
+    TBI.LiveIns.clear();
+    MBB = TBI.Succ;
+  } while (MBB);
+
+  // As we move upwards in the trace, keep track of instructions that are
+  // required by deeper trace instructions. Map MI -> height required so far.
+  MIHeightMap Heights;
+
+  // For physregs, the def isn't known when we see the use.
+  // Instead, keep track of the highest use of each regunit.
+  SparseSet<LiveRegUnit> RegUnits;
+  RegUnits.setUniverse(MTM.TRI->getNumRegUnits());
+
+  // If the bottom of the trace was already precomputed, initialize heights
+  // from its live-in list.
+  // MBB is the highest precomputed block in the trace.
+  if (MBB) {
+    TraceBlockInfo &TBI = BlockInfo[MBB->getNumber()];
+    for (unsigned i = 0, e = TBI.LiveIns.size(); i != e; ++i) {
+      LiveInReg LI = TBI.LiveIns[i];
+      if (TargetRegisterInfo::isVirtualRegister(LI.Reg)) {
+        // For virtual registers, the def latency is included.
+        unsigned &Height = Heights[MTM.MRI->getVRegDef(LI.Reg)];
+        if (Height < LI.Height)
+          Height = LI.Height;
+      } else {
+        // For register units, the def latency is not included because we don't
+        // know the def yet.
+        RegUnits[LI.Reg].Cycle = LI.Height;
+      }
+    }
+  }
+
+  // Go through the trace blocks in bottom-up order.
+  SmallVector<DataDep, 8> Deps;
+  for (;!Stack.empty(); Stack.pop_back()) {
+    MBB = Stack.back();
+    DEBUG(dbgs() << "Heights for BB#" << MBB->getNumber() << ":\n");
+    TraceBlockInfo &TBI = BlockInfo[MBB->getNumber()];
+    TBI.HasValidInstrHeights = true;
+
+    // Get dependencies from PHIs in the trace successor.
+    if (TBI.Succ) {
+      for (MachineBasicBlock::const_iterator
+           I = TBI.Succ->begin(), E = TBI.Succ->end();
+           I != E && !I->isPHI(); ++I) {
+        const MachineInstr *PHI = I;
+        Deps.clear();
+        getPHIDeps(PHI, Deps, MBB, MTM.MRI);
+        if (!Deps.empty())
+          if (pushDepHeight(Deps.front(), PHI, Cycles.lookup(PHI).Height,
+                        Heights, MTM.ItinData, MTM.TII))
+            addLiveIns(Deps.front().DefMI, Stack);
+      }
+    }
+
+    // Go through the block backwards.
+    for (MachineBasicBlock::const_iterator BI = MBB->end(), BB = MBB->begin();
+         BI != BB;) {
+      const MachineInstr *MI = --BI;
+
+      // Find the MI height as determined by virtual register uses in the
+      // trace below.
+      unsigned Cycle = 0;
+      MIHeightMap::iterator HeightI = Heights.find(MI);
+      if (HeightI != Heights.end()) {
+        Cycle = HeightI->second;
+        // We won't be seeing any more MI uses.
+        Heights.erase(HeightI);
+      }
+
+      // Don't process PHI deps. They depend on the specific predecessor, and
+      // we'll get them when visiting the predecessor.
+      Deps.clear();
+      bool HasPhysRegs = !MI->isPHI() && getDataDeps(MI, Deps, MTM.MRI);
+
+      // There may also be regunit dependencies to include in the height.
+      if (HasPhysRegs)
+        Cycle = updatePhysDepsUpwards(MI, Cycle, RegUnits,
+                                      MTM.ItinData, MTM.TII, MTM.TRI);
+
+      DEBUG(dbgs() << Cycle << '\t' << *MI);
+      Cycles[MI].Height = Cycle;
+
+      // Update the required height of any virtual registers read by MI.
+      for (unsigned i = 0, e = Deps.size(); i != e; ++i)
+        if (pushDepHeight(Deps[i], MI, Cycle, Heights, MTM.ItinData, MTM.TII))
+          addLiveIns(Deps[i].DefMI, Stack);
+    }
+
+    // Update virtual live-in heights. They were added by addLiveIns() with a 0
+    // height because the final height isn't known until now.
+    DEBUG(dbgs() << "BB#" << MBB->getNumber() <<  " Live-ins:");
+    for (unsigned i = 0, e = TBI.LiveIns.size(); i != e; ++i) {
+      LiveInReg &LIR = TBI.LiveIns[i];
+      const MachineInstr *DefMI = MTM.MRI->getVRegDef(LIR.Reg);
+      LIR.Height = Heights.lookup(DefMI);
+      DEBUG(dbgs() << ' ' << PrintReg(LIR.Reg) << '@' << LIR.Height);
+    }
+
+    // Transfer the live regunits to the live-in list.
+    for (SparseSet<LiveRegUnit>::const_iterator
+         RI = RegUnits.begin(), RE = RegUnits.end(); RI != RE; ++RI) {
+      TBI.LiveIns.push_back(LiveInReg(RI->RegUnit, RI->Cycle));
+      DEBUG(dbgs() << ' ' << PrintRegUnit(RI->RegUnit, MTM.TRI)
+                   << '@' << RI->Cycle);
+    }
+    DEBUG(dbgs() << '\n');
+  }
+}
+
 MachineTraceMetrics::Trace
 MachineTraceMetrics::Ensemble::getTrace(const MachineBasicBlock *MBB) {
   // FIXME: Check cache tags, recompute as needed.
   computeTrace(MBB);
   computeInstrDepths(MBB);
+  computeInstrHeights(MBB);
   return Trace(*this, BlockInfo[MBB->getNumber()]);
 }