Add experimental support for register unit liveness.

Instead of computing a live interval per physreg, LiveIntervals can
compute live intervals per register unit. This makes impossible the
confusing situation where aliasing registers could have overlapping live
intervals. It should also make fixed interferernce checking cheaper
since registers have fewer register units than aliases.

Live intervals for regunits are computed on demand, using MRI use-def
chains and the new LiveRangeCalc class. Only regunits live in to ABI
blocks are precomputed during LiveIntervals::runOnMachineFunction().

The regunit liveness computations don't depend on LiveVariables.

llvm-svn: 158029

include/llvm/CodeGen/LiveIntervalAnalysis.h

@@ -35,7 +35,9 @@
 namespace llvm {
 
   class AliasAnalysis;
+  class LiveRangeCalc;
   class LiveVariables;
+  class MachineDominatorTree;
   class MachineLoopInfo;
   class TargetRegisterInfo;
   class MachineRegisterInfo;
@@ -52,6 +54,8 @@ namespace llvm {
     AliasAnalysis *AA;
     LiveVariables* LV;
     SlotIndexes* Indexes;
+    MachineDominatorTree *DomTree;
+    LiveRangeCalc *LRCalc;
 
     /// Special pool allocator for VNInfo's (LiveInterval val#).
     ///
@@ -92,11 +96,14 @@ namespace llvm {
     /// block.
     SmallVector<std::pair<unsigned, unsigned>, 8> RegMaskBlocks;
 
+    /// RegUnitIntervals - Keep a live interval for each register unit as a way
+    /// of tracking fixed physreg interference.
+    SmallVector<LiveInterval*, 0> RegUnitIntervals;
+
   public:
     static char ID; // Pass identification, replacement for typeid
-    LiveIntervals() : MachineFunctionPass(ID) {
-      initializeLiveIntervalsPass(*PassRegistry::getPassRegistry());
-    }
+    LiveIntervals();
+    virtual ~LiveIntervals();
 
     // Calculate the spill weight to assign to a single instruction.
     static float getSpillWeight(bool isDef, bool isUse, unsigned loopDepth);
@@ -317,6 +324,34 @@ namespace llvm {
     bool checkRegMaskInterference(LiveInterval &LI,
                                   BitVector &UsableRegs);
 
+    // Register unit functions.
+    //
+    // Fixed interference occurs when MachineInstrs use physregs directly
+    // instead of virtual registers. This typically happens when passing
+    // arguments to a function call, or when instructions require operands in
+    // fixed registers.
+    //
+    // Each physreg has one or more register units, see MCRegisterInfo. We
+    // track liveness per register unit to handle aliasing registers more
+    // efficiently.
+
+    /// getRegUnit - Return the live range for Unit.
+    /// It will be computed if it doesn't exist.
+    LiveInterval &getRegUnit(unsigned Unit) {
+      LiveInterval *LI = RegUnitIntervals[Unit];
+      if (!LI) {
+        // Compute missing ranges on demand.
+        RegUnitIntervals[Unit] = LI = new LiveInterval(Unit, HUGE_VALF);
+        computeRegUnitInterval(LI);
+      }
+      return *LI;
+    }
+
+    /// trackingRegUnits - Does LiveIntervals curently track register units?
+    /// This function will be removed when regunit tracking is permanently
+    /// enabled.
+    bool trackingRegUnits() const { return !RegUnitIntervals.empty(); }
+
   private:
     /// computeIntervals - Compute live intervals.
     void computeIntervals();
@@ -360,6 +395,9 @@ namespace llvm {
     void printInstrs(raw_ostream &O) const;
     void dumpInstrs() const;
 
+    void computeLiveInRegUnits();
+    void computeRegUnitInterval(LiveInterval*);
+
     class HMEditor;
   };
 } // End llvm namespace

lib/CodeGen/LiveIntervalAnalysis.cpp

@@ -20,6 +20,7 @@
 #include "llvm/Value.h"
 #include "llvm/Analysis/AliasAnalysis.h"
 #include "llvm/CodeGen/LiveVariables.h"
+#include "llvm/CodeGen/MachineDominators.h"
 #include "llvm/CodeGen/MachineInstr.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/CodeGen/Passes.h"
@@ -33,6 +34,7 @@
 #include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/STLExtras.h"
+#include "LiveRangeCalc.h"
 #include <algorithm>
 #include <limits>
 #include <cmath>
@@ -42,6 +44,9 @@ using namespace llvm;
 static cl::opt<bool> DisableReMat("disable-rematerialization",
                                   cl::init(false), cl::Hidden);
 
+// Temporary option to enable regunit liveness.
+static cl::opt<bool> LiveRegUnits("live-regunits", cl::Hidden);
+
 STATISTIC(numIntervals , "Number of original intervals");
 
 char LiveIntervals::ID = 0;
@@ -61,12 +66,23 @@ void LiveIntervals::getAnalysisUsage(AnalysisUsage &AU) const {
   AU.addRequired<LiveVariables>();
   AU.addPreserved<LiveVariables>();
   AU.addPreservedID(MachineLoopInfoID);
+  if (LiveRegUnits)
+    AU.addRequiredTransitiveID(MachineDominatorsID);
   AU.addPreservedID(MachineDominatorsID);
   AU.addPreserved<SlotIndexes>();
   AU.addRequiredTransitive<SlotIndexes>();
   MachineFunctionPass::getAnalysisUsage(AU);
 }
 
+LiveIntervals::LiveIntervals() : MachineFunctionPass(ID),
+  DomTree(0), LRCalc(0) {
+  initializeLiveIntervalsPass(*PassRegistry::getPassRegistry());
+}
+
+LiveIntervals::~LiveIntervals() {
+  delete LRCalc;
+}
+
 void LiveIntervals::releaseMemory() {
   // Free the live intervals themselves.
   for (DenseMap<unsigned, LiveInterval*>::iterator I = R2IMap.begin(),
@@ -78,6 +94,10 @@ void LiveIntervals::releaseMemory() {
   RegMaskBits.clear();
   RegMaskBlocks.clear();
 
+  for (unsigned i = 0, e = RegUnitIntervals.size(); i != e; ++i)
+    delete RegUnitIntervals[i];
+  RegUnitIntervals.clear();
+
   // Release VNInfo memory regions, VNInfo objects don't need to be dtor'd.
   VNInfoAllocator.Reset();
 }
@@ -93,6 +113,10 @@ bool LiveIntervals::runOnMachineFunction(MachineFunction &fn) {
   AA = &getAnalysis<AliasAnalysis>();
   LV = &getAnalysis<LiveVariables>();
   Indexes = &getAnalysis<SlotIndexes>();
+  if (LiveRegUnits)
+    DomTree = &getAnalysis<MachineDominatorTree>();
+  if (LiveRegUnits && !LRCalc)
+    LRCalc = new LiveRangeCalc();
   AllocatableRegs = TRI->getAllocatableSet(fn);
   ReservedRegs = TRI->getReservedRegs(fn);
 
@@ -100,6 +124,10 @@ bool LiveIntervals::runOnMachineFunction(MachineFunction &fn) {
 
   numIntervals += getNumIntervals();
 
+  if (LiveRegUnits) {
+    computeLiveInRegUnits();
+  }
+
   DEBUG(dump());
   return true;
 }
@@ -115,6 +143,11 @@ void LiveIntervals::print(raw_ostream &OS, const Module* ) const {
       OS << '\n';
     }
 
+  // Dump the regunits.
+  for (unsigned i = 0, e = RegUnitIntervals.size(); i != e; ++i)
+    if (LiveInterval *LI = RegUnitIntervals[i])
+      OS << PrintRegUnit(i, TRI) << " = " << *LI << '\n';
+
   // Dump the virtregs.
   for (unsigned Reg = 0, RegE = MRI->getNumVirtRegs(); Reg != RegE; ++Reg)
     if (const LiveInterval *LI =
@@ -626,6 +659,103 @@ LiveInterval* LiveIntervals::createInterval(unsigned reg) {
   return new LiveInterval(reg, Weight);
 }
 
+
+//===----------------------------------------------------------------------===//
+//                           Register Unit Liveness
+//===----------------------------------------------------------------------===//
+//
+// Fixed interference typically comes from ABI boundaries: Function arguments
+// and return values are passed in fixed registers, and so are exception
+// pointers entering landing pads. Certain instructions require values to be
+// present in specific registers. That is also represented through fixed
+// interference.
+//
+
+/// computeRegUnitInterval - Compute the live interval of a register unit, based
+/// on the uses and defs of aliasing registers.  The interval should be empty,
+/// or contain only dead phi-defs from ABI blocks.
+void LiveIntervals::computeRegUnitInterval(LiveInterval *LI) {
+  unsigned Unit = LI->reg;
+
+  assert(LRCalc && "LRCalc not initialized.");
+  LRCalc->reset(MF, getSlotIndexes(), DomTree, &getVNInfoAllocator());
+
+  // The physregs aliasing Unit are the roots and their super-registers.
+  // Create all values as dead defs before extending to uses. Note that roots
+  // may share super-registers. That's OK because createDeadDefs() is
+  // idempotent. It is very rare for a register unit to have multiple roots, so
+  // uniquing super-registers is probably not worthwhile.
+  for (MCRegUnitRootIterator Roots(Unit, TRI); Roots.isValid(); ++Roots) {
+    unsigned Root = *Roots;
+    if (!MRI->reg_empty(Root))
+      LRCalc->createDeadDefs(LI, Root);
+    for (MCSuperRegIterator Supers(Root, TRI); Supers.isValid(); ++Supers) {
+      if (!MRI->reg_empty(*Supers))
+        LRCalc->createDeadDefs(LI, *Supers);
+    }
+  }
+
+  // Now extend LI to reach all uses.
+  // Ignore uses of reserved registers. We only track defs of those.
+  for (MCRegUnitRootIterator Roots(Unit, TRI); Roots.isValid(); ++Roots) {
+    unsigned Root = *Roots;
+    if (!isReserved(Root) && !MRI->reg_empty(Root))
+      LRCalc->extendToUses(LI, Root);
+    for (MCSuperRegIterator Supers(Root, TRI); Supers.isValid(); ++Supers) {
+      unsigned Reg = *Supers;
+      if (!isReserved(Reg) && !MRI->reg_empty(Reg))
+        LRCalc->extendToUses(LI, Reg);
+    }
+  }
+}
+
+
+/// computeLiveInRegUnits - Precompute the live ranges of any register units
+/// that are live-in to an ABI block somewhere. Register values can appear
+/// without a corresponding def when entering the entry block or a landing pad.
+///
+void LiveIntervals::computeLiveInRegUnits() {
+  RegUnitIntervals.resize(TRI->getNumRegUnits());
+  DEBUG(dbgs() << "Computing live-in reg-units in ABI blocks.\n");
+
+  // Keep track of the intervals allocated.
+  SmallVector<LiveInterval*, 8> NewIntvs;
+
+  // Check all basic blocks for live-ins.
+  for (MachineFunction::const_iterator MFI = MF->begin(), MFE = MF->end();
+       MFI != MFE; ++MFI) {
+    const MachineBasicBlock *MBB = MFI;
+
+    // We only care about ABI blocks: Entry + landing pads.
+    if ((MFI != MF->begin() && !MBB->isLandingPad()) || MBB->livein_empty())
+      continue;
+
+    // Create phi-defs at Begin for all live-in registers.
+    SlotIndex Begin = Indexes->getMBBStartIdx(MBB);
+    DEBUG(dbgs() << Begin << "\tBB#" << MBB->getNumber());
+    for (MachineBasicBlock::livein_iterator LII = MBB->livein_begin(),
+         LIE = MBB->livein_end(); LII != LIE; ++LII) {
+      for (MCRegUnitIterator Units(*LII, TRI); Units.isValid(); ++Units) {
+        unsigned Unit = *Units;
+        LiveInterval *Intv = RegUnitIntervals[Unit];
+        if (!Intv) {
+          Intv = RegUnitIntervals[Unit] = new LiveInterval(Unit, HUGE_VALF);
+          NewIntvs.push_back(Intv);
+        }
+        VNInfo *VNI = Intv->createDeadDef(Begin, getVNInfoAllocator());
+        DEBUG(dbgs() << ' ' << PrintRegUnit(Unit, TRI) << '#' << VNI->id);
+      }
+    }
+    DEBUG(dbgs() << '\n');
+  }
+  DEBUG(dbgs() << "Created " << NewIntvs.size() << " new intervals.\n");
+
+  // Compute the 'normal' part of the intervals.
+  for (unsigned i = 0, e = NewIntvs.size(); i != e; ++i)
+    computeRegUnitInterval(NewIntvs[i]);
+}
+
+
 /// shrinkToUses - After removing some uses of a register, shrink its live
 /// range to just the remaining uses. This method does not compute reaching
 /// defs for new uses, and it doesn't remove dead defs.