Also attempt trivial coalescing for live intervals that end in a copy.

The coalescer is supposed to clean these up, but when setting up parameters
for a function call, there may be copies to physregs. If the defining
instruction has been LICM'ed far away, the coalescer won't touch it.

The register allocation hint does not always work - when the register
allocator is backtracking, it clears the hints.

This patch is more conservative than r90502, and does not break
483.xalancbmk/i686. It still breaks the PowerPC bootstrap, so it is disabled
by default, and can be enabled with the -trivial-coalesce-ends option.

llvm-svn: 91049

include/llvm/CodeGen/LiveIntervalAnalysis.h

@@ -112,10 +112,13 @@ namespace llvm {
       return (unsigned)(IntervalPercentage * indexes_->getFunctionSize());
     }
 
-    /// conflictsWithPhysRegDef - Returns true if the specified register
-    /// is defined during the duration of the specified interval.
-    bool conflictsWithPhysRegDef(const LiveInterval &li, VirtRegMap &vrm,
-                                 unsigned reg);
+    /// conflictsWithPhysReg - Returns true if the specified register is used or
+    /// defined during the duration of the specified interval. Copies to and
+    /// from li.reg are allowed. This method is only able to analyze simple
+    /// ranges that stay within a single basic block. Anything else is
+    /// considered a conflict.
+    bool conflictsWithPhysReg(const LiveInterval &li, VirtRegMap &vrm,
+                              unsigned reg);
 
     /// conflictsWithPhysRegRef - Similar to conflictsWithPhysRegRef except
     /// it can check use as well.

lib/CodeGen/LiveIntervalAnalysis.cpp

@@ -149,42 +149,69 @@ void LiveIntervals::dumpInstrs() const {
   printInstrs(errs());
 }
 
-/// conflictsWithPhysRegDef - Returns true if the specified register
-/// is defined during the duration of the specified interval.
-bool LiveIntervals::conflictsWithPhysRegDef(const LiveInterval &li,
-                                            VirtRegMap &vrm, unsigned reg) {
-  for (LiveInterval::Ranges::const_iterator
-         I = li.ranges.begin(), E = li.ranges.end(); I != E; ++I) {
-    for (SlotIndex index = I->start.getBaseIndex(),
-           end = I->end.getPrevSlot().getBaseIndex().getNextIndex();
-           index != end;
-           index = index.getNextIndex()) {
-      MachineInstr *MI = getInstructionFromIndex(index);
-      if (!MI)
-        continue;               // skip deleted instructions
+bool LiveIntervals::conflictsWithPhysReg(const LiveInterval &li,
+                                         VirtRegMap &vrm, unsigned reg) {
+  // We don't handle fancy stuff crossing basic block boundaries
+  if (li.ranges.size() != 1)
+    return true;
+  const LiveRange &range = li.ranges.front();
+  SlotIndex idx = range.start.getBaseIndex();
+  SlotIndex end = range.end.getPrevSlot().getBaseIndex().getNextIndex();
 
-      unsigned SrcReg, DstReg, SrcSubReg, DstSubReg;
-      if (tii_->isMoveInstr(*MI, SrcReg, DstReg, SrcSubReg, DstSubReg))
-        if (SrcReg == li.reg || DstReg == li.reg)
+  // Skip deleted instructions
+  MachineInstr *firstMI = getInstructionFromIndex(idx);
+  while (!firstMI && idx != end) {
+    idx = idx.getNextIndex();
+    firstMI = getInstructionFromIndex(idx);
+  }
+  if (!firstMI)
+    return false;
+
+  // Find last instruction in range
+  SlotIndex lastIdx = end.getPrevIndex();
+  MachineInstr *lastMI = getInstructionFromIndex(lastIdx);
+  while (!lastMI && lastIdx != idx) {
+    lastIdx = lastIdx.getPrevIndex();
+    lastMI = getInstructionFromIndex(lastIdx);
+  }
+  if (!lastMI)
+    return false;
+
+  // Range cannot cross basic block boundaries or terminators
+  MachineBasicBlock *MBB = firstMI->getParent();
+  if (MBB != lastMI->getParent() || lastMI->getDesc().isTerminator())
+    return true;
+
+  MachineBasicBlock::const_iterator E = lastMI;
+  ++E;
+  for (MachineBasicBlock::const_iterator I = firstMI; I != E; ++I) {
+    const MachineInstr &MI = *I;
+
+    // Allow copies to and from li.reg
+    unsigned SrcReg, DstReg, SrcSubReg, DstSubReg;
+    if (tii_->isMoveInstr(MI, SrcReg, DstReg, SrcSubReg, DstSubReg))
+      if (SrcReg == li.reg || DstReg == li.reg)
+        continue;
+
+    // Check for operands using reg
+    for (unsigned i = 0, e = MI.getNumOperands(); i != e;  ++i) {
+      const MachineOperand& mop = MI.getOperand(i);
+      if (!mop.isReg())
+        continue;
+      unsigned PhysReg = mop.getReg();
+      if (PhysReg == 0 || PhysReg == li.reg)
+        continue;
+      if (TargetRegisterInfo::isVirtualRegister(PhysReg)) {
+        if (!vrm.hasPhys(PhysReg))
           continue;
-      for (unsigned i = 0; i != MI->getNumOperands(); ++i) {
-        MachineOperand& mop = MI->getOperand(i);
-        if (!mop.isReg())
-          continue;
-        unsigned PhysReg = mop.getReg();
-        if (PhysReg == 0 || PhysReg == li.reg)
-          continue;
-        if (TargetRegisterInfo::isVirtualRegister(PhysReg)) {
-          if (!vrm.hasPhys(PhysReg))
-            continue;
-          PhysReg = vrm.getPhys(PhysReg);
-        }
-        if (PhysReg && tri_->regsOverlap(PhysReg, reg))
-          return true;
+        PhysReg = vrm.getPhys(PhysReg);
       }
+      if (PhysReg && tri_->regsOverlap(PhysReg, reg))
+        return true;
     }
   }
 
+  // No conflicts found.
   return false;
 }
 

lib/CodeGen/RegAllocLinearScan.cpp

@@ -59,6 +59,11 @@ PreSplitIntervals("pre-alloc-split",
                   cl::desc("Pre-register allocation live interval splitting"),
                   cl::init(false), cl::Hidden);
 
+static cl::opt<bool>
+TrivCoalesceEnds("trivial-coalesce-ends",
+                  cl::desc("Attempt trivial coalescing of interval ends"),
+                  cl::init(false), cl::Hidden);
+
 static RegisterRegAlloc
 linearscanRegAlloc("linearscan", "linear scan register allocator",
                    createLinearScanRegisterAllocator);
@@ -390,66 +395,71 @@ void RALinScan::ComputeRelatedRegClasses() {
         RelatedRegClasses.unionSets(I->second, OneClassForEachPhysReg[*AS]);
 }
 
-/// attemptTrivialCoalescing - If a simple interval is defined by a copy,
-/// try allocate the definition the same register as the source register
-/// if the register is not defined during live time of the interval. This
-/// eliminate a copy. This is used to coalesce copies which were not
-/// coalesced away before allocation either due to dest and src being in
-/// different register classes or because the coalescer was overly
-/// conservative.
+/// attemptTrivialCoalescing - If a simple interval is defined by a copy, try
+/// allocate the definition the same register as the source register if the
+/// register is not defined during live time of the interval. If the interval is
+/// killed by a copy, try to use the destination register. This eliminates a
+/// copy. This is used to coalesce copies which were not coalesced away before
+/// allocation either due to dest and src being in different register classes or
+/// because the coalescer was overly conservative.
 unsigned RALinScan::attemptTrivialCoalescing(LiveInterval &cur, unsigned Reg) {
   unsigned Preference = vrm_->getRegAllocPref(cur.reg);
   if ((Preference && Preference == Reg) || !cur.containsOneValue())
     return Reg;
 
-  VNInfo *vni = cur.begin()->valno;
-  if ((vni->def == SlotIndex()) ||
-      vni->isUnused() || !vni->isDefAccurate())
+  // We cannot handle complicated live ranges. Simple linear stuff only.
+  if (cur.ranges.size() != 1)
     return Reg;
-  MachineInstr *CopyMI = li_->getInstructionFromIndex(vni->def);
-  unsigned SrcReg, DstReg, SrcSubReg, DstSubReg, PhysReg;
-  if (!CopyMI ||
-      !tii_->isMoveInstr(*CopyMI, SrcReg, DstReg, SrcSubReg, DstSubReg))
+
+  const LiveRange &range = cur.ranges.front();
+
+  VNInfo *vni = range.valno;
+  if (vni->isUnused())
     return Reg;
-  PhysReg = SrcReg;
-  if (TargetRegisterInfo::isVirtualRegister(SrcReg)) {
-    if (!vrm_->isAssignedReg(SrcReg))
+
+  unsigned CandReg;
+  {
+    MachineInstr *CopyMI;
+    unsigned SrcReg, DstReg, SrcSubReg, DstSubReg;
+    if (vni->def != SlotIndex() && vni->isDefAccurate() &&
+        (CopyMI = li_->getInstructionFromIndex(vni->def)) &&
+        tii_->isMoveInstr(*CopyMI, SrcReg, DstReg, SrcSubReg, DstSubReg))
+      // Defined by a copy, try to extend SrcReg forward
+      CandReg = SrcReg;
+    else if (TrivCoalesceEnds &&
+             (CopyMI =
+              li_->getInstructionFromIndex(range.end.getBaseIndex())) &&
+             tii_->isMoveInstr(*CopyMI, SrcReg, DstReg, SrcSubReg, DstSubReg) &&
+             cur.reg == SrcReg)
+      // Only used by a copy, try to extend DstReg backwards
+      CandReg = DstReg;
+    else
       return Reg;
-    PhysReg = vrm_->getPhys(SrcReg);
   }
-  if (Reg == PhysReg)
+
+  if (TargetRegisterInfo::isVirtualRegister(CandReg)) {
+    if (!vrm_->isAssignedReg(CandReg))
+      return Reg;
+    CandReg = vrm_->getPhys(CandReg);
+  }
+  if (Reg == CandReg)
     return Reg;
 
   const TargetRegisterClass *RC = mri_->getRegClass(cur.reg);
-  if (!RC->contains(PhysReg))
+  if (!RC->contains(CandReg))
+    return Reg;
+
+  if (li_->conflictsWithPhysReg(cur, *vrm_, CandReg))
     return Reg;
 
   // Try to coalesce.
-  if (!li_->conflictsWithPhysRegDef(cur, *vrm_, PhysReg)) {
-    DEBUG(errs() << "Coalescing: " << cur << " -> " << tri_->getName(PhysReg)
-                 << '\n');
-    vrm_->clearVirt(cur.reg);
-    vrm_->assignVirt2Phys(cur.reg, PhysReg);
+  DEBUG(errs() << "Coalescing: " << cur << " -> " << tri_->getName(CandReg)
+        << '\n');
+  vrm_->clearVirt(cur.reg);
+  vrm_->assignVirt2Phys(cur.reg, CandReg);
 
-    // Remove unnecessary kills since a copy does not clobber the register.
-    if (li_->hasInterval(SrcReg)) {
-      LiveInterval &SrcLI = li_->getInterval(SrcReg);
-      for (MachineRegisterInfo::use_iterator I = mri_->use_begin(cur.reg),
-             E = mri_->use_end(); I != E; ++I) {
-        MachineOperand &O = I.getOperand();
-        if (!O.isKill())
-          continue;
-        MachineInstr *MI = &*I;
-        if (SrcLI.liveAt(li_->getInstructionIndex(MI).getDefIndex()))
-          O.setIsKill(false);
-      }
-    }
-
-    ++NumCoalesce;
-    return PhysReg;
-  }
-
-  return Reg;
+  ++NumCoalesce;
+  return CandReg;
 }
 
 bool RALinScan::runOnMachineFunction(MachineFunction &fn) {