Do not consider subreg defs as reads when computing subrange liveness

Subregister definitions are considered uses for the purpose of tracking
liveness of the whole register. At the same time, when calculating live
interval subranges, subregister defs should not be treated as uses.

Differential Revision: https://reviews.llvm.org/D24190

llvm-svn: 280532

4 files changed, 13 insertions, 11 deletions

diff --git a/llvm/lib/CodeGen/LiveIntervalAnalysis.cpp b/llvm/lib/CodeGen/LiveIntervalAnalysis.cpp
index a3122763806..4195f007d43 100644
--- a/llvm/lib/CodeGen/LiveIntervalAnalysis.cpp
+++ b/llvm/lib/CodeGen/LiveIntervalAnalysis.cpp
@@ -506,20 +506,19 @@ void LiveIntervals::shrinkToUses(LiveInterval::SubRange &SR, unsigned Reg) {
 
   // Visit all instructions reading Reg.
   SlotIndex LastIdx;
-  for (MachineOperand &MO : MRI->reg_operands(Reg)) {
-    MachineInstr *UseMI = MO.getParent();
-    if (UseMI->isDebugValue() || !MO.readsReg())
+  for (MachineOperand &MO : MRI->use_nodbg_operands(Reg)) {
+    // Skip "undef" uses.
+    if (!MO.readsReg())
       continue;
     // Maybe the operand is for a subregister we don't care about.
     unsigned SubReg = MO.getSubReg();
     if (SubReg != 0) {
       LaneBitmask LaneMask = TRI->getSubRegIndexLaneMask(SubReg);
-      if (MO.isDef())
-        LaneMask = ~LaneMask & MRI->getMaxLaneMaskForVReg(Reg);
       if ((LaneMask & SR.LaneMask) == 0)
         continue;
     }
     // We only need to visit each instruction once.
+    MachineInstr *UseMI = MO.getParent();
     SlotIndex Idx = getInstructionIndex(*UseMI).getRegSlot();
     if (Idx == LastIdx)
       continue;
diff --git a/llvm/lib/CodeGen/LiveRangeCalc.cpp b/llvm/lib/CodeGen/LiveRangeCalc.cpp
index 25d22a02d33..98022d99cf2 100644
--- a/llvm/lib/CodeGen/LiveRangeCalc.cpp
+++ b/llvm/lib/CodeGen/LiveRangeCalc.cpp
@@ -163,13 +163,18 @@ void LiveRangeCalc::extendToUses(LiveRange &LR, unsigned Reg, LaneBitmask Mask,
     LI->computeSubRangeUndefs(Undefs, Mask, *MRI, *Indexes);
 
   // Visit all operands that read Reg. This may include partial defs.
+  bool IsSubRange = (Mask != ~0U);
   const TargetRegisterInfo &TRI = *MRI->getTargetRegisterInfo();
   for (MachineOperand &MO : MRI->reg_nodbg_operands(Reg)) {
     // Clear all kill flags. They will be reinserted after register allocation
     // by LiveIntervalAnalysis::addKillFlags().
     if (MO.isUse())
       MO.setIsKill(false);
-    if (!MO.readsReg())
+    // MO::readsReg returns "true" for subregister defs. This is for keeping
+    // liveness of the entire register (i.e. for the main range of the live
+    // interval). For subranges, definitions of non-overlapping subregisters
+    // do not count as uses.
+    if (!MO.readsReg() || (IsSubRange && MO.isDef()))
       continue;
 
     unsigned SubReg = MO.getSubReg();
diff --git a/llvm/lib/CodeGen/LiveRangeCalc.h b/llvm/lib/CodeGen/LiveRangeCalc.h
index 84cc1cb2be7..892f535a5ae 100644
--- a/llvm/lib/CodeGen/LiveRangeCalc.h
+++ b/llvm/lib/CodeGen/LiveRangeCalc.h
@@ -160,6 +160,7 @@ class LiveRangeCalc {
   /// all uses must be jointly dominated by the definitions from @p LR
   /// together with definitions of other lanes where @p LR becomes undefined
   /// (via <def,read-undef> operands).
+  /// If @p LR is a main range, the @p LaneMask should be set to ~0.
   void extendToUses(LiveRange &LR, unsigned Reg, LaneBitmask LaneMask,
                     LiveInterval *LI = nullptr);
 
diff --git a/llvm/lib/CodeGen/SplitKit.cpp b/llvm/lib/CodeGen/SplitKit.cpp
index 6d59f85d35f..e06bc4a3614 100644
--- a/llvm/lib/CodeGen/SplitKit.cpp
+++ b/llvm/lib/CodeGen/SplitKit.cpp
@@ -1206,7 +1206,8 @@ void SplitEditor::rewriteAssigned(bool ExtendRanges) {
       // defining the register. This is because a <def,read-undef> operand
       // will create an "undef" point, and we cannot extend any subranges
       // until all of them have been accounted for.
-      ExtPoints.push_back(ExtPoint(MO, RegIdx, Next));
+      if (MO.isUse())
+        ExtPoints.push_back(ExtPoint(MO, RegIdx, Next));
     } else {
       LiveRangeCalc &LRC = getLRCalc(RegIdx);
       LRC.extend(LI, Next, 0, ArrayRef<SlotIndex>());
@@ -1221,10 +1222,6 @@ void SplitEditor::rewriteAssigned(bool ExtendRanges) {
     unsigned Reg = EP.MO.getReg(), Sub = EP.MO.getSubReg();
     LaneBitmask LM = Sub != 0 ? TRI.getSubRegIndexLaneMask(Sub)
                               : MRI.getMaxLaneMaskForVReg(Reg);
-    // If this is a non-read-undef definition of a sub-register, extend
-    // subranges for everything except that sub-register.
-    if (Sub != 0 && EP.MO.isDef())
-      LM = MRI.getMaxLaneMaskForVReg(Reg) & ~LM;
     for (LiveInterval::SubRange &S : LI.subranges()) {
       if (!(S.LaneMask & LM))
         continue;