[llvm] r262767 - RegisterCoalescer: Need to check DstReg+SrcReg for missing undef flags

[Matthias Braun via llvm-commits]

Author: matze
Date: Fri Mar  4 22:36:10 2016
New Revision: 262767

URL: http://llvm.org/viewvc/llvm-project?rev=262767&view=rev
Log:
RegisterCoalescer: Need to check DstReg+SrcReg for missing undef flags

copy coalescing with enabled subregister liveness can reveal undef uses,
previously this was only checked for the SrcReg in updateRegDefsUses()
but we need to check DstReg as well.

Modified:
    llvm/trunk/lib/CodeGen/RegisterCoalescer.cpp

Modified: llvm/trunk/lib/CodeGen/RegisterCoalescer.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/RegisterCoalescer.cpp?rev=262767&r1=262766&r2=262767&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/RegisterCoalescer.cpp (original)
+++ llvm/trunk/lib/CodeGen/RegisterCoalescer.cpp Fri Mar  4 22:36:10 2016
@@ -203,6 +203,16 @@ namespace {
     /// make sure to set it to the correct physical subregister.
     void updateRegDefsUses(unsigned SrcReg, unsigned DstReg, unsigned SubIdx);
 
+    /// If the given machine operand reads only undefined lanes add an undef
+    /// flag.
+    /// This can happen when undef uses were previously concealed by a copy
+    /// which we coalesced. Example:
+    ///    %vreg0:sub0<def,read-undef> = ...
+    ///    %vreg1 = COPY %vreg0       <-- Coalescing COPY reveals undef
+    ///           = use %vreg1:sub1   <-- hidden undef use
+    void addUndefFlag(const LiveInterval &Int, SlotIndex UseIdx,
+                      MachineOperand &MO, unsigned SubRegIdx);
+
     /// Handle copies of undef values.
     /// Returns true if @p CopyMI was a copy of an undef value and eliminated.
     bool eliminateUndefCopy(MachineInstr *CopyMI);
@@ -1191,12 +1201,51 @@ bool RegisterCoalescer::eliminateUndefCo
   return true;
 }
 
+void RegisterCoalescer::addUndefFlag(const LiveInterval &Int, SlotIndex UseIdx,
+                                     MachineOperand &MO, unsigned SubRegIdx) {
+  LaneBitmask Mask = TRI->getSubRegIndexLaneMask(SubRegIdx);
+  if (MO.isDef())
+    Mask = ~Mask;
+  bool IsUndef = true;
+  for (const LiveInterval::SubRange &S : Int.subranges()) {
+    if ((S.LaneMask & Mask) == 0)
+      continue;
+    if (S.liveAt(UseIdx)) {
+      IsUndef = false;
+      break;
+    }
+  }
+  if (IsUndef) {
+    MO.setIsUndef(true);
+    // We found out some subregister use is actually reading an undefined
+    // value. In some cases the whole vreg has become undefined at this
+    // point so we have to potentially shrink the main range if the
+    // use was ending a live segment there.
+    LiveQueryResult Q = Int.Query(UseIdx);
+    if (Q.valueOut() == nullptr)
+      ShrinkMainRange = true;
+  }
+}
+
 void RegisterCoalescer::updateRegDefsUses(unsigned SrcReg,
                                           unsigned DstReg,
                                           unsigned SubIdx) {
   bool DstIsPhys = TargetRegisterInfo::isPhysicalRegister(DstReg);
   LiveInterval *DstInt = DstIsPhys ? nullptr : &LIS->getInterval(DstReg);
 
+  if (DstInt && DstInt->hasSubRanges() && DstReg != SrcReg) {
+    for (MachineOperand &MO : MRI->reg_operands(DstReg)) {
+      unsigned SubReg = MO.getSubReg();
+      if (SubReg == 0 || MO.isUndef())
+        continue;
+      MachineInstr &MI = *MO.getParent();
+      if (MI.isDebugValue())
+        continue;
+      SlotIndex UseIdx = LIS->getInstructionIndex(MI).getRegSlot(true);
+      addUndefFlag(*DstInt, UseIdx, MO, SubReg);
+    }
+  }
+
   SmallPtrSet<MachineInstr*, 8> Visited;
   for (MachineRegisterInfo::reg_instr_iterator
        I = MRI->reg_instr_begin(SrcReg), E = MRI->reg_instr_end();
@@ -1238,30 +1287,11 @@ void RegisterCoalescer::updateRegDefsUse
           LaneBitmask Mask = MRI->getMaxLaneMaskForVReg(DstInt->reg);
           DstInt->createSubRangeFrom(Allocator, Mask, *DstInt);
         }
-        LaneBitmask Mask = TRI->getSubRegIndexLaneMask(SubIdx);
-        bool IsUndef = true;
         SlotIndex MIIdx = UseMI->isDebugValue()
                               ? LIS->getSlotIndexes()->getIndexBefore(*UseMI)
                               : LIS->getInstructionIndex(*UseMI);
         SlotIndex UseIdx = MIIdx.getRegSlot(true);
-        for (LiveInterval::SubRange &S : DstInt->subranges()) {
-          if ((S.LaneMask & Mask) == 0)
-            continue;
-          if (S.liveAt(UseIdx)) {
-            IsUndef = false;
-            break;
-          }
-        }
-        if (IsUndef) {
-          MO.setIsUndef(true);
-          // We found out some subregister use is actually reading an undefined
-          // value. In some cases the whole vreg has become undefined at this
-          // point so we have to potentially shrink the main range if the
-          // use was ending a live segment there.
-          LiveQueryResult Q = DstInt->Query(MIIdx);
-          if (Q.valueOut() == nullptr)
-            ShrinkMainRange = true;
-        }
+        addUndefFlag(*DstInt, UseIdx, MO, SubIdx);
       }
 
       if (DstIsPhys)