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

1 files changed, 6 insertions, 1 deletions

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();