Revert "Reapply "RegisterCoalescer: Add implicit-def of super register when coalescing SUBREG_TO_REG""

This reverts commit 1f283a6.
Reason: breaks MSan buildbot
(https://lab.llvm.org/buildbot/#/builders/74/builds/24077)

Author: thurstond

llvm/lib/CodeGen/RegisterCoalescer.cpp

@@ -305,11 +305,7 @@ namespace {
     /// number if it is not zero. If DstReg is a physical register and the
     /// existing subregister number of the def / use being updated is not zero,
     /// make sure to set it to the correct physical subregister.
-    ///
-    /// If \p IsSubregToReg, we are coalescing a DstReg = SUBREG_TO_REG
-    /// SrcReg. This introduces an implicit-def of DstReg on coalesced users.
-    void updateRegDefsUses(Register SrcReg, Register DstReg, unsigned SubIdx,
-                           bool IsSubregToReg);
+    void updateRegDefsUses(Register SrcReg, Register DstReg, unsigned SubIdx);
 
     /// If the given machine operand reads only undefined lanes add an undef
     /// flag.
@@ -1332,7 +1328,8 @@ bool RegisterCoalescer::reMaterializeTrivialDef(const CoalescerPair &CP,
     if (DstReg.isPhysical()) {
       Register NewDstReg = DstReg;
 
-      unsigned NewDstIdx = TRI->composeSubRegIndices(CP.getSrcIdx(), DefSubIdx);
+      unsigned NewDstIdx = TRI->composeSubRegIndices(CP.getSrcIdx(),
+                                              DefMI->getOperand(0).getSubReg());
       if (NewDstIdx)
         NewDstReg = TRI->getSubReg(DstReg, NewDstIdx);
 
@@ -1481,7 +1478,7 @@ bool RegisterCoalescer::reMaterializeTrivialDef(const CoalescerPair &CP,
     MRI->setRegClass(DstReg, NewRC);
 
     // Update machine operands and add flags.
-    updateRegDefsUses(DstReg, DstReg, DstIdx, false);
+    updateRegDefsUses(DstReg, DstReg, DstIdx);
     NewMI.getOperand(0).setSubReg(NewIdx);
     // updateRegDefUses can add an "undef" flag to the definition, since
     // it will replace DstReg with DstReg.DstIdx. If NewIdx is 0, make
@@ -1803,7 +1800,7 @@ void RegisterCoalescer::addUndefFlag(const LiveInterval &Int, SlotIndex UseIdx,
 }
 
 void RegisterCoalescer::updateRegDefsUses(Register SrcReg, Register DstReg,
-                                          unsigned SubIdx, bool IsSubregToReg) {
+                                          unsigned SubIdx) {
   bool DstIsPhys = DstReg.isPhysical();
   LiveInterval *DstInt = DstIsPhys ? nullptr : &LIS->getInterval(DstReg);
 
@@ -1843,22 +1840,16 @@ void RegisterCoalescer::updateRegDefsUses(Register SrcReg, Register DstReg,
     if (DstInt && !Reads && SubIdx && !UseMI->isDebugInstr())
       Reads = DstInt->liveAt(LIS->getInstructionIndex(*UseMI));
 
-    bool FullDef = true;
-
     // Replace SrcReg with DstReg in all UseMI operands.
     for (unsigned i = 0, e = Ops.size(); i != e; ++i) {
       MachineOperand &MO = UseMI->getOperand(Ops[i]);
 
       // Adjust <undef> flags in case of sub-register joins. We don't want to
       // turn a full def into a read-modify-write sub-register def and vice
       // versa.
-      if (SubIdx && MO.isDef()) {
+      if (SubIdx && MO.isDef())
         MO.setIsUndef(!Reads);
 
-        if (!Reads)
-          FullDef = false;
-      }
-
       // A subreg use of a partially undef (super) register may be a complete
       // undef use now and then has to be marked that way.
       if (MO.isUse() && !DstIsPhys) {
@@ -1890,25 +1881,6 @@ void RegisterCoalescer::updateRegDefsUses(Register SrcReg, Register DstReg,
         MO.substVirtReg(DstReg, SubIdx, *TRI);
     }
 
-    if (IsSubregToReg && !FullDef) {
-      // If the coalesed instruction doesn't fully define the register, we need
-      // to preserve the original super register liveness for SUBREG_TO_REG.
-      //
-      // We pretended SUBREG_TO_REG was a regular copy for coalescing purposes,
-      // but it introduces liveness for other subregisters. Downstream users may
-      // have been relying on those bits, so we need to ensure their liveness is
-      // captured with a def of other lanes.
-
-      // FIXME: Need to add new subrange if tracking subranges. We could also
-      // skip adding this if we knew the other lanes are dead, and only for
-      // other lanes.
-
-      assert(!MRI->shouldTrackSubRegLiveness(DstReg) &&
-             "this should update subranges");
-      MachineInstrBuilder MIB(*MF, UseMI);
-      MIB.addReg(DstReg, RegState::ImplicitDefine);
-    }
-
     LLVM_DEBUG({
       dbgs() << "\t\tupdated: ";
       if (!UseMI->isDebugInstr())
@@ -2108,8 +2080,6 @@ bool RegisterCoalescer::joinCopy(MachineInstr *CopyMI, bool &Again) {
     });
   }
 
-  const bool IsSubregToReg = CopyMI->isSubregToReg();
-
   ShrinkMask = LaneBitmask::getNone();
   ShrinkMainRange = false;
 
@@ -2177,12 +2147,9 @@ bool RegisterCoalescer::joinCopy(MachineInstr *CopyMI, bool &Again) {
 
   // Rewrite all SrcReg operands to DstReg.
   // Also update DstReg operands to include DstIdx if it is set.
-  if (CP.getDstIdx()) {
-    assert(!IsSubregToReg && "can this happen?");
-    updateRegDefsUses(CP.getDstReg(), CP.getDstReg(), CP.getDstIdx(), false);
-  }
-  updateRegDefsUses(CP.getSrcReg(), CP.getDstReg(), CP.getSrcIdx(),
-                    IsSubregToReg);
+  if (CP.getDstIdx())
+    updateRegDefsUses(CP.getDstReg(), CP.getDstReg(), CP.getDstIdx());
+  updateRegDefsUses(CP.getSrcReg(), CP.getDstReg(), CP.getSrcIdx());
 
   // Shrink subregister ranges if necessary.
   if (ShrinkMask.any()) {