Revert "[DebugInfo] MachineSink: Insert undef DBG_VALUEs when sinking instructions"

This reverts commit ee50590e1684c197bc4336984795e48bf53c7a4e.

PR43855 reports a performance regression from this commit, which I'll
look into.

1 files changed, 3 insertions, 50 deletions

diff --git a/llvm/lib/CodeGen/MachineSink.cpp b/llvm/lib/CodeGen/MachineSink.cpp
index 4c55158a004..27a2e7023f2 100644
--- a/llvm/lib/CodeGen/MachineSink.cpp
+++ b/llvm/lib/CodeGen/MachineSink.cpp
@@ -736,9 +736,6 @@ static bool SinkingPreventsImplicitNullCheck(MachineInstr &MI,
 static void performSink(MachineInstr &MI, MachineBasicBlock &SuccToSinkTo,
                         MachineBasicBlock::iterator InsertPos,
                         SmallVectorImpl<MachineInstr *> *DbgVals = nullptr) {
-  const MachineRegisterInfo &MRI = MI.getMF()->getRegInfo();
-  const TargetInstrInfo &TII = *MI.getMF()->getSubtarget().getInstrInfo();
-
   // If debug values are provided use those, otherwise call collectDebugValues.
   SmallVector<MachineInstr *, 2> DbgValuesToSink;
   if (DbgVals)
@@ -761,57 +758,13 @@ static void performSink(MachineInstr &MI, MachineBasicBlock &SuccToSinkTo,
   SuccToSinkTo.splice(InsertPos, ParentBlock, MI,
                       ++MachineBasicBlock::iterator(MI));
 
-  // Sink a copy of debug users to the insert position. Mark the original
-  // DBG_VALUE location as 'undef', indicating that any earlier variable
-  // location should be terminated as we've optimised away the value at this
-  // point.
-  // If the sunk instruction is a copy, try to forward the copy instead of
-  // leaving an 'undef' DBG_VALUE in the original location. Don't do this if
-  // there's any subregister weirdness involved.
+  // Move previously adjacent debug value instructions to the insert position.
   for (SmallVectorImpl<MachineInstr *>::iterator DBI = DbgValuesToSink.begin(),
                                                  DBE = DbgValuesToSink.end();
        DBI != DBE; ++DBI) {
     MachineInstr *DbgMI = *DBI;
-    MachineInstr *NewDbgMI = DbgMI->getMF()->CloneMachineInstr(*DBI);
-    SuccToSinkTo.insert(InsertPos, NewDbgMI);
-
-    // Copy DBG_VALUE operand and set the original to undef. We then check to
-    // see whether this is something that can be copy-forwarded. If it isn't,
-    // continue around the loop.
-    MachineOperand DbgMO = DbgMI->getOperand(0);
-    DbgMI->getOperand(0).setReg(0);
-
-    const MachineOperand *SrcMO = nullptr, *DstMO = nullptr;
-    if (!TII.isCopyInstr(MI, SrcMO, DstMO))
-      continue;
-
-    // Check validity of forwarding this copy.
-    bool PostRA = MRI.getNumVirtRegs() == 0;
-
-    // Trying to forward between physical and virtual registers is too hard.
-    if (DbgMO.getReg().isVirtual() != SrcMO->getReg().isVirtual())
-      continue;
-
-    // Only try virtual register copy-forwarding before regalloc, and physical
-    // register copy-forwarding after regalloc.
-    bool arePhysRegs = !DbgMO.getReg().isVirtual();
-    if (arePhysRegs != PostRA)
-      continue;
-
-    // Pre-regalloc, only forward if all subregisters agree (or there are no
-    // subregs at all). More analysis might recover some forwardable copies.
-    if (!PostRA && (DbgMO.getSubReg() != SrcMO->getSubReg() ||
-                    DbgMO.getSubReg() != DstMO->getSubReg()))
-      continue;
-
-    // Post-regalloc, we may be sinking a DBG_VALUE of a sub or super-register
-    // of this copy. Only forward the copy if the DBG_VALUE operand exactly
-    // matches the copy destination.
-    if (PostRA && DbgMO.getReg() != DstMO->getReg())
-      continue;
-
-    DbgMI->getOperand(0).setReg(SrcMO->getReg());
-    DbgMI->getOperand(0).setSubReg(SrcMO->getSubReg());
+    SuccToSinkTo.splice(InsertPos, ParentBlock, DbgMI,
+                        ++MachineBasicBlock::iterator(DbgMI));
   }
 }