[LiveDebugValues] Do not insert DBG_VALUEs after a MBB terminator

This fixes a miscompile that happened because a DBG_VALUE interfered
with the MachineOutliner's liveness analysis.

Inserting a DBG_VALUE after a terminator breaks predicates on MBB such
as isReturnBlock(). And the resulting DBG_VALUE cannot be "live".

I plan to introduce a MachineVerifier check for this situation in a
follow up.

rdar://59859175

Testing: check-llvm, LNT build with a stage2 compiler & entry values
enabled

Differential Revision: https://reviews.llvm.org/D75548

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/LiveDebugValues.cpp b/llvm/lib/CodeGen/LiveDebugValues.cpp
index 4d0c246..94c5cc5 100644
--- a/llvm/lib/CodeGen/LiveDebugValues.cpp
+++ b/llvm/lib/CodeGen/LiveDebugValues.cpp
@@ -959,6 +959,10 @@ void LiveDebugValues::emitEntryValues(MachineInstr &MI,
                                       VarLocMap &VarLocIDs,
                                       TransferMap &Transfers,
                                       VarLocSet &KillSet) {
+  // Do not insert entry value locations after a terminator.
+  if (MI.isTerminator())
+    return;
+
   for (uint64_t ID : KillSet) {
     LocIndex Idx = LocIndex::fromRawInteger(ID);
     const VarLoc &VL = VarLocIDs[Idx];
@@ -1003,6 +1007,7 @@ void LiveDebugValues::insertTransferDebugPair(
     // Record the new location as an open range, and a postponed transfer
     // inserting a DBG_VALUE for this location.
     OpenRanges.insert(LocId, VL);
+    assert(!MI.isTerminator() && "Cannot insert DBG_VALUE after terminator");
     TransferDebugPair MIP = {&MI, LocId};
     Transfers.push_back(MIP);
   };
@@ -1778,6 +1783,8 @@ bool LiveDebugValues::ExtendRanges(MachineFunction &MF) {
 
   // Add any DBG_VALUE instructions created by location transfers.
   for (auto &TR : Transfers) {
+    assert(!TR.TransferInst->isTerminator() &&
+           "Cannot insert DBG_VALUE after terminator");
     MachineBasicBlock *MBB = TR.TransferInst->getParent();
     const VarLoc &VL = VarLocIDs[TR.LocationID];
     MachineInstr *MI = VL.BuildDbgValue(MF);