[DebugInfo] LiveDebugValues: defer DBG_VALUE creation during analysis

When transfering variable locations from one place to another,
LiveDebugValues immediately creates a DBG_VALUE representing that
transfer. This causes trouble if the variable location should
subsequently be invalidated by a loop back-edge, such as in the added
test case: the transfer DBG_VALUE from a now-invalid location is used
as proof that the variable location is correct. This is effectively a
self-fulfilling prophesy.

To avoid this, defer the insertion of transfer DBG_VALUEs until after
analysis has completed. Some of those transfers are still sketchy, but
we don't propagate them into other blocks now.

Differential Revision: https://reviews.llvm.org/D67393

llvm-svn: 373720

1 files changed, 7 insertions, 8 deletions

diff --git a/llvm/lib/CodeGen/LiveDebugValues.cpp b/llvm/lib/CodeGen/LiveDebugValues.cpp
index 0bdc62a345c..8d523377fb4 100644
--- a/llvm/lib/CodeGen/LiveDebugValues.cpp
+++ b/llvm/lib/CodeGen/LiveDebugValues.cpp
@@ -986,9 +986,6 @@ bool LiveDebugValues::transferTerminator(MachineBasicBlock *CurMBB,
                                          const VarLocMap &VarLocIDs) {
   bool Changed = false;
 
-  if (OpenRanges.empty())
-    return false;
-
   LLVM_DEBUG(for (unsigned ID
                   : OpenRanges.getVarLocs()) {
     // Copy OpenRanges to OutLocs, if not already present.
@@ -1362,11 +1359,6 @@ bool LiveDebugValues::ExtendRanges(MachineFunction &MF) {
                       DebugEntryVals, OverlapFragments, SeenFragments);
         OLChanged |= transferTerminator(MBB, OpenRanges, OutLocs, VarLocIDs);
 
-        // Add any DBG_VALUE instructions necessitated by spills.
-        for (auto &TR : Transfers)
-          MBB->insertAfterBundle(TR.TransferInst->getIterator(), TR.DebugInst);
-        Transfers.clear();
-
         LLVM_DEBUG(printVarLocInMBB(MF, OutLocs, VarLocIDs,
                                     "OutLocs after propagating", dbgs()));
         LLVM_DEBUG(printVarLocInMBB(MF, InLocs, VarLocIDs,
@@ -1387,6 +1379,13 @@ bool LiveDebugValues::ExtendRanges(MachineFunction &MF) {
     assert(Pending.empty() && "Pending should be empty");
   }
 
+  // Add any DBG_VALUE instructions created by location transfers.
+  for (auto &TR : Transfers) {
+    auto *MBB = TR.TransferInst->getParent();
+    MBB->insertAfterBundle(TR.TransferInst->getIterator(), TR.DebugInst);
+  }
+  Transfers.clear();
+
   // Deferred inlocs will not have had any DBG_VALUE insts created; do
   // that now.
   flushPendingLocs(PendingInLocs, VarLocIDs);