[PHIEliminate] Move dbg values after phi and label

If there are DBG_VALUEs between phi and label (after phi and before label),
DBG_VALUE will block PHI lowering after the LABEL. Moving all DBG_VALUEs
after Labels in the function ScheduleDAGSDNodes::EmitSchedule to avoid
impacting PHI lowering.

  before:
     PHI
     DBG_VALUE
     LABEL
  after: (move DBG_VALUE after label)
     PHI
     LABEL
     DBG_VALUE
  then: (phi lowering after label)
     LABEL
     COPY
     DBG_VALUE

Fixes the issue: https://bugs.llvm.org/show_bug.cgi?id=43859

Differential Revision: https://reviews.llvm.org/D70597

1 files changed, 22 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/MachineVerifier.cpp b/llvm/lib/CodeGen/MachineVerifier.cpp
index 6c0402d..24fb330 100644
--- a/llvm/lib/CodeGen/MachineVerifier.cpp
+++ b/llvm/lib/CodeGen/MachineVerifier.cpp
@@ -273,6 +273,7 @@ namespace {
     void markReachable(const MachineBasicBlock *MBB);
     void calcRegsPassed();
     void checkPHIOps(const MachineBasicBlock &MBB);
+    void checkPHIDbg(const MachineBasicBlock &MBB);
 
     void calcRegsRequired();
     void verifyLiveVariables();
@@ -2262,11 +2263,31 @@ void MachineVerifier::checkPHIOps(const MachineBasicBlock &MBB) {
   }
 }
 
+// Check there must be no DBG_VALUEs between PHIs and LABELs, so that
+// the PHIs could lower after LABELs successfully without dbg barriers.
+void MachineVerifier::checkPHIDbg(const MachineBasicBlock &MBB) {
+
+  // Quick exit for basic blocks without PHIs.
+  if (MBB.empty() || !MBB.front().isPHI())
+    return;
+
+  MachineBasicBlock &MBB2 = const_cast<MachineBasicBlock &>(MBB);
+  MachineBasicBlock::iterator LastPHIIt =
+    std::prev(MBB2.SkipPHIsAndLabels(MBB2.begin()));
+  // Backward to skip debug if the lastPHIIt is DBG_VALUE, the LastPHIIt
+  // should be the same if no DBG_VALUEs between PHIs and LABELs.
+  if (LastPHIIt != skipDebugInstructionsBackward(
+          std::prev(MBB2.SkipPHIsLabelsAndDebug(MBB2.begin())), MBB2.begin()))
+    report("Unexpected DBG_VALUEs between PHI and LABEL", &MBB2);
+}
+
 void MachineVerifier::visitMachineFunctionAfter() {
   calcRegsPassed();
 
-  for (const MachineBasicBlock &MBB : *MF)
+  for (const MachineBasicBlock &MBB : *MF) {
     checkPHIOps(MBB);
+    checkPHIDbg(MBB);
+  }
 
   // Now check liveness info if available
   calcRegsRequired();