Propagate DBG_VALUE entries when there are unvisited predecessors

Variables are sometimes missing their debug location information in
blocks in which the variables should be available. This would occur
when one or more predecessor blocks had not yet been visited by the
routine which propagated the information from predecessor blocks.

This is addressed by only considering predecessor blocks which have
already been visited.

The solution to this problem was suggested by Daniel Berlin on the
LLVM developer mailing list.

Differential Revision: https://reviews.llvm.org/D24927

llvm-svn: 282506

1 files changed, 24 insertions, 10 deletions

diff --git a/llvm/lib/CodeGen/LiveDebugValues.cpp b/llvm/lib/CodeGen/LiveDebugValues.cpp
index 41af34e0099..7d9461214ef 100644
--- a/llvm/lib/CodeGen/LiveDebugValues.cpp
+++ b/llvm/lib/CodeGen/LiveDebugValues.cpp
@@ -201,7 +201,8 @@ private:
                 VarLocInMBB &OutLocs, VarLocMap &VarLocIDs);
 
   bool join(MachineBasicBlock &MBB, VarLocInMBB &OutLocs, VarLocInMBB &InLocs,
-            const VarLocMap &VarLocIDs);
+            const VarLocMap &VarLocIDs,
+            SmallPtrSet<const MachineBasicBlock *, 16> &Visited);
 
   bool ExtendRanges(MachineFunction &MF);
 
@@ -368,7 +369,8 @@ bool LiveDebugValues::transfer(MachineInstr &MI, OpenRangesSet &OpenRanges,
 /// inserting a new DBG_VALUE instruction at the start of the @MBB - if the same
 /// source variable in all the predecessors of @MBB reside in the same location.
 bool LiveDebugValues::join(MachineBasicBlock &MBB, VarLocInMBB &OutLocs,
-                           VarLocInMBB &InLocs, const VarLocMap &VarLocIDs) {
+                           VarLocInMBB &InLocs, const VarLocMap &VarLocIDs,
+                           SmallPtrSet<const MachineBasicBlock *, 16> &Visited) {
   DEBUG(dbgs() << "join MBB: " << MBB.getName() << "\n");
   bool Changed = false;
 
@@ -376,21 +378,32 @@ bool LiveDebugValues::join(MachineBasicBlock &MBB, VarLocInMBB &OutLocs,
 
   // For all predecessors of this MBB, find the set of VarLocs that
   // can be joined.
+  int NumVisited = 0;
   for (auto p : MBB.predecessors()) {
+    // Ignore unvisited predecessor blocks.  As we are processing
+    // the blocks in reverse post-order any unvisited block can
+    // be considered to not remove any incoming values.
+    if (!Visited.count(p))
+      continue;
     auto OL = OutLocs.find(p);
     // Join is null in case of empty OutLocs from any of the pred.
     if (OL == OutLocs.end())
       return false;
 
-    // Just copy over the Out locs to incoming locs for the first predecessor.
-    if (p == *MBB.pred_begin()) {
+    // Just copy over the Out locs to incoming locs for the first visited
+    // predecessor, and for all other predecessors join the Out locs.
+    if (!NumVisited)
       InLocsT = OL->second;
-      continue;
-    }
-    // Join with this predecessor.
-    InLocsT &= OL->second;
+    else
+      InLocsT &= OL->second;
+    NumVisited++;
   }
 
+  // As we are processing blocks in reverse post-order we
+  // should have processed at least one predecessor, unless it
+  // is the entry block which has no predecessor.
+  assert((NumVisited || MBB.pred_empty()) &&
+         "Should have processed at least one predecessor");
   if (InLocsT.empty())
     return false;
 
@@ -463,6 +476,7 @@ bool LiveDebugValues::ExtendRanges(MachineFunction &MF) {
   // To solve it, we perform join() and transfer() using the two worklist method
   // until the ranges converge.
   // Ranges have converged when both worklists are empty.
+  SmallPtrSet<const MachineBasicBlock *, 16> Visited;
   while (!Worklist.empty() || !Pending.empty()) {
     // We track what is on the pending worklist to avoid inserting the same
     // thing twice.  We could avoid this with a custom priority queue, but this
@@ -472,8 +486,8 @@ bool LiveDebugValues::ExtendRanges(MachineFunction &MF) {
     while (!Worklist.empty()) {
       MachineBasicBlock *MBB = OrderToBB[Worklist.top()];
       Worklist.pop();
-      MBBJoined = join(*MBB, OutLocs, InLocs, VarLocIDs);
-
+      MBBJoined = join(*MBB, OutLocs, InLocs, VarLocIDs, Visited);
+      Visited.insert(MBB);
       if (MBBJoined) {
         MBBJoined = false;
         Changed = true;