[DebugInfo] Handle joins PHI+Def values in InstrRef LiveDebugValues

In the InstrRefBasedImpl for LiveDebugValues, we attempt to propagate
debug values through basic blocks in part by checking to see whether all
a variable's incoming debug values to a BB "agree", i.e. whether their
properties match and they refer to the same underlying value.

Prior to this patch, the check for agreement between incoming values
relied on exact equality, which meant that a VPHI and a Def DbgValue
that referred to the same underlying value would be seen as disagreeing.
This patch changes this behaviour to treat them as referring to the same
value, allowing the shared value to propagate into the BB.

Differential Revision: https://reviews.llvm.org/D125953

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp b/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
index 23fde21..91d5ac2 100644
--- a/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
+++ b/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
@@ -2533,6 +2533,12 @@ bool InstrRefBasedLDV::vlocJoin(
     if (*V.second == FirstVal)
       continue; // No disagreement.
 
+    // If both values are not equal but have equal non-empty IDs then they refer
+    // to the same value from different sources (e.g. one is VPHI and the other
+    // is Def), which does not cause disagreement.
+    if (V.second->ID != ValueIDNum::EmptyValue && V.second->ID == FirstVal.ID)
+      continue;
+
     // Eliminate if a backedge feeds a VPHI back into itself.
     if (V.second->Kind == DbgValue::VPHI &&
         V.second->BlockNo == MBB.getNumber() &&