[RDA] Fix getUniqueReachingDef for self loops

We've fixed the case where this could return an instruction after the
given instruction, but also means that we can falsely return a
'unique' def when they could be one coming from the backedge of a
loop.

Differential Revision: https://reviews.llvm.org/D87751

1 files changed, 6 insertions, 10 deletions

diff --git a/llvm/lib/CodeGen/ReachingDefAnalysis.cpp b/llvm/lib/CodeGen/ReachingDefAnalysis.cpp
index 5a48370..86c2f63 100644
--- a/llvm/lib/CodeGen/ReachingDefAnalysis.cpp
+++ b/llvm/lib/CodeGen/ReachingDefAnalysis.cpp
@@ -397,7 +397,6 @@ ReachingDefAnalysis::getGlobalReachingDefs(MachineInstr *MI, int PhysReg,
     return;
   }
 
-  SmallPtrSet<MachineBasicBlock *, 2> Visited;
   for (auto *MBB : MI->getParent()->predecessors())
     getLiveOuts(MBB, PhysReg, Defs);
 }
@@ -437,18 +436,15 @@ MachineInstr *ReachingDefAnalysis::getUniqueReachingMIDef(MachineInstr *MI,
   SmallPtrSet<MachineBasicBlock*, 4> VisitedBBs;
   SmallPtrSet<MachineInstr*, 2> Incoming;
   MachineBasicBlock *Parent = MI->getParent();
-  VisitedBBs.insert(Parent);
   for (auto *Pred : Parent->predecessors())
-    getLiveOuts(Pred, PhysReg, Incoming, VisitedBBs);
+    getLiveOuts(Pred, PhysReg, Incoming);
 
-  // If we have a local def and an incoming instruction, then there's not a
-  // unique instruction def.
-  if (!Incoming.empty() && LocalDef)
-    return nullptr;
-  else if (Incoming.size() == 1)
+  // Check that we have a single incoming value and that it does not
+  // come from the same block as MI - since it would mean that the def
+  // is executed after MI.
+  if (Incoming.size() == 1 && (*Incoming.begin())->getParent() != Parent)
     return *Incoming.begin();
-  else
-    return LocalDef;
+  return nullptr;
 }
 
 MachineInstr *ReachingDefAnalysis::getMIOperand(MachineInstr *MI,