ART: Fix bug in DeadPhiHandling

When reviving dead phis for --debuggable, the DeadPhiHandling algorithm
could produce two phis of the same type, which causes the SSAChecker
graph verifier to fail. This patch fixes the bug.

Note that the code is currently not exercised because compilation is
delegated to Quick for --debuggable.

Bug: 24129675
Change-Id: I26b6dcf3071b325cc7871b989a36c505279ae681

1 files changed, 25 insertions, 5 deletions

diff --git a/compiler/optimizing/ssa_builder.cc b/compiler/optimizing/ssa_builder.cc
index 6f71ea3d6b..acc734a6b2 100644
--- a/compiler/optimizing/ssa_builder.cc
+++ b/compiler/optimizing/ssa_builder.cc
@@ -57,8 +57,13 @@ class DeadPhiHandling : public ValueObject {
 };
 
 bool DeadPhiHandling::UpdateType(HPhi* phi) {
+  if (phi->IsDead()) {
+    // Phi was rendered dead while waiting in the worklist because it was replaced
+    // with an equivalent.
+    return false;
+  }
+
   Primitive::Type existing = phi->GetType();
-  DCHECK(phi->IsLive());
 
   bool conflict = false;
   Primitive::Type new_type = existing;
@@ -112,11 +117,26 @@ bool DeadPhiHandling::UpdateType(HPhi* phi) {
     phi->SetType(Primitive::kPrimVoid);
     phi->SetDead();
     return true;
-  } else {
-    DCHECK(phi->IsLive());
-    phi->SetType(new_type);
-    return existing != new_type;
+  } else if (existing == new_type) {
+    return false;
   }
+
+  DCHECK(phi->IsLive());
+  phi->SetType(new_type);
+
+  // There might exist a `new_type` equivalent of `phi` already. In that case,
+  // we replace the equivalent with the, now live, `phi`.
+  HPhi* equivalent = phi->GetNextEquivalentPhiWithSameType();
+  if (equivalent != nullptr) {
+    // There cannot be more than two equivalents with the same type.
+    DCHECK(equivalent->GetNextEquivalentPhiWithSameType() == nullptr);
+    // If doing fix-point iteration, the equivalent might be in `worklist_`.
+    // Setting it dead will make UpdateType skip it.
+    equivalent->SetDead();
+    equivalent->ReplaceWith(phi);
+  }
+
+  return true;
 }
 
 void DeadPhiHandling::VisitBasicBlock(HBasicBlock* block) {