Index: llvm/trunk/lib/Transforms/Scalar/NaryReassociate.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Scalar/NaryReassociate.cpp
+++ llvm/trunk/lib/Transforms/Scalar/NaryReassociate.cpp
@@ -240,10 +240,17 @@
           Changed = true;
           SE->forgetValue(&*I);
           I->replaceAllUsesWith(NewI);
-          // If SeenExprs constains I's WeakTrackingVH, that entry will be
-          // replaced with
-          // nullptr.
+          WeakVH NewIExist = NewI;
+          // If SeenExprs/NewIExist contains I's WeakTrackingVH/WeakVH, that
+          // entry will be replaced with nullptr if deleted.
           RecursivelyDeleteTriviallyDeadInstructions(&*I, TLI);
+          if (!NewIExist) {
+            // Rare occation where the new instruction (NewI) have been removed,
+            // probably due to parts of the input code was dead from the
+            // beginning, reset the iterator and start over from the beginning
+            I = BB->begin();
+            continue;
+          }
           I = NewI->getIterator();
         }
         // Add the rewritten instruction to SeenExprs; the original instruction
Index: llvm/trunk/test/Transforms/NaryReassociate/pr37539.ll
===================================================================
--- llvm/trunk/test/Transforms/NaryReassociate/pr37539.ll
+++ llvm/trunk/test/Transforms/NaryReassociate/pr37539.ll
@@ -0,0 +1,29 @@
+; RUN: opt < %s -nary-reassociate -S -o - | FileCheck %s
+
+; The test check that compilation does not segv (see pr37539).
+
+define void @f1() {
+; CHECK-LABEL: @f1(
+; CHECK-NEXT:    br label %[[BB1:.*]]
+; CHECK:         [[BB1]]
+; CHECK-NEXT:    [[P1:%.*]] = phi i16 [ 0, [[TMP0:%.*]] ], [ [[A1:%.*]], %[[BB1]] ]
+; CHECK-NEXT:    [[SCEVGEP_OFFS:%.*]] = add i16 2, 0
+; CHECK-NEXT:    [[A1]] = add i16 [[P1]], [[SCEVGEP_OFFS]]
+; CHECK-NEXT:    br i1 false, label %[[BB1]], label %[[BB7:.*]]
+; CHECK:         [[BB7]]
+; CHECK-NEXT:    ret void
+;
+  br label %bb1
+
+bb1:
+  %p1 = phi i16 [ 0, %0 ], [ %a1, %bb1 ]
+  %p2 = phi i16 [ 0, %0 ], [ %a2, %bb1 ]
+  %scevgep.offs = add i16 2, 0
+  %a1 = add i16 %p1, %scevgep.offs
+  %scevgep.offs5 = add i16 2, 0
+  %a2 = add i16 %p2, %scevgep.offs5
+  br i1 false, label %bb1, label %bb7
+
+bb7:
+  ret void
+}