Index: lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
===================================================================
--- lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
+++ lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
@@ -1740,8 +1740,9 @@
 /// well by cloning the loop if the result is small enough.
 static bool
 unswitchLoop(Loop &L, DominatorTree &DT, LoopInfo &LI, AssumptionCache &AC,
-             TargetTransformInfo &TTI, bool NonTrivial,
+             TargetTransformInfo &TTI, ScalarEvolution *SE, bool NonTrivial,
              function_ref<void(bool, ArrayRef<Loop *>)> UnswitchCB) {
+  assert(!L.isInvalid() && "Unswitching invalid loop?");
   assert(L.isRecursivelyLCSSAForm(DT, LI) &&
          "Loops must be in LCSSA form before unswitching.");
 
@@ -1754,6 +1755,9 @@
     // If we unswitched successfully we will want to clean up the loop before
     // processing it further so just mark it as unswitched and return.
     UnswitchCB(/*CurrentLoopValid*/ true, {});
+    assert(!L.isInvalid() && "Invalid loop after unswitching?");
+    if (SE)
+      SE->forgetTopmostLoop(&L);
     return true;
   }
 
@@ -1898,8 +1902,18 @@
   LLVM_DEBUG(dbgs() << "  Trying to unswitch non-trivial (cost = "
                     << BestUnswitchCost << ") branch: " << *BestUnswitchTI
                     << "\n");
-  return unswitchInvariantBranch(L, cast<BranchInst>(*BestUnswitchTI), DT, LI,
-                                 AC, UnswitchCB);
+  bool Changed = unswitchInvariantBranch(L, cast<BranchInst>(*BestUnswitchTI),
+                                         DT, LI, AC, UnswitchCB);
+  // FIXME: There should be assert on !L.isInvalid() here. However it seems that
+  // we have a bug that may lead to construction of an invalid loop in the call
+  // of unswitchInvariantBranch. Specifically, without this check two existing
+  // unit tests will fail this assert:
+  //   LLVM :: Transforms/SimpleLoopUnswitch/nontrivial-unswitch-cost.ll
+  //   LLVM :: Transforms/SimpleLoopUnswitch/nontrivial-unswitch.ll
+  // Once it is fixed, the restriction should be lifted and turned into assert.
+  if (Changed && SE && !L.isInvalid())
+    SE->forgetTopmostLoop(&L);
+  return Changed;
 }
 
 PreservedAnalyses SimpleLoopUnswitchPass::run(Loop &L, LoopAnalysisManager &AM,
@@ -1929,7 +1943,7 @@
       U.markLoopAsDeleted(L, LoopName);
   };
 
-  if (!unswitchLoop(L, AR.DT, AR.LI, AR.AC, AR.TTI, NonTrivial,
+  if (!unswitchLoop(L, AR.DT, AR.LI, AR.AC, AR.TTI, &AR.SE, NonTrivial,
                     UnswitchCB))
     return PreservedAnalyses::all();
 
@@ -1993,8 +2007,11 @@
       LPM.markLoopAsDeleted(*L);
   };
 
+  ScalarEvolution *SE = nullptr;
+  if (auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>())
+    SE = &SEWP->getSE();
   bool Changed =
-      unswitchLoop(*L, DT, LI, AC, TTI, NonTrivial, UnswitchCB);
+      unswitchLoop(*L, DT, LI, AC, TTI, SE, NonTrivial, UnswitchCB);
 
   // If anything was unswitched, also clear any cached information about this
   // loop.
Index: test/Transforms/SimpleLoopUnswitch/pr37651.ll
===================================================================
--- /dev/null
+++ test/Transforms/SimpleLoopUnswitch/pr37651.ll
@@ -0,0 +1,37 @@
+; REQUIRES: asserts
+; RUN: opt -iv-users -simple-loop-unswitch -indvars -S < %s | FileCheck %s
+; RUN: opt -passes='print<scalar-evolution>,loop(unswitch),print<scalar-evolution>' -S < %s | FileCheck %s
+; PR37651. Exercise two scenarios: the original scenario how the bug was
+; reported and the scenario that shows that the problem lies in unswitching.
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+
+; Make sure that SCEV gets invalidated properly after simple unswitching.
+define void @f1() {
+
+; CHECK-LABEL: @f1
+
+vector.ph:
+  br label %bb3
+
+bb3:                                              ; preds = %bb13, %vector.ph
+  %_tmp5329 = phi i8 [ %_tmp54, %bb13 ], [ undef, %vector.ph ]
+  %_tmp40 = icmp eq i8 undef, 0
+  br label %bb11
+
+bb11:                                             ; preds = %bb12, %bb3
+  br i1 %_tmp40, label %bb12, label %bb13
+
+bb12:                                             ; preds = %bb11
+  br i1 false, label %bb11, label %bb13
+
+bb13:                                             ; preds = %bb12, %bb11
+  %_tmp54 = add nsw i8 %_tmp5329, 1
+  %_tmp57 = icmp eq i8 %_tmp54, 0
+  br i1 %_tmp57, label %bb14.loopexit, label %bb3
+
+bb14.loopexit:                                    ; preds = %bb13
+  ret void
+}