Index: lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
===================================================================
--- lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
+++ lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
@@ -304,10 +304,10 @@
     formLCSSA(*OldContainingL, DT, &LI, nullptr);
 
     // We shouldn't need to form dedicated exits because the exit introduced
-    // here is the (just split by unswitching) preheader. As such, it is
-    // necessarily dedicated.
-    assert(OldContainingL->hasDedicatedExits() &&
-           "Unexpected predecessor of hoisted loop preheader!");
+    // here is the (just split by unswitching) preheader. However, after trivial
+    // switches, we may have non-dedicated exits, so let's conservatively
+    // form dedicated exit blocks and figure out if we can optimize later.
+    formDedicatedExitBlocks(OldContainingL, &DT, &LI, /*PreserveLCSSA*/ true);
   }
 }
 
@@ -516,6 +516,7 @@
   if (FullUnswitch)
     hoistLoopToNewParent(L, *NewPH, DT, LI);
 
+  LLVM_DEBUG(dbgs() << "    done: unswitching trivial branch...\n");
   ++NumTrivial;
   ++NumBranches;
   return true;
@@ -574,7 +575,7 @@
   else if (ExitCaseIndices.empty())
     return false;
 
-  LLVM_DEBUG(dbgs() << "    unswitching trivial cases...\n");
+  LLVM_DEBUG(dbgs() << "    unswitching trivial switch...\n");
 
   if (MSSAU && VerifyMemorySSA)
     MSSAU->getMemorySSA()->verifyMemorySSA();
@@ -786,6 +787,7 @@
 
   ++NumTrivial;
   ++NumSwitches;
+  LLVM_DEBUG(dbgs() << "    done: unswitching trivial switch...\n");
   return true;
 }
 
Index: test/Transforms/SimpleLoopUnswitch/formDedicatedAfterTrivial1.ll
===================================================================
--- /dev/null
+++ test/Transforms/SimpleLoopUnswitch/formDedicatedAfterTrivial1.ll
@@ -0,0 +1,25 @@
+; RUN: opt < %s -simple-loop-unswitch -disable-output
+
+; PR38283
+; PR38737
+define void @f1() {
+for.cond1thread-pre-split.lr.ph.lr.ph:
+  %tobool4 = icmp eq i16 undef, 0
+  br label %for.cond1thread-pre-split
+
+for.cond1thread-pre-split:                        ; preds = %if.end, %for.cond1thread-pre-split.lr.ph.lr.ph
+  %tobool3 = icmp eq i16 undef, 0
+  br label %for.body2
+
+for.body2:                                        ; preds = %if.end6, %for.cond1thread-pre-split
+  br i1 %tobool3, label %if.end, label %for.end
+
+if.end:                                           ; preds = %for.body2
+  br i1 %tobool4, label %if.end6, label %for.cond1thread-pre-split
+
+if.end6:                                          ; preds = %if.end
+  br i1 undef, label %for.body2, label %for.end
+
+for.end:                                          ; preds = %if.end6, %for.body2
+  ret void
+}
Index: test/Transforms/SimpleLoopUnswitch/formDedicatedAfterTrivial2.ll
===================================================================
--- /dev/null
+++ test/Transforms/SimpleLoopUnswitch/formDedicatedAfterTrivial2.ll
@@ -0,0 +1,22 @@
+; RUN: opt < %s -simple-loop-unswitch -disable-output
+
+; PR38283
+; PR38737
+define void @Test(i32) {
+entry:
+  %trunc = trunc i32 %0 to i3
+  br label %outer
+outer:
+  br label %inner
+inner:
+  switch i3 %trunc, label %crit_edge [
+    i3 2, label %break
+    i3 1, label %loopexit
+  ]
+crit_edge:
+  br i1 true, label %loopexit, label %inner
+loopexit:
+  ret void
+break:
+  br label %outer
+}
Index: test/Transforms/SimpleLoopUnswitch/formDedicatedAfterTrivial3.ll
===================================================================
--- /dev/null
+++ test/Transforms/SimpleLoopUnswitch/formDedicatedAfterTrivial3.ll
@@ -0,0 +1,105 @@
+; RUN: opt <%s -mtriple=s390x-linux-gnu -O3 -mcpu=z13 -enable-simple-loop-unswitch -disable-output
+
+; PR38283
+; PR38737
+source_filename = "crash0.c"
+target datalayout = "E-m:e-i1:8:16-i8:8:16-i64:64-f128:64-v128:64-a:8:16-n32:64"
+target triple = "s390x-ibm-linux"
+
+@g_450 = external dso_local global [8 x [8 x [4 x i32]]], align 4
+
+; Function Attrs: nounwind
+define dso_local void @main() #0 {
+bb:
+  call void @func_1()
+  unreachable
+}
+
+; Function Attrs: nounwind
+define dso_local void @func_1() #0 {
+bb:
+  %tmp = load i32, i32* getelementptr inbounds ([8 x [8 x [4 x i32]]], [8 x [8 x [4 x i32]]]* @g_450, i64 0, i64 3, i64 7, i64 2), align 4, !tbaa !1
+  call void @func_9(i32 signext %tmp)
+  unreachable
+}
+
+; Function Attrs: nounwind
+define dso_local void @func_9(i32 signext %arg) #0 {
+bb:
+  %tmp = alloca i32, align 4
+  %tmp1 = alloca i32, align 4
+  %tmp2 = alloca i32, align 4
+  %tmp3 = alloca i32, align 4
+  %tmp4 = alloca i32*, align 8
+  store i32 %arg, i32* %tmp, align 4, !tbaa !1
+  store i32* %tmp3, i32** %tmp4, align 8, !tbaa !5
+  br label %bb5
+
+bb5:                                              ; preds = %bb24, %bb
+  store i32 0, i32* %tmp2, align 4, !tbaa !1
+  br label %bb6
+
+bb6:                                              ; preds = %bb19, %bb5
+  %tmp7 = load i32, i32* %tmp2, align 4, !tbaa !1
+  %tmp8 = icmp ne i32 %tmp7, 25
+  br i1 %tmp8, label %bb9, label %bb22
+
+bb9:                                              ; preds = %bb6
+  %tmp10 = load i32, i32* %tmp, align 4, !tbaa !1
+  %tmp11 = icmp ne i32 %tmp10, 0
+  br i1 %tmp11, label %bb12, label %bb13
+
+bb12:                                             ; preds = %bb9
+  store i32 9, i32* %tmp1, align 4
+  br label %bb17
+
+bb13:                                             ; preds = %bb9
+  %tmp14 = load i32, i32* %tmp3, align 4, !tbaa !1
+  %tmp15 = icmp ne i32 %tmp14, 0
+  br i1 %tmp15, label %bb16, label %bb17
+
+bb16:                                             ; preds = %bb13
+  store i32 49, i32* %tmp1, align 4
+  br label %bb17
+
+bb17:                                             ; preds = %bb16, %bb13, %bb12
+  %tmp18 = load i32, i32* %tmp1, align 4
+  switch i32 %tmp18, label %bb24 [
+    i32 0, label %bb19
+    i32 49, label %bb22
+  ]
+
+bb19:                                             ; preds = %bb17
+  %tmp20 = load i32, i32* %tmp2, align 4, !tbaa !1
+  %tmp21 = add nsw i32 %tmp20, 1
+  store i32 %tmp21, i32* %tmp2, align 4, !tbaa !1
+  br label %bb6
+
+bb22:                                             ; preds = %bb17, %bb6
+  br label %bb23
+
+bb23:                                             ; preds = %bb23, %bb22
+  br label %bb23
+
+bb24:                                             ; preds = %bb17
+  %tmp25 = icmp sge i32 0, undef
+  %tmp26 = zext i1 %tmp25 to i32
+  %tmp27 = load i32*, i32** %tmp4, align 8, !tbaa !5
+  %tmp28 = load i32, i32* %tmp27, align 4, !tbaa !1
+  %tmp29 = or i32 %tmp28, %tmp26
+  store i32 %tmp29, i32* %tmp27, align 4, !tbaa !1
+  store i32 0, i32* %tmp1, align 4
+  br label %bb5
+}
+
+attributes #0 = { nounwind "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="z13" "target-features"="+transactional-execution,+vector" "unsafe-fp-math"="false" "use-soft-float"="false" }
+
+!llvm.ident = !{!0}
+
+!0 = !{!"clang version 7.0.0 (http://llvm.org/git/clang.git 31fe70446bc81aa70778c3d3eaa20263eba3c4e5) (http://llvm.org/git/llvm.git 7a4d8f6dc1a41dca2f39a4327b3a4fdd5263ad88)"}
+!1 = !{!2, !2, i64 0}
+!2 = !{!"int", !3, i64 0}
+!3 = !{!"omnipotent char", !4, i64 0}
+!4 = !{!"Simple C/C++ TBAA"}
+!5 = !{!6, !6, i64 0}
+!6 = !{!"any pointer", !3, i64 0}