Index: lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
===================================================================
--- lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
+++ lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
@@ -302,10 +302,11 @@
     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
+    // unswitching it is possible to get new non-dedicated exits out of parent
+    // loop so let's conservatively form dedicated exit blocks and figure out
+    // if we can optimize later.
+    formDedicatedExitBlocks(OldContainingL, &DT, &LI, /*PreserveLCSSA*/ true);
   }
 }
 
@@ -482,6 +483,7 @@
   if (FullUnswitch)
     hoistLoopToNewParent(L, *NewPH, DT, LI);
 
+  LLVM_DEBUG(dbgs() << "    done: unswitching trivial branch...\n");
   ++NumTrivial;
   ++NumBranches;
   return true;
@@ -539,7 +541,7 @@
   else if (ExitCaseIndices.empty())
     return false;
 
-  LLVM_DEBUG(dbgs() << "    unswitching trivial cases...\n");
+  LLVM_DEBUG(dbgs() << "    unswitching trivial switch...\n");
 
   // We may need to invalidate SCEVs for the outermost loop reached by any of
   // the exits.
@@ -739,6 +741,7 @@
 
   ++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 +1,37 @@
+; RUN: opt < %s -simple-loop-unswitch -disable-output
+
+; PR38283
+; PR38737
+declare void @func_1()
+
+define void @func_9(i32 signext %arg) {
+bb:
+  br label %bb5
+bb5:                                              ; preds = %bb24, %bb
+  %tmp3.0 = phi i32 [ undef, %bb ], [ %tmp29, %bb24 ]
+  %tmp11 = icmp eq i32 %arg, 0
+  %tmp15 = icmp eq i32 %tmp3.0, 0
+  %spec.select = select i1 %tmp15, i32 0, i32 49
+  %tmp1.2 = select i1 %tmp11, i32 %spec.select, i32 9
+  %trunc = trunc i32 %tmp1.2 to i6
+  br label %bb9
+
+bb9:                                              ; preds = %bb5, %bb19
+  %tmp2.03 = phi i32 [ 0, %bb5 ], [ %tmp21, %bb19 ]
+  switch i6 %trunc, label %bb24 [
+    i6 0, label %bb19
+    i6 -15, label %bb22
+  ]
+
+bb19:                                             ; preds = %bb9
+  %tmp21 = add nuw nsw i32 %tmp2.03, 1
+  %tmp8 = icmp eq i32 %tmp21, 25
+  br i1 %tmp8, label %bb22, label %bb9
+
+bb22:                                             ; preds = %bb19, %bb9
+  unreachable
+
+bb24:                                             ; preds = %bb9
+  %tmp29 = or i32 %tmp3.0, 1
+  br label %bb5
+}