Index: llvm/trunk/include/llvm/Analysis/LoopInfo.h
===================================================================
--- llvm/trunk/include/llvm/Analysis/LoopInfo.h
+++ llvm/trunk/include/llvm/Analysis/LoopInfo.h
@@ -402,6 +402,9 @@
   /// isLCSSAForm - Return true if the Loop is in LCSSA form
   bool isLCSSAForm(DominatorTree &DT) const;
 
+  /// \brief Return true if this Loop and all inner subloops are in LCSSA form.
+  bool isRecursivelyLCSSAForm(DominatorTree &DT) const;
+
   /// isLoopSimplifyForm - Return true if the Loop is in the form that
   /// the LoopSimplify form transforms loops to, which is sometimes called
   /// normal form.
Index: llvm/trunk/lib/Analysis/LoopInfo.cpp
===================================================================
--- llvm/trunk/lib/Analysis/LoopInfo.cpp
+++ llvm/trunk/lib/Analysis/LoopInfo.cpp
@@ -200,6 +200,15 @@
   return true;
 }
 
+bool Loop::isRecursivelyLCSSAForm(DominatorTree &DT) const {
+  if (!isLCSSAForm(DT))
+    return false;
+
+  return std::all_of(begin(), end(), [&](const Loop *L) {
+    return L->isRecursivelyLCSSAForm(DT);
+  });
+}
+
 /// isLoopSimplifyForm - Return true if the Loop is in the form that
 /// the LoopSimplify form transforms loops to, which is sometimes called
 /// normal form.
Index: llvm/trunk/lib/Transforms/Scalar/IndVarSimplify.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Scalar/IndVarSimplify.cpp
+++ llvm/trunk/lib/Transforms/Scalar/IndVarSimplify.cpp
@@ -50,6 +50,7 @@
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
 #include "llvm/Transforms/Utils/Local.h"
+#include "llvm/Transforms/Utils/LoopUtils.h"
 #include "llvm/Transforms/Utils/SimplifyIndVar.h"
 using namespace llvm;
 
@@ -215,7 +216,7 @@
 /// loop. For PHI nodes, there may be multiple uses, so compute the nearest
 /// common dominator for the incoming blocks.
 static Instruction *getInsertPointForUses(Instruction *User, Value *Def,
-                                          DominatorTree *DT) {
+                                          DominatorTree *DT, LoopInfo *LI) {
   PHINode *PHI = dyn_cast<PHINode>(User);
   if (!PHI)
     return User;
@@ -234,10 +235,21 @@
     InsertPt = InsertBB->getTerminator();
   }
   assert(InsertPt && "Missing phi operand");
-  assert((!isa<Instruction>(Def) ||
-          DT->dominates(cast<Instruction>(Def), InsertPt)) &&
-         "def does not dominate all uses");
-  return InsertPt;
+
+  auto *DefI = dyn_cast<Instruction>(Def);
+  if (!DefI)
+    return InsertPt;
+
+  assert(DT->dominates(DefI, InsertPt) && "def does not dominate all uses");
+
+  auto *L = LI->getLoopFor(DefI->getParent());
+  assert(!L || L->contains(LI->getLoopFor(InsertPt->getParent())));
+
+  for (auto *DTN = (*DT)[InsertPt->getParent()]; DTN; DTN = DTN->getIDom())
+    if (LI->getLoopFor(DTN->getBlock()) == L)
+      return DTN->getBlock()->getTerminator();
+
+  llvm_unreachable("DefI dominates InsertPt!");
 }
 
 //===----------------------------------------------------------------------===//
@@ -528,8 +540,8 @@
 /// able to brute-force evaluate arbitrary instructions as long as they have
 /// constant operands at the beginning of the loop.
 void IndVarSimplify::rewriteLoopExitValues(Loop *L, SCEVExpander &Rewriter) {
-  // Verify the input to the pass in already in LCSSA form.
-  assert(L->isLCSSAForm(*DT));
+  // Check a pre-condition.
+  assert(L->isRecursivelyLCSSAForm(*DT) && "Indvars did not preserve LCSSA!");
 
   SmallVector<BasicBlock*, 8> ExitBlocks;
   L->getUniqueExitBlocks(ExitBlocks);
@@ -1167,10 +1179,11 @@
 
 /// This IV user cannot be widen. Replace this use of the original narrow IV
 /// with a truncation of the new wide IV to isolate and eliminate the narrow IV.
-static void truncateIVUse(NarrowIVDefUse DU, DominatorTree *DT) {
+static void truncateIVUse(NarrowIVDefUse DU, DominatorTree *DT, LoopInfo *LI) {
   DEBUG(dbgs() << "INDVARS: Truncate IV " << *DU.WideDef
         << " for user " << *DU.NarrowUse << "\n");
-  IRBuilder<> Builder(getInsertPointForUses(DU.NarrowUse, DU.NarrowDef, DT));
+  IRBuilder<> Builder(
+      getInsertPointForUses(DU.NarrowUse, DU.NarrowDef, DT, LI));
   Value *Trunc = Builder.CreateTrunc(DU.WideDef, DU.NarrowDef->getType());
   DU.NarrowUse->replaceUsesOfWith(DU.NarrowDef, Trunc);
 }
@@ -1207,7 +1220,8 @@
   assert (CastWidth <= IVWidth && "Unexpected width while widening compare.");
 
   // Widen the compare instruction.
-  IRBuilder<> Builder(getInsertPointForUses(DU.NarrowUse, DU.NarrowDef, DT));
+  IRBuilder<> Builder(
+      getInsertPointForUses(DU.NarrowUse, DU.NarrowDef, DT, LI));
   DU.NarrowUse->replaceUsesOfWith(DU.NarrowDef, DU.WideDef);
 
   // Widen the other operand of the compare, if necessary.
@@ -1229,7 +1243,7 @@
       // After SimplifyCFG most loop exit targets have a single predecessor.
       // Otherwise fall back to a truncate within the loop.
       if (UsePhi->getNumOperands() != 1)
-        truncateIVUse(DU, DT);
+        truncateIVUse(DU, DT, LI);
       else {
         PHINode *WidePhi =
           PHINode::Create(DU.WideDef->getType(), 1, UsePhi->getName() + ".wide",
@@ -1297,7 +1311,7 @@
     // This user does not evaluate to a recurence after widening, so don't
     // follow it. Instead insert a Trunc to kill off the original use,
     // eventually isolating the original narrow IV so it can be removed.
-    truncateIVUse(DU, DT);
+    truncateIVUse(DU, DT, LI);
     return nullptr;
   }
   // Assume block terminators cannot evaluate to a recurrence. We can't to
@@ -2165,9 +2179,9 @@
 
   // Clean up dead instructions.
   Changed |= DeleteDeadPHIs(L->getHeader(), TLI);
+
   // Check a post-condition.
-  assert(L->isLCSSAForm(*DT) &&
-         "Indvars did not leave the loop in lcssa form!");
+  assert(L->isRecursivelyLCSSAForm(*DT) && "Indvars did not preserve LCSSA!");
 
   // Verify that LFTR, and any other change have not interfered with SCEV's
   // ability to compute trip count.
Index: llvm/trunk/test/Transforms/IndVarSimplify/pr25578.ll
===================================================================
--- llvm/trunk/test/Transforms/IndVarSimplify/pr25578.ll
+++ llvm/trunk/test/Transforms/IndVarSimplify/pr25578.ll
@@ -0,0 +1,45 @@
+; RUN: opt < %s -indvars -S | FileCheck %s
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+
+; CHECK-LABEL: @foo
+define void @foo() {
+entry:
+  br label %L1_header
+
+L1_header:
+  br label %L2_header
+
+; CHECK: L2_header:
+; CHECK: %[[INDVAR:.*]] = phi i64
+; CHECK: %[[TRUNC:.*]] = trunc i64 %[[INDVAR]] to i32
+L2_header:
+  %i = phi i32 [ 0, %L1_header ], [ %i_next, %L2_latch ]
+  %i_prom = sext i32 %i to i64
+  br label %L3_header
+
+L3_header:
+  br i1 undef, label %L3_latch, label %L2_exiting_1
+
+L3_latch:
+  br i1 undef, label %L3_header, label %L2_exiting_2
+
+L2_exiting_1:
+  br i1 undef, label %L2_latch, label %L1_latch
+
+L2_exiting_2:
+  br i1 undef, label %L2_latch, label %L1_latch
+
+L2_latch:
+  %i_next = add nsw i32 %i, 1
+  br label %L2_header
+
+L1_latch:
+; CHECK: L1_latch:
+; CHECK: %i_lcssa = phi i32 [ %[[TRUNC]], %L2_exiting_1 ], [ %[[TRUNC]], %L2_exiting_2 ]
+
+  %i_lcssa = phi i32 [ %i, %L2_exiting_1 ], [ %i, %L2_exiting_2 ]
+  br i1 undef, label %exit, label %L1_header
+
+exit:
+  ret void
+}