Index: lib/Transforms/Scalar/LoopSimplifyCFG.cpp
===================================================================
--- lib/Transforms/Scalar/LoopSimplifyCFG.cpp
+++ lib/Transforms/Scalar/LoopSimplifyCFG.cpp
@@ -87,6 +87,8 @@
   DominatorTree &DT;
   MemorySSAUpdater *MSSAU;
 
+  // Whether or not the current loop has irreducible CFG.
+  bool HasIrreducibleCFG = false;
   // Whether or not the current loop will still exist after terminator constant
   // folding will be done. In theory, there are two ways how it can happen:
   // 1. Loop's latch(es) become unreachable from loop header;
@@ -145,6 +147,26 @@
                   BlocksInLoopAfterFolding);
   }
 
+  /// Whether or not the current loop has irreducible CFG.
+  bool hasIrreducibleCFG(LoopBlocksDFS &DFS) {
+    // Index of a basic block in RPO traversal.
+    DenseMap<const BasicBlock *, unsigned> RPO;
+    unsigned Current = 0;
+    for (auto I = DFS.beginRPO(), E = DFS.endRPO(); I != E; ++I)
+      RPO[*I] = Current++;
+
+    for (auto I = DFS.beginRPO(), E = DFS.endRPO(); I != E; ++I) {
+      BasicBlock *BB = *I;
+      for (auto *Succ : successors(BB))
+        if (L.contains(Succ) && !LI.isLoopHeader(Succ) && RPO[BB] > RPO[Succ])
+          // If an edge goes from a block with greater order number into a block
+          // with lesses number, and it is not a loop backedge, then it can only
+          // be a part of irreducible non-loop cycle.
+          return true;
+    }
+    return false;
+  }
+
   /// Fill all information about status of blocks and exits of the current loop
   /// if constant folding of all branches will be done.
   void analyze() {
@@ -152,6 +174,18 @@
     DFS.perform(&LI);
     assert(DFS.isComplete() && "DFS is expected to be finished");
 
+    // TODO: The algorithm below relies on both RPO and Postorder traversals.
+    // When the loop has only reducible CFG inside, then the invariant "all
+    // predecessors of X are processed before X in RPO" is preserved. However
+    // an irreducible loop can break this invariant (e.g. latch does not have to
+    // be the last block in the traversal in this case, and the algorithm relies
+    // on this). We can later decide to support such cases by altering the
+    // algorithms, but so far we just give up analyzing them.
+    if (hasIrreducibleCFG(DFS)) {
+      HasIrreducibleCFG = true;
+      return;
+    }
+
     // Collect live and dead loop blocks and exits.
     LiveLoopBlocks.insert(L.getHeader());
     for (auto I = DFS.beginRPO(), E = DFS.endRPO(); I != E; ++I) {
@@ -323,6 +357,11 @@
     LLVM_DEBUG(dbgs() << "In function " << L.getHeader()->getParent()->getName()
                       << ": ");
 
+    if (HasIrreducibleCFG) {
+      LLVM_DEBUG(dbgs() << "Loops with irreducible CFG are not supported!\n");
+      return false;
+    }
+
     // Nothing to constant-fold.
     if (FoldCandidates.empty()) {
       LLVM_DEBUG(
Index: test/Transforms/LoopSimplifyCFG/irreducible_cfg.ll
===================================================================
--- /dev/null
+++ test/Transforms/LoopSimplifyCFG/irreducible_cfg.ll
@@ -0,0 +1,51 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; REQUIRES: asserts
+; RUN: opt -S -enable-loop-simplifycfg-term-folding=true -loop-simplifycfg -debug-only=loop-simplifycfg -verify-loop-info -verify-dom-info -verify-loop-lcssa 2>&1 < %s | FileCheck %s
+; RUN: opt -S -enable-loop-simplifycfg-term-folding=true -passes='require<domtree>,loop(simplify-cfg)' -debug-only=loop-simplifycfg -verify-loop-info -verify-dom-info -verify-loop-lcssa 2>&1 < %s | FileCheck %s
+; RUN: opt -S -enable-loop-simplifycfg-term-folding=true -loop-simplifycfg -enable-mssa-loop-dependency=true -verify-memoryssa -debug-only=loop-simplifycfg -verify-loop-info -verify-dom-info -verify-loop-lcssa 2>&1 < %s | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128-ni:1"
+
+; This test has irreducible CFG, and RPO may be the following:
+; Header, Dead, Irreducible2, Latch, Irreducible3, Irreducible1.
+; As result, we will process Irreducible2 before its predecessor Irreducible1.
+; The current algorithm gets confused in this case. We may support irreducible
+; CFG in the future.
+define void @irreducible_cfg(i1 %cond) {
+; CHECK-LABEL: @irreducible_cfg(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[HEADER:%.*]]
+; CHECK:       header:
+; CHECK-NEXT:    br i1 false, label [[DEAD:%.*]], label [[IRREDUCIBLE1:%.*]]
+; CHECK:       dead:
+; CHECK-NEXT:    br label [[IRREDUCIBLE2:%.*]]
+; CHECK:       irreducible2:
+; CHECK-NEXT:    br i1 [[COND:%.*]], label [[LATCH:%.*]], label [[IRREDUCIBLE3:%.*]]
+; CHECK:       latch:
+; CHECK-NEXT:    br label [[HEADER]]
+; CHECK:       irreducible3:
+; CHECK-NEXT:    br label [[IRREDUCIBLE1]]
+; CHECK:       irreducible1:
+; CHECK-NEXT:    br label [[IRREDUCIBLE2]]
+;
+entry:
+  br label %header
+
+header:                                             ; preds = %latch, %entry
+  br i1 false, label %dead, label %irreducible1
+
+dead:                                          ; preds = %header
+  br label %irreducible2
+
+irreducible2:                                             ; preds = %irreducible1, %dead
+  br i1 %cond, label %latch, label %irreducible3
+
+latch:                                         ; preds = %irreducible2
+  br label %header
+
+irreducible3:                                           ; preds = %irreducible2
+  br label %irreducible1
+
+irreducible1:                                          ; preds = %irreducible3, %header
+  br label %irreducible2
+}