Index: lib/Transforms/InstCombine/InstCombineCasts.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -1799,6 +1799,31 @@
 /// All the related PHI nodes can be replaced by new PHI nodes with type A.
 /// The uses of \p CI can be changed to the new PHI node corresponding to \p PN.
 Instruction *InstCombiner::optimizeBitCastFromPhi(CastInst &CI, PHINode *PN) {
+  // Do not try to optimize the following case:
+  //   ...
+  //   Z = PHI [X, ...
+  //   Y = BitCast (Z)
+  //   store Y
+  //   store Z
+  // As the optimization converts the case to:
+  //   ...
+  //   Zn = PHI [Xn, ...
+  //   X = BitCast (Xn)
+  //   store Zn
+  //   store X
+  // Potentially leading to infinite loop.
+  bool Store = false;
+  for (User *U : PN->users()) {
+    auto *SI = dyn_cast<StoreInst>(U);
+    if (SI && SI->isSimple() && SI->getOperand(0) == PN)
+      Store = true;
+  }
+  for (User *U : CI.users()) {
+    auto *SI = dyn_cast<StoreInst>(U);
+    if (SI && SI->isSimple() && SI->getOperand(0) == &CI && Store)
+      return nullptr;
+  }
+
   Value *Src = CI.getOperand(0);
   Type *SrcTy = Src->getType();         // Type B
   Type *DestTy = CI.getType();          // Type A
Index: test/Transforms/InstCombine/bitcast-cycle.ll
===================================================================
--- test/Transforms/InstCombine/bitcast-cycle.ll
+++ test/Transforms/InstCombine/bitcast-cycle.ll
@@ -0,0 +1,43 @@
+; RUN: opt < %s -S -instcombine | FileCheck %s
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+
+; This test case checks that BitCast optimization do not cycle.
+
+; CHECK: define i32 @foo
+
+@i = constant i32 1, align 4
+@f = constant float 0x3FF19999A0000000, align 4
+@cmp = common global i32 0, align 4
+@resf = common global float* null, align 8
+@resi = common global i32* null, align 8
+
+; Function Attrs: norecurse nounwind uwtable
+define i32 @foo() #0 {
+entry:
+  br label %while.cond
+
+while.cond:                                       ; preds = %if.then, %if.else, %entry
+  %res.0 = phi i32* [ null, %entry ], [ @i, %if.then ], [ bitcast (float* @f to i32*), %if.else ]
+  %0 = load i32, i32* @cmp, align 4
+  %shr = ashr i32 %0, 1
+  store i32 %shr, i32* @cmp, align 4
+  %tobool = icmp ne i32 %shr, 0
+  br i1 %tobool, label %while.body, label %while.end
+
+while.body:                                       ; preds = %while.cond
+  %and = and i32 %shr, 1
+  %tobool1 = icmp ne i32 %and, 0
+  br i1 %tobool1, label %if.then, label %if.else
+
+if.then:                                          ; preds = %while.body
+  br label %while.cond
+
+if.else:                                          ; preds = %while.body
+  br label %while.cond
+
+while.end:                                        ; preds = %while.cond
+  %1 = bitcast i32* %res.0 to float*
+  store float* %1, float** @resf, align 8
+  store i32* %res.0, i32** @resi, align 8
+  ret i32 0
+}