Index: lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -969,19 +969,6 @@
   return nullptr;
 }
 
-/// dyn_castZExtVal - Checks if V is a zext or constant that can
-/// be truncated to Ty without losing bits.
-static Value *dyn_castZExtVal(Value *V, Type *Ty) {
-  if (ZExtInst *Z = dyn_cast<ZExtInst>(V)) {
-    if (Z->getSrcTy() == Ty)
-      return Z->getOperand(0);
-  } else if (ConstantInt *C = dyn_cast<ConstantInt>(V)) {
-    if (C->getValue().getActiveBits() <= cast<IntegerType>(Ty)->getBitWidth())
-      return ConstantExpr::getTrunc(C, Ty);
-  }
-  return nullptr;
-}
-
 namespace {
 const unsigned MaxDepth = 6;
 typedef Instruction *(*FoldUDivOperandCb)(Value *Op0, Value *Op1,
@@ -1095,6 +1082,43 @@
   return 0;
 }
 
+/// If we have zero-extended operands of an unsigned div or rem, we may be able
+/// to narrow the operation (sink the zext below the math).
+static Instruction *narrowUDivURem(BinaryOperator &I,
+                                   InstCombiner::BuilderTy &Builder) {
+  Instruction::BinaryOps Opcode = I.getOpcode();
+  Value *N = I.getOperand(0);
+  Value *D = I.getOperand(1);
+  Type *Ty = I.getType();
+  Value *X, *Y;
+  if (match(N, m_ZExt(m_Value(X))) && match(D, m_ZExt(m_Value(Y))) &&
+      X->getType() == Y->getType() && (N->hasOneUse() || D->hasOneUse())) {
+    // udiv (zext X), (zext Y) --> zext (udiv X, Y)
+    // urem (zext X), (zext Y) --> zext (urem X, Y)
+    Value *NarrowOp = Builder.CreateBinOp(Opcode, X, Y);
+    return new ZExtInst(NarrowOp, Ty);
+  }
+
+  Constant *C;
+  if ((match(N, m_OneUse(m_ZExt(m_Value(X)))) && match(D, m_Constant(C))) ||
+      (match(D, m_OneUse(m_ZExt(m_Value(X)))) && match(N, m_Constant(C)))) {
+    // If the constant is the same in the smaller type, use the narrow version.
+    Constant *TruncC = ConstantExpr::getTrunc(C, X->getType());
+    if (ConstantExpr::getZExt(TruncC, Ty) != C)
+      return nullptr;
+
+    // udiv (zext X), C --> zext (udiv X, C')
+    // urem (zext X), C --> zext (urem X, C')
+    // udiv C, (zext X) --> zext (udiv C', X)
+    // urem C, (zext X) --> zext (urem C', X)
+    Value *NarrowOp = isa<Constant>(D) ? Builder.CreateBinOp(Opcode, X, TruncC)
+                                       : Builder.CreateBinOp(Opcode, TruncC, X);
+    return new ZExtInst(NarrowOp, Ty);
+  }
+
+  return nullptr;
+}
+
 Instruction *InstCombiner::visitUDiv(BinaryOperator &I) {
   Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
 
@@ -1127,12 +1151,8 @@
     }
   }
 
-  // (zext A) udiv (zext B) --> zext (A udiv B)
-  if (ZExtInst *ZOp0 = dyn_cast<ZExtInst>(Op0))
-    if (Value *ZOp1 = dyn_castZExtVal(Op1, ZOp0->getSrcTy()))
-      return new ZExtInst(
-          Builder.CreateUDiv(ZOp0->getOperand(0), ZOp1, "div", I.isExact()),
-          I.getType());
+  if (Instruction *NarrowDiv = narrowUDivURem(I, Builder))
+    return NarrowDiv;
 
   // (LHS udiv (select (select (...)))) -> (LHS >> (select (select (...))))
   SmallVector<UDivFoldAction, 6> UDivActions;
@@ -1477,11 +1497,8 @@
   if (Instruction *common = commonIRemTransforms(I))
     return common;
 
-  // (zext A) urem (zext B) --> zext (A urem B)
-  if (ZExtInst *ZOp0 = dyn_cast<ZExtInst>(Op0))
-    if (Value *ZOp1 = dyn_castZExtVal(Op1, ZOp0->getSrcTy()))
-      return new ZExtInst(Builder.CreateURem(ZOp0->getOperand(0), ZOp1),
-                          I.getType());
+  if (Instruction *NarrowRem = narrowUDivURem(I, Builder))
+    return NarrowRem;
 
   // X urem Y -> X and Y-1, where Y is a power of 2,
   if (isKnownToBeAPowerOfTwo(Op1, /*OrZero*/ true, 0, &I)) {
Index: test/Transforms/InstCombine/udivrem-change-width.ll
===================================================================
--- test/Transforms/InstCombine/udivrem-change-width.ll
+++ test/Transforms/InstCombine/udivrem-change-width.ll
@@ -79,10 +79,9 @@
 ; CHECK-LABEL: @udiv_i32_multiuse(
 ; CHECK-NEXT:    [[ZA:%.*]] = zext i8 %a to i32
 ; CHECK-NEXT:    [[ZB:%.*]] = zext i8 %b to i32
-; CHECK-NEXT:    [[DIV:%.*]] = udiv i8 %a, %b
-; CHECK-NEXT:    [[UDIV:%.*]] = zext i8 [[DIV]] to i32
+; CHECK-NEXT:    [[UDIV:%.*]] = udiv i32 [[ZA]], [[ZB]]
 ; CHECK-NEXT:    [[EXTRA_USES:%.*]] = add nuw nsw i32 [[ZA]], [[ZB]]
-; CHECK-NEXT:    [[R:%.*]] = mul nuw nsw i32 [[EXTRA_USES]], [[UDIV]]
+; CHECK-NEXT:    [[R:%.*]] = mul nuw nsw i32 [[UDIV]], [[EXTRA_USES]]
 ; CHECK-NEXT:    ret i32 [[R]]
 ;
   %za = zext i8 %a to i32
@@ -133,10 +132,9 @@
 ; CHECK-LABEL: @urem_i32_multiuse(
 ; CHECK-NEXT:    [[ZA:%.*]] = zext i8 %a to i32
 ; CHECK-NEXT:    [[ZB:%.*]] = zext i8 %b to i32
-; CHECK-NEXT:    [[TMP1:%.*]] = urem i8 %a, %b
-; CHECK-NEXT:    [[UREM:%.*]] = zext i8 [[TMP1]] to i32
+; CHECK-NEXT:    [[UREM:%.*]] = urem i32 [[ZA]], [[ZB]]
 ; CHECK-NEXT:    [[EXTRA_USES:%.*]] = add nuw nsw i32 [[ZA]], [[ZB]]
-; CHECK-NEXT:    [[R:%.*]] = mul nuw nsw i32 [[EXTRA_USES]], [[UREM]]
+; CHECK-NEXT:    [[R:%.*]] = mul nuw nsw i32 [[UREM]], [[EXTRA_USES]]
 ; CHECK-NEXT:    ret i32 [[R]]
 ;
   %za = zext i8 %a to i32
@@ -172,8 +170,8 @@
 
 define <2 x i32> @udiv_i32_c_vec(<2 x i8> %a) {
 ; CHECK-LABEL: @udiv_i32_c_vec(
-; CHECK-NEXT:    [[ZA:%.*]] = zext <2 x i8> %a to <2 x i32>
-; CHECK-NEXT:    [[UDIV:%.*]] = udiv <2 x i32> [[ZA]], <i32 10, i32 17>
+; CHECK-NEXT:    [[TMP1:%.*]] = udiv <2 x i8> %a, <i8 10, i8 17>
+; CHECK-NEXT:    [[UDIV:%.*]] = zext <2 x i8> [[TMP1]] to <2 x i32>
 ; CHECK-NEXT:    ret <2 x i32> [[UDIV]]
 ;
   %za = zext <2 x i8> %a to <2 x i32>
@@ -184,9 +182,8 @@
 define i32 @udiv_i32_c_multiuse(i8 %a) {
 ; CHECK-LABEL: @udiv_i32_c_multiuse(
 ; CHECK-NEXT:    [[ZA:%.*]] = zext i8 %a to i32
-; CHECK-NEXT:    [[DIV:%.*]] = udiv i8 %a, 10
-; CHECK-NEXT:    [[UDIV:%.*]] = zext i8 [[DIV]] to i32
-; CHECK-NEXT:    [[EXTRA_USE:%.*]] = add nuw nsw i32 [[ZA]], [[UDIV]]
+; CHECK-NEXT:    [[UDIV:%.*]] = udiv i32 [[ZA]], 10
+; CHECK-NEXT:    [[EXTRA_USE:%.*]] = add nuw nsw i32 [[UDIV]], [[ZA]]
 ; CHECK-NEXT:    ret i32 [[EXTRA_USE]]
 ;
   %za = zext i8 %a to i32
@@ -219,8 +216,8 @@
 
 define <2 x i32> @urem_i32_c_vec(<2 x i8> %a) {
 ; CHECK-LABEL: @urem_i32_c_vec(
-; CHECK-NEXT:    [[ZA:%.*]] = zext <2 x i8> %a to <2 x i32>
-; CHECK-NEXT:    [[UREM:%.*]] = urem <2 x i32> [[ZA]], <i32 10, i32 17>
+; CHECK-NEXT:    [[TMP1:%.*]] = urem <2 x i8> %a, <i8 10, i8 17>
+; CHECK-NEXT:    [[UREM:%.*]] = zext <2 x i8> [[TMP1]] to <2 x i32>
 ; CHECK-NEXT:    ret <2 x i32> [[UREM]]
 ;
   %za = zext <2 x i8> %a to <2 x i32>
@@ -231,9 +228,8 @@
 define i32 @urem_i32_c_multiuse(i8 %a) {
 ; CHECK-LABEL: @urem_i32_c_multiuse(
 ; CHECK-NEXT:    [[ZA:%.*]] = zext i8 %a to i32
-; CHECK-NEXT:    [[TMP1:%.*]] = urem i8 %a, 10
-; CHECK-NEXT:    [[UREM:%.*]] = zext i8 [[TMP1]] to i32
-; CHECK-NEXT:    [[EXTRA_USE:%.*]] = add nuw nsw i32 [[ZA]], [[UREM]]
+; CHECK-NEXT:    [[UREM:%.*]] = urem i32 [[ZA]], 10
+; CHECK-NEXT:    [[EXTRA_USE:%.*]] = add nuw nsw i32 [[UREM]], [[ZA]]
 ; CHECK-NEXT:    ret i32 [[EXTRA_USE]]
 ;
   %za = zext i8 %a to i32
@@ -255,8 +251,8 @@
 
 define i32 @udiv_c_i32(i8 %a) {
 ; CHECK-LABEL: @udiv_c_i32(
-; CHECK-NEXT:    [[ZA:%.*]] = zext i8 %a to i32
-; CHECK-NEXT:    [[UDIV:%.*]] = udiv i32 10, [[ZA]]
+; CHECK-NEXT:    [[TMP1:%.*]] = udiv i8 10, %a
+; CHECK-NEXT:    [[UDIV:%.*]] = zext i8 [[TMP1]] to i32
 ; CHECK-NEXT:    ret i32 [[UDIV]]
 ;
   %za = zext i8 %a to i32
@@ -266,8 +262,8 @@
 
 define i32 @urem_c_i32(i8 %a) {
 ; CHECK-LABEL: @urem_c_i32(
-; CHECK-NEXT:    [[ZA:%.*]] = zext i8 %a to i32
-; CHECK-NEXT:    [[UREM:%.*]] = urem i32 10, [[ZA]]
+; CHECK-NEXT:    [[TMP1:%.*]] = urem i8 10, %a
+; CHECK-NEXT:    [[UREM:%.*]] = zext i8 [[TMP1]] to i32
 ; CHECK-NEXT:    ret i32 [[UREM]]
 ;
   %za = zext i8 %a to i32
@@ -275,3 +271,18 @@
   ret i32 %urem
 }
 
+; Make sure constexpr is handled.
+
+@b = external global [1 x i8]
+
+define i32 @udiv_constexpr(i8 %a) {
+; CHECK-LABEL: @udiv_constexpr(
+; CHECK-NEXT:    [[TMP1:%.*]] = udiv i8 %a, ptrtoint ([1 x i8]* @b to i8)
+; CHECK-NEXT:    [[D:%.*]] = zext i8 [[TMP1]] to i32
+; CHECK-NEXT:    ret i32 [[D]]
+;
+  %za = zext i8 %a to i32
+  %d = udiv i32 %za, zext (i8 ptrtoint ([1 x i8]* @b to i8) to i32)
+  ret i32 %d
+}
+