Index: lib/Transforms/InstCombine/InstCombineCompares.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -3737,6 +3737,11 @@
           const APInt &CVal = CI->getValue();
           if (CVal.getBitWidth() - CVal.countLeadingZeros() > MulWidth)
             return nullptr;
+        } else {
+          // In this case we could have the operand of the binary operation
+          // being defined in another block, and performing the replacement
+          // could break the dominance relation.
+          return nullptr;
         }
       } else {
         // Other uses prohibit this transformation.
@@ -3856,18 +3861,17 @@
       } else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(U)) {
         assert(BO->getOpcode() == Instruction::And);
         // Replace (mul & mask) --> zext (mul.with.overflow & short_mask)
-        Value *ShortMask =
-            Builder.CreateTrunc(BO->getOperand(1), Builder.getIntNTy(MulWidth));
+        ConstantInt *CI = cast<ConstantInt>(BO->getOperand(1));
+        APInt ShortMask = CI->getValue().trunc(MulWidth);
         Value *ShortAnd = Builder.CreateAnd(Mul, ShortMask);
-        Value *Zext = Builder.CreateZExt(ShortAnd, BO->getType());
-        if (auto *ZextI = dyn_cast<Instruction>(Zext))
-          IC.Worklist.Add(ZextI);
+        Instruction *Zext =
+            cast<Instruction>(Builder.CreateZExt(ShortAnd, BO->getType()));
+        IC.Worklist.Add(Zext);
         IC.replaceInstUsesWith(*BO, Zext);
       } else {
         llvm_unreachable("Unexpected Binary operation");
       }
-      if (auto *UI = dyn_cast<Instruction>(U))
-        IC.Worklist.Add(UI);
+      IC.Worklist.Add(cast<Instruction>(U));
     }
   }
   if (isa<Instruction>(OtherVal))
Index: test/Transforms/InstCombine/2017-07-07-UMul-ZExt.ll
===================================================================
--- test/Transforms/InstCombine/2017-07-07-UMul-ZExt.ll
+++ test/Transforms/InstCombine/2017-07-07-UMul-ZExt.ll
@@ -1,7 +1,29 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt < %s -instcombine -S | FileCheck %s
 
-; CHECK: llvm.umul.with.overflow
 define i32 @sterix(i32, i8, i64) {
+; CHECK-LABEL: @sterix(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CONV:%.*]] = zext i32 [[TMP0:%.*]] to i64
+; CHECK-NEXT:    [[CONV1:%.*]] = sext i8 [[TMP1:%.*]] to i32
+; CHECK-NEXT:    [[MUL:%.*]] = mul i32 [[CONV1]], 1945964878
+; CHECK-NEXT:    [[SH_PROM:%.*]] = trunc i64 [[TMP2:%.*]] to i32
+; CHECK-NEXT:    [[SHR:%.*]] = lshr i32 [[MUL]], [[SH_PROM]]
+; CHECK-NEXT:    [[CONV2:%.*]] = zext i32 [[SHR]] to i64
+; CHECK-NEXT:    [[MUL3:%.*]] = mul nuw nsw i64 [[CONV]], [[CONV2]]
+; CHECK-NEXT:    [[CONV6:%.*]] = and i64 [[MUL3]], 4294967295
+; CHECK-NEXT:    [[TOBOOL:%.*]] = icmp eq i64 [[CONV6]], [[MUL3]]
+; CHECK-NEXT:    br i1 [[TOBOOL]], label [[LOR_RHS:%.*]], label [[LOR_END:%.*]]
+; CHECK:       lor.rhs:
+; CHECK-NEXT:    [[AND:%.*]] = and i64 [[MUL3]], [[TMP2]]
+; CHECK-NEXT:    [[CONV4:%.*]] = trunc i64 [[AND]] to i32
+; CHECK-NEXT:    [[TOBOOL7:%.*]] = icmp eq i32 [[CONV4]], 0
+; CHECK-NEXT:    [[PHITMP:%.*]] = zext i1 [[TOBOOL7]] to i32
+; CHECK-NEXT:    br label [[LOR_END]]
+; CHECK:       lor.end:
+; CHECK-NEXT:    [[TMP3:%.*]] = phi i32 [ 1, [[ENTRY:%.*]] ], [ [[PHITMP]], [[LOR_RHS]] ]
+; CHECK-NEXT:    ret i32 [[TMP3]]
+;
 entry:
   %conv = zext i32 %0 to i64
   %conv1 = sext i8 %1 to i32
Index: test/Transforms/InstCombine/pr33765.ll
===================================================================
--- /dev/null
+++ test/Transforms/InstCombine/pr33765.ll
@@ -0,0 +1,32 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S %s -instcombine | FileCheck %s
+
+@glob = external global i16
+
+define void @patatino(i8 %beth) {
+; CHECK-LABEL: @patatino(
+; CHECK-NEXT:    [[CONV:%.*]] = zext i8 [[BETH:%.*]] to i32
+; CHECK-NEXT:    br i1 undef, label [[IF_THEN9:%.*]], label [[IF_THEN9]]
+; CHECK:       if.then9:
+; CHECK-NEXT:    [[MUL:%.*]] = mul nuw nsw i32 [[CONV]], [[CONV]]
+; CHECK-NEXT:    [[TINKY:%.*]] = load i16, i16* @glob, align 2
+; CHECK-NEXT:    [[CONV131:%.*]] = zext i16 [[TINKY]] to i32
+; CHECK-NEXT:    [[AND:%.*]] = and i32 [[MUL]], [[CONV131]]
+; CHECK-NEXT:    [[CONV14:%.*]] = trunc i32 [[AND]] to i16
+; CHECK-NEXT:    store i16 [[CONV14]], i16* @glob, align 2
+; CHECK-NEXT:    ret void
+;
+  %conv = zext i8 %beth to i32
+  %mul = mul nuw nsw i32 %conv, %conv
+  %conv3 = and i32 %mul, 255
+  %tobool8 = icmp ne i32 %mul, %conv3
+  br i1 %tobool8, label %if.then9, label %if.then9
+
+if.then9:
+  %tinky = load i16, i16* @glob
+  %conv13 = sext i16 %tinky to i32
+  %and = and i32 %mul, %conv13
+  %conv14 = trunc i32 %and to i16
+  store i16 %conv14, i16* @glob
+  ret void
+}