Index: include/llvm/Analysis/MemoryBuiltins.h
===================================================================
--- include/llvm/Analysis/MemoryBuiltins.h
+++ include/llvm/Analysis/MemoryBuiltins.h
@@ -224,6 +224,9 @@
   SizeOffsetType visitSelectInst(SelectInst &I);
   SizeOffsetType visitUndefValue(UndefValue&);
   SizeOffsetType visitInstruction(Instruction &I);
+
+private:
+  bool CheckedZextOrTrunc(APInt &I);
 };
 
 typedef std::pair<Value*, Value*> SizeOffsetEvalType;
Index: lib/Analysis/MemoryBuiltins.cpp
===================================================================
--- lib/Analysis/MemoryBuiltins.cpp
+++ lib/Analysis/MemoryBuiltins.cpp
@@ -505,6 +505,22 @@
   return unknown();
 }
 
+/// When we're compiling N-bit code, and the user uses parameters that are
+/// greater than N bits (e.g. uint64_t on a 32-bit build), we can run into
+/// trouble with APInt size issues. This function handles resizing + overflow
+/// checks for us. Check and zext or trunc /p I depending on IntTyBits and
+/// I's value.
+bool ObjectSizeOffsetVisitor::CheckedZextOrTrunc(APInt &I) {
+  // More bits than we can handle. Checking the bit width isn't necessary, but
+  // it's faster than checking active bits, and should give `false` in the
+  // vast majority of cases.
+  if (I.getBitWidth() > IntTyBits && I.getActiveBits() > IntTyBits)
+    return false;
+  if (I.getBitWidth() != IntTyBits)
+    I = I.zextOrTrunc(IntTyBits);
+  return true;
+};
+
 SizeOffsetType ObjectSizeOffsetVisitor::visitAllocaInst(AllocaInst &I) {
   if (!I.getAllocatedType()->isSized())
     return unknown();
@@ -515,8 +531,15 @@
 
   Value *ArraySize = I.getArraySize();
   if (const ConstantInt *C = dyn_cast<ConstantInt>(ArraySize)) {
-    Size *= C->getValue().zextOrSelf(IntTyBits);
-    return std::make_pair(align(Size, I.getAlignment()), Zero);
+    APInt NumElems = C->getValue();
+    if (!CheckedZextOrTrunc(NumElems)) {
+      return unknown();
+    }
+
+    bool Overflow;
+    Size = Size.umul_ov(NumElems, Overflow);
+    return Overflow ? unknown() : std::make_pair(align(Size, I.getAlignment()),
+                                                 Zero);
   }
   return unknown();
 }
@@ -561,21 +584,6 @@
   if (!Arg)
     return unknown();
 
-  // When we're compiling N-bit code, and the user uses parameters that are
-  // greater than N bits (e.g. uint64_t on a 32-bit build), we can run into
-  // trouble with APInt size issues. This function handles resizing + overflow
-  // checks for us.
-  auto CheckedZextOrTrunc = [&](APInt &I) {
-    // More bits than we can handle. Checking the bit width isn't necessary, but
-    // it's faster than checking active bits, and should give `false` in the
-    // vast majority of cases.
-    if (I.getBitWidth() > IntTyBits && I.getActiveBits() > IntTyBits)
-      return false;
-    if (I.getBitWidth() != IntTyBits)
-      I = I.zextOrTrunc(IntTyBits);
-    return true;
-  };
-
   APInt Size = Arg->getValue();
   if (!CheckedZextOrTrunc(Size))
     return unknown();
Index: test/Transforms/InstCombine/pr33689_same_bitwidth.ll
===================================================================
--- /dev/null
+++ test/Transforms/InstCombine/pr33689_same_bitwidth.ll
@@ -0,0 +1,53 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S -instcombine %s -o - | FileCheck %s
+
+; All the "useless" instructions should be removed and we shouldn't crash.
+
+target datalayout = "p:16:16"
+
+%i64_t = type i64
+
+@a = external global i16
+@b = external global i16*
+
+define void @f() {
+; CHECK-LABEL: @f(
+; CHECK-NEXT:  bb0:
+; CHECK-NEXT:    [[TMP12:%.*]] = alloca [2 x i32], align 8
+; CHECK-NEXT:    [[TMP12_SUB:%.*]] = getelementptr inbounds [2 x i32], [2 x i32]* [[TMP12]], i16 0, i16 0
+; CHECK-NEXT:    br i1 undef, label [[BB1:%.*]], label [[BB2:%.*]]
+; CHECK:       bb1:
+; CHECK-NEXT:    [[TMP8:%.*]] = ptrtoint [2 x i32]* [[TMP12]] to i16
+; CHECK-NEXT:    store i16 [[TMP8]], i16* @a, align 2
+; CHECK-NEXT:    unreachable
+; CHECK:       bb2:
+; CHECK-NEXT:    [[TMP9:%.*]] = load i16*, i16** @b, align 2
+; CHECK-NEXT:    store i16 0, i16* [[TMP9]], align 2
+; CHECK-NEXT:    [[TMP10:%.*]] = load i32, i32* [[TMP12_SUB]], align 8
+; CHECK-NEXT:    [[TMP11:%.*]] = add i32 [[TMP10]], -1
+; CHECK-NEXT:    store i32 [[TMP11]], i32* [[TMP12_SUB]], align 8
+; CHECK-NEXT:    ret void
+;
+bb0:
+  %tmp1 = alloca %i64_t
+  %tmp2 = bitcast %i64_t* %tmp1 to i32*
+  %useless3 = bitcast %i64_t* %tmp1 to i16*
+  %useless4 = getelementptr inbounds i16, i16* %useless3, i16 undef
+  %useless5 = bitcast i16* %useless4 to i32*
+  br i1 undef, label %bb1, label %bb2
+
+bb1:                                              ; preds = %bb0
+  %useless6 = insertvalue [1 x i32*] undef, i32* %tmp2, 0
+  %useless7 = insertvalue [1 x i32*] %useless6, i32* null, 0
+  %tmp8 = ptrtoint i32* %tmp2 to i16
+  store i16 %tmp8, i16* @a
+  unreachable
+
+bb2:                                              ; preds = %bb0
+  %tmp9 = load i16*, i16** @b
+  store i16 0, i16* %tmp9
+  %tmp10 = load i32, i32* %tmp2
+  %tmp11 = sub i32 %tmp10, 1
+  store i32 %tmp11, i32* %tmp2
+  ret void
+}