Index: include/llvm/IR/IRBuilder.h
===================================================================
--- include/llvm/IR/IRBuilder.h
+++ include/llvm/IR/IRBuilder.h
@@ -54,6 +54,9 @@
 class MDNode;
 class Use;
 
+
+enum class AlignmentSign { Signed, Unsigned};
+
 /// This provides the default implementation of the IRBuilder
 /// 'InsertHelper' method that is called whenever an instruction is created by
 /// IRBuilder and needs to be inserted.
@@ -2182,7 +2185,7 @@
   /// This overload handles the condition where the Alignment is dependent
   /// on an existing value rather than a static value.
   CallInst *CreateAlignmentAssumption(const DataLayout &DL, Value *PtrValue,
-                                      Value *Alignment,
+                                      Value *Alignment, AlignmentSign Sign,
                                       Value *OffsetValue = nullptr) {
     assert(isa<PointerType>(PtrValue->getType()) &&
            "trying to create an alignment assumption on a non-pointer?");
@@ -2190,15 +2193,30 @@
     Type *IntPtrTy = getIntPtrTy(DL, PtrTy->getAddressSpace());
 
     if (Alignment->getType() != IntPtrTy)
-      Alignment = CreateIntCast(Alignment, IntPtrTy, /*isSigned*/ true,
+      Alignment = CreateIntCast(Alignment, IntPtrTy,
+                                /*isSigned*/ Sign == AlignmentSign::Signed,
                                 "alignmentcast");
-    Value *IsPositive =
-        CreateICmp(CmpInst::ICMP_SGT, Alignment,
-                   ConstantInt::get(Alignment->getType(), 0), "ispositive");
-    Value *PositiveMask =
-        CreateSub(Alignment, ConstantInt::get(IntPtrTy, 1), "positivemask");
-    Value *Mask = CreateSelect(IsPositive, PositiveMask,
-                               ConstantInt::get(IntPtrTy, 0), "mask");
+
+    // Alignments are only valid if  > 0 && IsPowerOf2.
+    Value *IsPositive = CreateICmp(
+        Sign == AlignmentSign::Signed ? CmpInst::ICMP_SGT : CmpInst::ICMP_UGT,
+        Alignment, ConstantInt::get(Alignment->getType(), 0), "ispositive");
+
+    // Save alignment - 1, which is valuable for both IsPowerOf2 and creating
+    // the mask.
+    Value *Sub1 = CreateSub(
+        Alignment, ConstantInt::get(Alignment->getType(), 1), "alignsubone");
+
+    // Calculate Alignment & (Alignment - 1) == 0
+    Value *AlignAndMinus1 = CreateAnd(Alignment, Sub1, "alignandsubone");
+    Value *IsPowerOf2 =
+        CreateICmpEQ(AlignAndMinus1, ConstantInt::get(Alignment->getType(), 0),
+                     "ispowertwo");
+
+    Value *ValidMask = CreateAnd(IsPositive, IsPowerOf2, "alignisvalid");
+
+    Value *Mask = CreateSelect(
+        ValidMask, Sub1, ConstantInt::get(Alignment->getType(), 0), "mask");
 
     return CreateAlignmentAssumptionHelper(DL, PtrValue, Mask, IntPtrTy,
                                            OffsetValue);