[IRBuilder] Remove positivity check from CreateAlignmentAssumption()

LebedevRI · LebedevRI · commit a95a7105efde · 2019-01-24T19:32:48.000Z
Summary: An alignment should be non-zero positive power-of-two, anything and everything else is UB. We should not have that check for all these prerequisites here, it's just UB. Also, that was likely confusing middle-end passes. While there, `CreateIntCast()` should be called with `/*isSigned*/ false`. Think about it, there are two explanations: "An alignment should be positive", therefore the sign bit is unset, so `zext` and `sext` is equivalent. Or a second one: you have `i2 0b10` - a valid alignment, now you `sext` it: `i2 0b110` - no longer valid alignment. Reviewers: craig.topper, jyknight, hfinkel, erichkeane, rjmccall Reviewed By: hfinkel, rjmccall Subscribers: hfinkel, llvm-commits Differential Revision: https://reviews.llvm.org/D54653 llvm-svn: 352089
diff --git a/llvm/include/llvm/IR/IRBuilder.h b/llvm/include/llvm/IR/IRBuilder.h
@@ -2279,10 +2279,11 @@ class IRBuilder : public IRBuilderBase, public Inserter {
                                       Value **TheCheck = nullptr) {
     assert(isa<PointerType>(PtrValue->getType()) &&
            "trying to create an alignment assumption on a non-pointer?");
+    assert(Alignment != 0 && "Invalid Alignment");
     auto *PtrTy = cast<PointerType>(PtrValue->getType());
     Type *IntPtrTy = getIntPtrTy(DL, PtrTy->getAddressSpace());
 
-    Value *Mask = ConstantInt::get(IntPtrTy, Alignment > 0 ? Alignment - 1 : 0);
+    Value *Mask = ConstantInt::get(IntPtrTy, Alignment - 1);
     return CreateAlignmentAssumptionHelper(DL, PtrValue, Mask, IntPtrTy,
                                            OffsetValue, TheCheck);
   }
@@ -2309,15 +2310,10 @@ class IRBuilder : public IRBuilderBase, public Inserter {
     Type *IntPtrTy = getIntPtrTy(DL, PtrTy->getAddressSpace());
 
     if (Alignment->getType() != IntPtrTy)
-      Alignment = CreateIntCast(Alignment, IntPtrTy, /*isSigned*/ true,
+      Alignment = CreateIntCast(Alignment, IntPtrTy, /*isSigned*/ false,
                                 "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");
+
+    Value *Mask = CreateSub(Alignment, ConstantInt::get(IntPtrTy, 1), "mask");
 
     return CreateAlignmentAssumptionHelper(DL, PtrValue, Mask, IntPtrTy,
                                            OffsetValue, TheCheck);
