[llvm] r352089 - [IRBuilder] Remove positivity check from CreateAlignmentAssumption()

[Roman Lebedev via llvm-commits]

Author: lebedevri
Date: Thu Jan 24 11:32:48 2019
New Revision: 352089

URL: http://llvm.org/viewvc/llvm-project?rev=352089&view=rev
Log:
[IRBuilder] Remove positivity check from CreateAlignmentAssumption()

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now you `sext` it: `i2 0b110` - no longer valid alignment.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

Modified:
    llvm/trunk/include/llvm/IR/IRBuilder.h

Modified: llvm/trunk/include/llvm/IR/IRBuilder.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/IR/IRBuilder.h?rev=352089&r1=352088&r2=352089&view=diff
==============================================================================
--- llvm/trunk/include/llvm/IR/IRBuilder.h (original)
+++ llvm/trunk/include/llvm/IR/IRBuilder.h Thu Jan 24 11:32:48 2019
@@ -2279,10 +2279,11 @@ public:
                                       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 @@ public:
     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);