From: Roman Lebedev <lebedev.ri@gmail.com>
Date: Thu, 24 Jan 2019 19:32:48 +0000 (+0000)
Subject: [IRBuilder] Remove positivity check from CreateAlignmentAssumption()
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=dea0eae9a50dc3ae9132d5d751683edb3c209bd2;p=llvm

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now you `sext` it: `i2 0b110` - no longer valid alignment.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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@352089 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/include/llvm/IR/IRBuilder.h b/include/llvm/IR/IRBuilder.h
index e14864924fe..c7cfee6e093 100644
--- a/include/llvm/IR/IRBuilder.h
+++ b/include/llvm/IR/IRBuilder.h
@@ -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);