[Alignment] Add a None() member function

Summary:
This will allow writing `if(A != llvm::Align::None())` which is clearer than `if(A > llvm::Align(1))`

This is patch is part of a series to introduce an Alignment type.
See this thread for context: http://lists.llvm.org/pipermail/llvm-dev/2019-July/133851.html
See this patch for the introduction of the type: https://reviews.llvm.org/D64790

Reviewers: courbet

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67697

llvm-svn: 372207

Author: gchatelet

llvm/include/llvm/Support/Alignment.h

@@ -55,7 +55,7 @@ struct Align {
 
 public:
   /// Default is byte-aligned.
-  Align() = default;
+  constexpr Align() = default;
   /// Do not perform checks in case of copy/move construct/assign, because the
   /// checks have been performed when building `Other`.
   Align(const Align &Other) = default;
@@ -73,6 +73,13 @@ struct Align {
   /// This is a hole in the type system and should not be abused.
   /// Needed to interact with C for instance.
   uint64_t value() const { return uint64_t(1) << ShiftValue; }
+
+  /// Returns a default constructed Align which corresponds to no alignment.
+  /// This is useful to test for unalignment as it conveys clear semantic.
+  /// `if (A != llvm::Align::None())`
+  /// would be better than
+  /// `if (A > llvm::Align(1))`
+  constexpr static const Align None() { return llvm::Align(); }
 };
 
 /// Treats the value 0 as a 1, so Align is always at least 1.

llvm/unittests/Support/AlignmentTest.cpp

@@ -28,7 +28,10 @@ std::vector<uint64_t> getValidAlignments() {
   return Out;
 }
 
-TEST(AlignmentTest, AlignDefaultCTor) { EXPECT_EQ(Align().value(), 1ULL); }
+TEST(AlignmentTest, AlignDefaultCTor) {
+  EXPECT_EQ(Align().value(), 1ULL);
+  EXPECT_EQ(Align::None().value(), 1ULL);
+}
 
 TEST(AlignmentTest, MaybeAlignDefaultCTor) {
   EXPECT_FALSE(MaybeAlign().hasValue());