[Alignment][NFC] Allow constexpr Align

Summary:
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: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68329

llvm-svn: 373580

llvm/include/llvm/Support/Alignment.h

@@ -58,10 +58,10 @@ public:
   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;
-  Align &operator=(const Align &Other) = default;
-  Align(Align &&Other) = default;
-  Align &operator=(Align &&Other) = default;
+  constexpr Align(const Align &Other) = default;
+  constexpr Align &operator=(const Align &Other) = default;
+  constexpr Align(Align &&Other) = default;
+  constexpr Align &operator=(Align &&Other) = default;
 
   explicit Align(uint64_t Value) {
     assert(Value > 0 && "Value must not be 0");
@@ -80,6 +80,24 @@ public:
   /// would be better than
   /// `if (A > Align(1))`
   constexpr static const Align None() { return Align(); }
+
+  /// This function is useful when initializing constexpr Align constants.
+  /// e.g. static constexpr Align kAlign16 = Align::Constant<16>();
+  /// Most compilers (clang, gcc, icc) will be able to compute `ShiftValue`
+  /// at compile time with `Align::Align(uint64_t Value)` but to be
+  /// able to use Align as a constexpr constant use this method.
+  /// FIXME: When LLVM is C++17 ready `Align::Align(uint64_t Value)`
+  /// can be constexpr and we can dispatch between runtime (Log2_64) vs
+  /// compile time (CTLog2) versions using constexpr-if. Then this
+  /// function is no more necessary and we can add user defined literals
+  /// for convenience.
+  template <uint64_t kValue> constexpr static Align Constant() {
+    static_assert(kValue > 0 && llvm::isPowerOf2_64(kValue),
+                  "Not a valid alignment");
+    Align A;
+    A.ShiftValue = CTLog2<kValue>();
+    return A;
+  }
 };
 
 /// Treats the value 0 as a 1, so Align is always at least 1.

llvm/include/llvm/Support/MathExtras.h

@@ -532,6 +532,15 @@ inline double Log2(double Value) {
 #endif
 }
 
+/// Return the compile time log base 2 of the specified Value.
+/// `kValue` has to be a power of two.
+template <uint64_t kValue> static constexpr inline uint8_t CTLog2() {
+  static_assert(kValue > 0 && llvm::isPowerOf2_64(kValue),
+                "Value is not a valid power of 2");
+  return 1 + CTLog2<kValue / 2>();
+}
+template <> constexpr inline uint8_t CTLog2<1>() { return 0; }
+
 /// Return the floor log base 2 of the specified value, -1 if the value is zero.
 /// (32 bit edition.)
 /// Ex. Log2_32(32) == 5, Log2_32(1) == 0, Log2_32(0) == -1, Log2_32(6) == 2

llvm/lib/Target/AArch64/AArch64StackTagging.cpp

@@ -62,7 +62,7 @@ static cl::opt<bool> ClMergeInit(
 static cl::opt<unsigned> ClScanLimit("stack-tagging-merge-init-scan-limit",
                                      cl::init(40), cl::Hidden);
 
-static const Align kTagGranuleSize = Align(16);
+static constexpr Align kTagGranuleSize = Align::Constant<16>();
 
 namespace {
 

llvm/unittests/Support/AlignmentTest.cpp

@@ -44,6 +44,16 @@ TEST(AlignmentTest, ValidCTors) {
   }
 }
 
+TEST(AlignmentTest, CompileTimeConstant) {
+  EXPECT_EQ(Align::Constant<1>(), Align(1));
+  EXPECT_EQ(Align::Constant<2>(), Align(2));
+  EXPECT_EQ(Align::Constant<4>(), Align(4));
+  EXPECT_EQ(Align::Constant<8>(), Align(8));
+  EXPECT_EQ(Align::Constant<16>(), Align(16));
+  EXPECT_EQ(Align::Constant<32>(), Align(32));
+  EXPECT_EQ(Align::Constant<64>(), Align(64));
+}
+
 TEST(AlignmentTest, CheckMaybeAlignHasValue) {
   EXPECT_TRUE(MaybeAlign(1));
   EXPECT_TRUE(MaybeAlign(1).hasValue());

llvm/unittests/Support/MathExtrasTest.cpp

@@ -203,6 +203,25 @@ TEST(MathExtras, PowerOf2Floor) {
   EXPECT_EQ(4U, PowerOf2Floor(7U));
 }
 
+TEST(MathExtras, CTLog2) {
+  EXPECT_EQ(CTLog2<1ULL << 0>(), 0);
+  EXPECT_EQ(CTLog2<1ULL << 1>(), 1);
+  EXPECT_EQ(CTLog2<1ULL << 2>(), 2);
+  EXPECT_EQ(CTLog2<1ULL << 3>(), 3);
+  EXPECT_EQ(CTLog2<1ULL << 4>(), 4);
+  EXPECT_EQ(CTLog2<1ULL << 5>(), 5);
+  EXPECT_EQ(CTLog2<1ULL << 6>(), 6);
+  EXPECT_EQ(CTLog2<1ULL << 7>(), 7);
+  EXPECT_EQ(CTLog2<1ULL << 8>(), 8);
+  EXPECT_EQ(CTLog2<1ULL << 9>(), 9);
+  EXPECT_EQ(CTLog2<1ULL << 10>(), 10);
+  EXPECT_EQ(CTLog2<1ULL << 11>(), 11);
+  EXPECT_EQ(CTLog2<1ULL << 12>(), 12);
+  EXPECT_EQ(CTLog2<1ULL << 13>(), 13);
+  EXPECT_EQ(CTLog2<1ULL << 14>(), 14);
+  EXPECT_EQ(CTLog2<1ULL << 15>(), 15);
+}
+
 TEST(MathExtras, ByteSwap_32) {
   EXPECT_EQ(0x44332211u, ByteSwap_32(0x11223344));
   EXPECT_EQ(0xDDCCBBAAu, ByteSwap_32(0xAABBCCDD));