[LLVM][NFC] Adding an Alignment type to LLVM

Summary:
This patch introduces a type to straighten LLVM's alignment management.
See this thread for context: http://lists.llvm.org/pipermail/llvm-dev/2019-July/133851.html

The next step is to use this type throughout LLVM

Reviewers: jfb, jakehehrlich

Subscribers: mgorny, mgrang, dexonsmith, llvm-commits, courbet

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64790

llvm-svn: 367393

llvm/include/llvm/Support/Alignment.h

@@ -0,0 +1,345 @@
+//===-- llvm/Support/Alignment.h - Useful alignment functions ---*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file contains types to represent alignments.
+// They are instrumented to guarantee some invariants are preserved and prevent
+// invalid manipulations.
+//
+// - Align represents an alignment in bytes, it is always set and always a valid
+// power of two, its minimum value is 1 which means no alignment requirements.
+//
+// - MaybeAlign is an optional type, it may be undefined or set. When it's set
+// you can get the underlying Align type by using the getValue() method.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_SUPPORT_ALIGNMENT_H_
+#define LLVM_SUPPORT_ALIGNMENT_H_
+
+#include "llvm/ADT/Optional.h"
+#include "llvm/Support/Compiler.h"
+#include "llvm/Support/MathExtras.h"
+#include <cassert>
+#include <limits>
+
+namespace llvm {
+
+#define ALIGN_CHECK_ISPOSITIVE(decl)                                           \
+  assert(decl > 0 && (#decl " should be defined"))
+#define ALIGN_CHECK_ISSET(decl)                                                \
+  assert(decl.hasValue() && (#decl " should be defined"))
+
+// This struct is a compact representation of a valid (non-zero power of two)
+// alignment.
+// It is suitable for use as static global constants.
+struct Align {
+private:
+  uint8_t ShiftValue = 0; // The log2 of the required alignment.
+                          // ShiftValue is less than 64 by construction.
+
+  friend struct MaybeAlign;
+  friend unsigned Log2(Align);
+  friend bool operator==(Align Lhs, Align Rhs);
+  friend bool operator!=(Align Lhs, Align Rhs);
+  friend bool operator<=(Align Lhs, Align Rhs);
+  friend bool operator>=(Align Lhs, Align Rhs);
+  friend bool operator<(Align Lhs, Align Rhs);
+  friend bool operator>(Align Lhs, Align Rhs);
+  friend unsigned encode(struct MaybeAlign A);
+  friend struct MaybeAlign decodeMaybeAlign(unsigned Value);
+
+public:
+  // Default is byte-aligned.
+  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;
+
+  explicit Align(uint64_t Value) {
+    assert(Value > 0 && "Value must not be 0");
+    assert(llvm::isPowerOf2_64(Value) && "Alignment is not a power of 2");
+    ShiftValue = Log2_64(Value);
+    assert(ShiftValue < 64 && "Broken invariant");
+  }
+
+  // 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; }
+};
+
+// Treats the value 0 as a 1, so Align is always at least 1.
+inline Align assumeAligned(uint64_t Value) {
+  return Value ? Align(Value) : Align();
+}
+
+// This struct is a compact representation of a valid (power of two) or
+// undefined (0) alignment.
+struct MaybeAlign : public llvm::Optional<Align> {
+private:
+  using UP = llvm::Optional<Align>;
+
+public:
+  // Do not perform checks in case of copy/move construct/assign, because the
+  // checks have been performed when building `Other`.
+  MaybeAlign(const MaybeAlign &Other) = default;
+  MaybeAlign &operator=(const MaybeAlign &Other) = default;
+  MaybeAlign(MaybeAlign &&Other) = default;
+  MaybeAlign &operator=(MaybeAlign &&Other) = default;
+
+  // Use llvm::Optional<Align> constructor.
+  using UP::UP;
+
+  explicit MaybeAlign(uint64_t Value) {
+    assert((Value == 0 || llvm::isPowerOf2_64(Value)) &&
+           "Alignment is not 0 or a power of 2");
+    if (Value)
+      emplace(Value);
+  }
+
+  // For convenience, returns a valid alignment or 1 if undefined.
+  Align valueOrOne() const { return hasValue() ? getValue() : Align(); }
+};
+
+// -----------------------------------------------------------------------------
+// isAligned: Checks that SizeInBytes is a multiple of the alignment.
+// -----------------------------------------------------------------------------
+
+inline bool isAligned(Align Lhs, uint64_t SizeInBytes) {
+  return SizeInBytes % Lhs.value() == 0;
+}
+
+// Returns false if the alignment is undefined.
+inline bool isAligned(MaybeAlign Lhs, uint64_t SizeInBytes) {
+  ALIGN_CHECK_ISSET(Lhs);
+  return SizeInBytes % (*Lhs).value() == 0;
+}
+
+// -----------------------------------------------------------------------------
+// alignTo: Returns a multiple of A needed to store `Size` bytes.
+// -----------------------------------------------------------------------------
+
+inline uint64_t alignTo(uint64_t Size, Align A) {
+  return (Size + A.value() - 1) / A.value() * A.value();
+}
+
+// Returns `Size` if current alignment is undefined.
+inline uint64_t alignTo(uint64_t Size, MaybeAlign A) {
+  return A ? alignTo(Size, A.getValue()) : Size;
+}
+
+// -----------------------------------------------------------------------------
+// log2: Returns the log2 of the alignment.
+// e.g. Align(16).log2() == 4
+// -----------------------------------------------------------------------------
+
+inline unsigned Log2(Align A) { return A.ShiftValue; }
+
+/// \pre A must be defined.
+inline unsigned Log2(MaybeAlign A) {
+  ALIGN_CHECK_ISSET(A);
+  return Log2(A.getValue());
+}
+
+// -----------------------------------------------------------------------------
+// commonAlignment: returns the alignment that satisfies both alignments.
+// Same semantic as MinAlign.
+// -----------------------------------------------------------------------------
+
+inline Align commonAlignment(Align A, Align B) { return std::min(A, B); }
+
+inline Align commonAlignment(Align A, uint64_t Offset) {
+  return Align(MinAlign(A.value(), Offset));
+}
+
+inline MaybeAlign commonAlignment(MaybeAlign A, MaybeAlign B) {
+  return A && B ? commonAlignment(*A, *B) : A ? A : B;
+}
+
+inline MaybeAlign commonAlignment(MaybeAlign A, uint64_t Offset) {
+  return MaybeAlign(MinAlign((*A).value(), Offset));
+}
+
+// -----------------------------------------------------------------------------
+// Encode/Decode
+// -----------------------------------------------------------------------------
+
+// Returns a more compact representation of the alignment.
+// An undefined MaybeAlign is encoded as 0.
+inline unsigned encode(MaybeAlign A) { return A ? A->ShiftValue + 1 : 0; }
+
+// Dual operation of the encode function above.
+inline MaybeAlign decodeMaybeAlign(unsigned Value) {
+  if (Value == 0)
+    return MaybeAlign();
+  Align Out;
+  Out.ShiftValue = Value - 1;
+  return Out;
+}
+
+// Returns a more compact representation of the alignment.
+// The encoded value is positive by definition.
+// e.g. Align(1).encode() == 1
+// e.g. Align(16).encode() == 5
+inline unsigned encode(Align A) { return encode(MaybeAlign(A)); }
+
+// -----------------------------------------------------------------------------
+// Comparisons
+// -----------------------------------------------------------------------------
+
+// Comparisons between Align and scalars. Rhs must be positive.
+inline bool operator==(Align Lhs, uint64_t Rhs) {
+  ALIGN_CHECK_ISPOSITIVE(Rhs);
+  return Lhs.value() == Rhs;
+}
+inline bool operator!=(Align Lhs, uint64_t Rhs) {
+  ALIGN_CHECK_ISPOSITIVE(Rhs);
+  return Lhs.value() != Rhs;
+}
+inline bool operator<=(Align Lhs, uint64_t Rhs) {
+  ALIGN_CHECK_ISPOSITIVE(Rhs);
+  return Lhs.value() <= Rhs;
+}
+inline bool operator>=(Align Lhs, uint64_t Rhs) {
+  ALIGN_CHECK_ISPOSITIVE(Rhs);
+  return Lhs.value() >= Rhs;
+}
+inline bool operator<(Align Lhs, uint64_t Rhs) {
+  ALIGN_CHECK_ISPOSITIVE(Rhs);
+  return Lhs.value() < Rhs;
+}
+inline bool operator>(Align Lhs, uint64_t Rhs) {
+  ALIGN_CHECK_ISPOSITIVE(Rhs);
+  return Lhs.value() > Rhs;
+}
+
+// Comparisons between MaybeAlign and scalars.
+inline bool operator==(MaybeAlign Lhs, uint64_t Rhs) {
+  return Lhs ? (*Lhs).value() == Rhs : Rhs == 0;
+}
+inline bool operator!=(MaybeAlign Lhs, uint64_t Rhs) {
+  return Lhs ? (*Lhs).value() != Rhs : Rhs != 0;
+}
+inline bool operator<=(MaybeAlign Lhs, uint64_t Rhs) {
+  ALIGN_CHECK_ISSET(Lhs);
+  ALIGN_CHECK_ISPOSITIVE(Rhs);
+  return (*Lhs).value() <= Rhs;
+}
+inline bool operator>=(MaybeAlign Lhs, uint64_t Rhs) {
+  ALIGN_CHECK_ISSET(Lhs);
+  ALIGN_CHECK_ISPOSITIVE(Rhs);
+  return (*Lhs).value() >= Rhs;
+}
+inline bool operator<(MaybeAlign Lhs, uint64_t Rhs) {
+  ALIGN_CHECK_ISSET(Lhs);
+  ALIGN_CHECK_ISPOSITIVE(Rhs);
+  return (*Lhs).value() < Rhs;
+}
+inline bool operator>(MaybeAlign Lhs, uint64_t Rhs) {
+  ALIGN_CHECK_ISSET(Lhs);
+  ALIGN_CHECK_ISPOSITIVE(Rhs);
+  return (*Lhs).value() > Rhs;
+}
+
+// Comparisons operators between Align.
+inline bool operator==(Align Lhs, Align Rhs) {
+  return Lhs.ShiftValue == Rhs.ShiftValue;
+}
+inline bool operator!=(Align Lhs, Align Rhs) {
+  return Lhs.ShiftValue != Rhs.ShiftValue;
+}
+inline bool operator<=(Align Lhs, Align Rhs) {
+  return Lhs.ShiftValue <= Rhs.ShiftValue;
+}
+inline bool operator>=(Align Lhs, Align Rhs) {
+  return Lhs.ShiftValue >= Rhs.ShiftValue;
+}
+inline bool operator<(Align Lhs, Align Rhs) {
+  return Lhs.ShiftValue < Rhs.ShiftValue;
+}
+inline bool operator>(Align Lhs, Align Rhs) {
+  return Lhs.ShiftValue > Rhs.ShiftValue;
+}
+
+// Comparisons operators between Align and MaybeAlign.
+inline bool operator==(Align Lhs, MaybeAlign Rhs) {
+  ALIGN_CHECK_ISSET(Rhs);
+  return Lhs.value() == (*Rhs).value();
+}
+inline bool operator!=(Align Lhs, MaybeAlign Rhs) {
+  ALIGN_CHECK_ISSET(Rhs);
+  return Lhs.value() != (*Rhs).value();
+}
+inline bool operator<=(Align Lhs, MaybeAlign Rhs) {
+  ALIGN_CHECK_ISSET(Rhs);
+  return Lhs.value() <= (*Rhs).value();
+}
+inline bool operator>=(Align Lhs, MaybeAlign Rhs) {
+  ALIGN_CHECK_ISSET(Rhs);
+  return Lhs.value() >= (*Rhs).value();
+}
+inline bool operator<(Align Lhs, MaybeAlign Rhs) {
+  ALIGN_CHECK_ISSET(Rhs);
+  return Lhs.value() < (*Rhs).value();
+}
+inline bool operator>(Align Lhs, MaybeAlign Rhs) {
+  ALIGN_CHECK_ISSET(Rhs);
+  return Lhs.value() > (*Rhs).value();
+}
+
+// Comparisons operators between MaybeAlign and Align.
+inline bool operator==(MaybeAlign Lhs, Align Rhs) {
+  ALIGN_CHECK_ISSET(Lhs);
+  return Lhs && (*Lhs).value() == Rhs.value();
+}
+inline bool operator!=(MaybeAlign Lhs, Align Rhs) {
+  ALIGN_CHECK_ISSET(Lhs);
+  return Lhs && (*Lhs).value() != Rhs.value();
+}
+inline bool operator<=(MaybeAlign Lhs, Align Rhs) {
+  ALIGN_CHECK_ISSET(Lhs);
+  return Lhs && (*Lhs).value() <= Rhs.value();
+}
+inline bool operator>=(MaybeAlign Lhs, Align Rhs) {
+  ALIGN_CHECK_ISSET(Lhs);
+  return Lhs && (*Lhs).value() >= Rhs.value();
+}
+inline bool operator<(MaybeAlign Lhs, Align Rhs) {
+  ALIGN_CHECK_ISSET(Lhs);
+  return Lhs && (*Lhs).value() < Rhs.value();
+}
+inline bool operator>(MaybeAlign Lhs, Align Rhs) {
+  ALIGN_CHECK_ISSET(Lhs);
+  return Lhs && (*Lhs).value() > Rhs.value();
+}
+
+// -----------------------------------------------------------------------------
+// Division
+// -----------------------------------------------------------------------------
+
+inline Align operator/(Align Lhs, uint64_t Divisor) {
+  assert(llvm::isPowerOf2_64(Divisor) &&
+         "Divisor must be positive and a power of 2");
+  assert(Lhs != 1 && "Can't halve byte alignment");
+  return Align(Lhs.value() / Divisor);
+}
+
+inline MaybeAlign operator/(MaybeAlign Lhs, uint64_t Divisor) {
+  assert(llvm::isPowerOf2_64(Divisor) &&
+         "Divisor must be positive and a power of 2");
+  return Lhs ? Lhs.getValue() / Divisor : MaybeAlign();
+}
+
+#undef ALIGN_CHECK_ISPOSITIVE
+#undef ALIGN_CHECK_ISSET
+
+} // namespace llvm
+
+#endif // LLVM_SUPPORT_ALIGNMENT_H_