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[Support] Clean up Align #160644

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Sep 25, 2025
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[Support] Clean up Align #160644

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23 changes: 6 additions & 17 deletions llvm/include/llvm/Support/Alignment.h
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Original file line number Diff line number Diff line change
Expand Up @@ -52,16 +52,8 @@ struct Align {
friend unsigned encode(struct MaybeAlign A);
friend struct MaybeAlign decodeMaybeAlign(unsigned Value);

/// A trivial type to allow construction of constexpr Align.
/// This is currently needed to workaround a bug in GCC 5.3 which prevents
/// definition of constexpr assign operators.
/// https://stackoverflow.com/questions/46756288/explicitly-defaulted-function-cannot-be-declared-as-constexpr-because-the-implic
/// FIXME: Remove this, make all assign operators constexpr and introduce user
/// defined literals when we don't have to support GCC 5.3 anymore.
/// https://llvm.org/docs/GettingStarted.html#getting-a-modern-host-c-toolchain
struct LogValue {
uint8_t Log;
};
struct FromShiftValue {};
constexpr Align(FromShiftValue, uint8_t Shift) : ShiftValue(Shift) {}

public:
/// Default is byte-aligned.
Expand All @@ -70,8 +62,8 @@ struct Align {
/// checks have been performed when building `Other`.
constexpr Align(const Align &Other) = default;
constexpr Align(Align &&Other) = default;
Align &operator=(const Align &Other) = default;
Align &operator=(Align &&Other) = default;
constexpr Align &operator=(const Align &Other) = default;
constexpr Align &operator=(Align &&Other) = default;

explicit Align(uint64_t Value) {
assert(Value > 0 && "Value must not be 0");
Expand All @@ -82,7 +74,7 @@ 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; }
constexpr uint64_t value() const { return uint64_t(1) << ShiftValue; }

// Returns the previous alignment.
Align previous() const {
Expand All @@ -94,17 +86,14 @@ struct Align {

/// Allow constructions of constexpr Align.
template <size_t kValue> constexpr static Align Constant() {
return LogValue{static_cast<uint8_t>(ConstantLog2<kValue>())};
return Align(FromShiftValue{}, ConstantLog2<kValue>());
}

/// Allow constructions of constexpr Align from types.
/// Compile time equivalent to Align(alignof(T)).
template <typename T> constexpr static Align Of() {
return Constant<std::alignment_of_v<T>>();
}

/// Constexpr constructor from LogValue type.
constexpr Align(LogValue CA) : ShiftValue(CA.Log) {}
};

/// Treats the value 0 as a 1, so Align is always at least 1.
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10 changes: 10 additions & 0 deletions llvm/unittests/Support/AlignmentTest.cpp
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Expand Up @@ -44,6 +44,16 @@ TEST(AlignmentTest, AlignConstexprConstant) {
EXPECT_EQ(Align(alignof(uint64_t)), kConstantAlign);
}

TEST(AlignmentTest, ConstexprAssign) {
constexpr auto assignAndGet = []() constexpr {
Align A = Align::Constant<8>();
Align B = Align::Constant<16>();
A = B;
return A.value();
};
static_assert(assignAndGet() == 16);
}

std::vector<uint64_t> getValidAlignments() {
std::vector<uint64_t> Out;
for (size_t Shift = 0; Shift < 64; ++Shift)
Expand Down