Release Notes

Unreleased

Breaking: int128_t::high is now std::uint64_t

int128_t previously stored std::uint64_t low and std::int64_t high. Both words are now std::uint64_t; the value is still interpreted as two’s complement across the pair, and no bit of the representation moved.

The reason is codegen. When the two halves of the struct have different types, a compiler’s data-reference analysis cannot merge them into a single wide access, so loops over int128_t are emitted with shuffles or scalarized outright. Measured with GCC 14 at -O3 -march=znver3 on r[i] = x[i] | y[i] over an std::array<int128_t, 128>:

before after

|, &, ^

96 instructions, 14 shuffles

24 instructions, 0 shuffles

~

74 instructions, 10 shuffles

20 instructions, 0 shuffles

Those are now exactly the numbers uint128_t has always produced, since it already had uniform word types. Scalar code is unaffected or better: a + b drops from 20 instructions to 10, a << n from 17 to 12, and a >> n from 18 to 13.

What still works

  • The two-word constructor is unchanged and still takes its high word as std::int64_t, so int128_t{-1, 0} and int128_t{INT64_MIN, 0} compile and mean exactly what they did.

  • sizeof, alignof, and the in-memory byte order are unchanged. The type remains trivially copyable and standard layout, so anything that memcpy`s an `int128_t is unaffected.

  • Every operator produces bit-identical results. Conversions to and from uint128_t and the native __int128 are unchanged.

What to change

Code that reads the high member and depends on its signedness. The member is unsigned now, so a sign test on it silently becomes false:

// Before: true for negative values. Now always false.
if (value.high < 0) { ... }

// Use the accessor
if (value.signed_high() < 0) { ... }

// Or, usually clearer, compare the value
if (value < 0) { ... }

signed_high() is a new public member:

BOOST_INT128_HOST_DEVICE constexpr std::int64_t signed_high() const noexcept;

It is a pure reinterpretation of the stored bits, constexpr everywhere, and available on device. Passing high to something expecting a std::int64_t also needs it, since the conversion is now the other direction.

See Storage and signed_high for details.

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