diff options
Diffstat (limited to 'absl/numeric/int128.cc')
| -rw-r--r-- | absl/numeric/int128.cc | 22 |
1 files changed, 22 insertions, 0 deletions
diff --git a/absl/numeric/int128.cc b/absl/numeric/int128.cc index 33f528ce9cb7..93b62c52901e 100644 --- a/absl/numeric/int128.cc +++ b/absl/numeric/int128.cc @@ -123,6 +123,28 @@ uint128 MakeUint128FromFloat(T v) { return MakeUint128(0, static_cast<uint64_t>(v)); } + +#if defined(__clang__) && !defined(__SSE3__) +// Workaround for clang bug: https://bugs.llvm.org/show_bug.cgi?id=38289 +// Casting from long double to uint64_t is miscompiled and drops bits. +// It is more work, so only use when we need the workaround. +uint128 MakeUint128FromFloat(long double v) { + // Go 50 bits at a time, that fits in a double + static_assert(std::numeric_limits<double>::digits >= 50, ""); + static_assert(std::numeric_limits<long double>::digits <= 150, ""); + // Undefined behavior if v is not finite or cannot fit into uint128. + assert(std::isfinite(v) && v > -1 && v < std::ldexp(1.0L, 128)); + + v = std::ldexp(v, -100); + uint64_t w0 = static_cast<uint64_t>(static_cast<double>(std::trunc(v))); + v = std::ldexp(v - static_cast<double>(w0), 50); + uint64_t w1 = static_cast<uint64_t>(static_cast<double>(std::trunc(v))); + v = std::ldexp(v - static_cast<double>(w1), 50); + uint64_t w2 = static_cast<uint64_t>(static_cast<double>(std::trunc(v))); + return (static_cast<uint128>(w0) << 100) | (static_cast<uint128>(w1) << 50) | + static_cast<uint128>(w2); +} +#endif // __clang__ && !__SSE3__ } // namespace uint128::uint128(float v) : uint128(MakeUint128FromFloat(v)) {} |