diff options
Diffstat (limited to 'absl/numeric/int128_no_intrinsic.inc')
| -rw-r--r-- | absl/numeric/int128_no_intrinsic.inc | 12 |
1 files changed, 9 insertions, 3 deletions
diff --git a/absl/numeric/int128_no_intrinsic.inc b/absl/numeric/int128_no_intrinsic.inc index 66f6809f..8834804c 100644 --- a/absl/numeric/int128_no_intrinsic.inc +++ b/absl/numeric/int128_no_intrinsic.inc @@ -279,7 +279,7 @@ constexpr int128 operator^(int128 lhs, int128 rhs) { } constexpr int128 operator<<(int128 lhs, int amount) { - // uint64_t shifts of >= 64 are undefined, so we need some special-casing. + // int64_t shifts of >= 64 are undefined, so we need some special-casing. return amount >= 64 ? MakeInt128( static_cast<int64_t>(Int128Low64(lhs) << (amount - 64)), 0) @@ -292,10 +292,16 @@ constexpr int128 operator<<(int128 lhs, int amount) { } constexpr int128 operator>>(int128 lhs, int amount) { - // uint64_t shifts of >= 64 are undefined, so we need some special-casing. + // int64_t shifts of >= 64 are undefined, so we need some special-casing. + // The (Int128High64(lhs) >> 32) >> 32 "trick" causes the the most significant + // int64 to be inititialized with all zeros or all ones correctly. It takes + // into account whether the number is negative or positive, and whether the + // current architecture does arithmetic or logical right shifts for negative + // numbers. return amount >= 64 ? MakeInt128( - 0, static_cast<uint64_t>(Int128High64(lhs) >> (amount - 64))) + (Int128High64(lhs) >> 32) >> 32, + static_cast<uint64_t>(Int128High64(lhs) >> (amount - 64))) : amount == 0 ? lhs : MakeInt128(Int128High64(lhs) >> amount, |