Fix Randen and PCG on Big Endian platforms (#1031)

pull/1050/head
Milad Fa 3 years ago committed by GitHub
parent cc413f8b67
commit 022527c50e
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GPG Key ID: 4AEE18F83AFDEB23
  1. 2
      absl/random/internal/explicit_seed_seq.h
  2. 7
      absl/random/internal/randen_engine.h
  3. 21
      absl/random/internal/randen_slow.cc

@ -74,7 +74,7 @@ class ExplicitSeedSeq {
template <typename OutIterator>
void generate(OutIterator begin, OutIterator end) {
for (size_t index = 0; begin != end; begin++) {
*begin = state_.empty() ? 0 : little_endian::FromHost32(state_[index++]);
*begin = state_.empty() ? 0 : state_[index++];
if (index >= state_.size()) {
index = 0;
}

@ -121,6 +121,13 @@ class alignas(16) randen_engine {
const size_t requested_entropy = (entropy_size == 0) ? 8u : entropy_size;
std::fill(std::begin(buffer) + requested_entropy, std::end(buffer), 0);
seq.generate(std::begin(buffer), std::begin(buffer) + requested_entropy);
#ifdef ABSL_IS_BIG_ENDIAN
// Randen expects the seed buffer to be in Little Endian; reverse it on
// Big Endian platforms.
for (sequence_result_type& e : buffer) {
e = absl::little_endian::FromHost(e);
}
#endif
// The Randen paper suggests preferentially initializing even-numbered
// 128-bit vectors of the randen state (there are 16 such vectors).
// The seed data is merged into the state offset by 128-bits, which

@ -395,6 +395,23 @@ inline ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE void Permute(
}
}
// Enables native loads in the round loop by pre-swapping.
inline ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE void SwapEndian(
absl::uint128* state) {
#ifdef ABSL_IS_BIG_ENDIAN
for (uint32_t block = 0; block < RandenTraits::kFeistelBlocks; ++block) {
uint64_t new_lo = absl::little_endian::ToHost64(
static_cast<uint64_t>(state[block] >> 64));
uint64_t new_hi = absl::little_endian::ToHost64(
static_cast<uint64_t>((state[block] << 64) >> 64));
state[block] = (static_cast<absl::uint128>(new_hi) << 64) | new_lo;
}
#else
// Avoid warning about unused variable.
(void)state;
#endif
}
} // namespace
namespace absl {
@ -439,8 +456,12 @@ void RandenSlow::Generate(const void* keys_void, void* state_void) {
const absl::uint128 prev_inner = state[0];
SwapEndian(state);
Permute(state, keys);
SwapEndian(state);
// Ensure backtracking resistance.
*state ^= prev_inner;
}

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