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

Forwarded: https://github.com/abseil/abseil-cpp/pull/1031
Origin: upstream, https://github.com/abseil/abseil-cpp/commit/022527c50e0e2bc937f9fa3c516e3e36cbba0845

Gbp-Pq: Name big-endian-random4.diff

diff --git a/absl/random/internal/explicit_seed_seq.h b/absl/random/internal/explicit_seed_seq.h
index e3aa31a184a1ff195daab3ed538bf34517a7d853..25f791535f81abb118757bd40ab147525e543c5b 100644 (file)
--- a/absl/random/internal/explicit_seed_seq.h
+++ b/absl/random/internal/explicit_seed_seq.h
@@ -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;
       }

diff --git a/absl/random/internal/randen_engine.h b/absl/random/internal/randen_engine.h
index 92bb8905f738a03a2e7087c1f8625fedb7a81a1f..372c3ac2bd2cdb13a9c0722b5ea142f0a6ee5660 100644 (file)
--- a/absl/random/internal/randen_engine.h
+++ b/absl/random/internal/randen_engine.h
@@ -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

diff --git a/absl/random/internal/randen_slow.cc b/absl/random/internal/randen_slow.cc
index d5c9347ba9844fcfe20f33cbbf4b90bf01af20f5..9bfd2a40922bfc0f5158017431c542f513b3bc0d 100644 (file)
--- a/absl/random/internal/randen_slow.cc
+++ b/absl/random/internal/randen_slow.cc
@@ -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;
 }