From d910383b492a2941168ee458ed93abd3c4f1ff37 Mon Sep 17 00:00:00 2001
From: Evan Brown <ezb@google.com>
Date: Fri, 3 Jan 2025 09:49:48 -0800
Subject: [PATCH] Mix pointers twice in build modes in which the
 PointerAlignment test is flaky if we mix once.

This reverts to the behavior from before a recent hashing change in these build modes. Note that load tests showed a positive impact for that change, but the load tests weren't run in these build modes with lower pointer entropy.

PiperOrigin-RevId: 711779528
Change-Id: Ie5a777c9a7e32a1417968ef048c496b17524b8e3
---
 absl/hash/hash_test.cc    | 13 ++-----------
 absl/hash/internal/hash.h |  5 +++++
 2 files changed, 7 insertions(+), 11 deletions(-)

diff --git a/absl/hash/hash_test.cc b/absl/hash/hash_test.cc
index 7bf5a80cc04..66efd2cf2fc 100644
--- a/absl/hash/hash_test.cc
+++ b/absl/hash/hash_test.cc
@@ -164,8 +164,6 @@ TEST(HashValueTest, Pointer) {
       std::make_tuple(&i, ptr, nullptr, ptr + 1, n)));
 }
 
-// The test is flaky in ASan and on Apple platforms.
-#if !defined(ABSL_HAVE_ADDRESS_SANITIZER) && !defined(__APPLE__)
 TEST(HashValueTest, PointerAlignment) {
   // We want to make sure that pointer alignment will not cause too many bits to
   // be stuck.
@@ -192,18 +190,11 @@ TEST(HashValueTest, PointerAlignment) {
     // Limit the scope to the bits we would be using for Swisstable.
     constexpr size_t kMask = (1 << (kLog2NumValues + 7)) - 1;
     size_t stuck_bits = (~bits_or | bits_and) & kMask;
-    // Test that there are less than 3 stuck bits. Sometimes we see stuck_bits
+    // Test that there are at most 2 stuck bits. Sometimes we see stuck_bits
     // of 0x3.
-    size_t stuck_bits_threshold = 3;
-#ifdef __ANDROID__
-    // On Android, we sometimes see stuck_bits of 0x780 when align is 11520.
-    stuck_bits_threshold = 5;
-#endif
-    EXPECT_LT(absl::popcount(stuck_bits), stuck_bits_threshold)
-        << "0x" << std::hex << stuck_bits;
+    EXPECT_LE(absl::popcount(stuck_bits), 2) << "0x" << std::hex << stuck_bits;
   }
 }
-#endif  // !defined(ABSL_HAVE_ADDRESS_SANITIZER) && !defined(__APPLE__)
 
 TEST(HashValueTest, PointerToMember) {
   struct Bass {
diff --git a/absl/hash/internal/hash.h b/absl/hash/internal/hash.h
index f70170b122d..797183adeb9 100644
--- a/absl/hash/internal/hash.h
+++ b/absl/hash/internal/hash.h
@@ -460,6 +460,11 @@ std::enable_if_t<std::is_pointer<T>::value, H> AbslHashValue(H hash_state,
   // Due to alignment, pointers tend to have low bits as zero, and the next few
   // bits follow a pattern since they are also multiples of some base value. The
   // byte swap in WeakMix helps ensure we still have good entropy in low bits.
+#if defined(__APPLE__) || defined(__ANDROID__) || \
+    defined(ABSL_HAVE_ADDRESS_SANITIZER)
+  // In these build modes, pointers may have less entropy so mix pointers twice.
+  return H::combine(std::move(hash_state), v, v);
+#endif
   return H::combine(std::move(hash_state), v);
 }