From 01892c70d5f8043d35b5eaca42db3e69bcbab05e Mon Sep 17 00:00:00 2001
From: Vadim Skipin <vadim.skipin@clickhouse.com>
Date: Thu, 7 May 2026 10:50:05 +0000
Subject: [PATCH] Fix waiter-table hashing

---
 include/silk/util/platform.h | 11 +++++++++++
 src/fibers/fiber.cpp         |  7 ++++---
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/include/silk/util/platform.h b/include/silk/util/platform.h
index 28a9270..945eec5 100644
--- a/include/silk/util/platform.h
+++ b/include/silk/util/platform.h
@@ -118,6 +118,17 @@ static inline uint64_t getTimeNanoseconds() noexcept
     return static_cast<uint64_t>(ts.tv_sec) * 1'000'000'000 + static_cast<uint64_t>(ts.tv_nsec);
 }
 
+/** 64-bit integer hash (Murmur3 fmix64 finalizer). */
+static constexpr uint64_t intHash(uint64_t key) noexcept
+{
+    key ^= key >> 33;
+    key *= 0xff51afd7ed558ccdULL;
+    key ^= key >> 33;
+    key *= 0xc4ceb9fe1a85ec53ULL;
+    key ^= key >> 33;
+    return key;
+}
+
 /**
  * Initialize librseq. Must be called before any rseq critical sections run.
  * Idempotent -- safe to call multiple times.
diff --git a/src/fibers/fiber.cpp b/src/fibers/fiber.cpp
index f9e5191..361ff0a 100644
--- a/src/fibers/fiber.cpp
+++ b/src/fibers/fiber.cpp
@@ -22,7 +22,6 @@
 #include <cerrno>
 #include <cstdint>
 #include <format>
-#include <functional>
 #include <memory>
 #include <mutex>
 #include <random>
@@ -58,6 +57,8 @@ static constexpr uint64_t CQE_TAG_CANCEL = 0;
 static constexpr uint64_t CQE_TAG_TIMEOUT = 1;
 static constexpr uint64_t CQE_TAG_DOORBELL = 2;
 
+static_assert((WAITER_TABLE_SIZE & (WAITER_TABLE_SIZE - 1)) == 0);
+
 // clang-format off
 #define FIBER_SIMPLE_COUNTERS(x) \
     x(FIBER_STARTED,           "FiberStarted") \
@@ -1192,7 +1193,7 @@ void FiberScheduler::suspend(SuspendCallback * callback, void * context) noexcep
 
 void FiberScheduler::enqueueWaiter(uint64_t key, Fiber * fiber) noexcept
 {
-    uint64_t index = std::hash<uint64_t>{}(key) % WAITER_TABLE_SIZE;
+    uint64_t index = intHash(key) & (WAITER_TABLE_SIZE - 1);
     scheduler->waiterTable[index].push(fiber);
 
     // seq_cst fence pairs with the one in releaseWaiters to prevent the
@@ -1207,7 +1208,7 @@ void FiberScheduler::releaseWaiters(uint64_t key) noexcept
     // seq_cst fence pairs with the one in enqueueWaiter.
     std::atomic_thread_fence(std::memory_order_seq_cst);
 
-    uint64_t index = std::hash<uint64_t>{}(key) % WAITER_TABLE_SIZE;
+    uint64_t index = intHash(key) & (WAITER_TABLE_SIZE - 1);
     Fiber * fiber = scheduler->waiterTable[index].popAll();
     while (fiber)
     {