From 5dc4e88bfea47e5f5d54fd10d17ee2e19ed1f4d7 Mon Sep 17 00:00:00 2001
From: Nikita Koval <nikita.koval@jetbrains.com>
Date: Wed, 20 Mar 2024 01:18:29 +0100
Subject: [PATCH] Optimize spin-wait loop in Spinner (#287)

---------

Signed-off-by: Evgeniy Moiseenko <evg.moiseenko94@gmail.com>
Co-authored-by: Evgeniy Moiseenko <evg.moiseenko94@gmail.com>
---
 .../kotlinx/lincheck/util/Spinner.kt          | 42 +++++++++++++++----
 .../linearizability/LockFreeSetTest.kt        |  2 +-
 2 files changed, 36 insertions(+), 8 deletions(-)

diff --git a/src/jvm/main/org/jetbrains/kotlinx/lincheck/util/Spinner.kt b/src/jvm/main/org/jetbrains/kotlinx/lincheck/util/Spinner.kt
index a5d7d3f39..6dc585015 100644
--- a/src/jvm/main/org/jetbrains/kotlinx/lincheck/util/Spinner.kt
+++ b/src/jvm/main/org/jetbrains/kotlinx/lincheck/util/Spinner.kt
@@ -130,10 +130,11 @@ class Spinner(val nThreads: Int = -1) {
      *   for example, to fall back into a blocking synchronization.
      */
     fun spin(): Boolean {
-        // spin a few iterations
-        repeat(spinLoopIterationsPerCall) {
-            counter++
-        }
+        // perform spin waiting
+        Thread.onSpinWait()
+        spinWait()
+        // update the counter
+        counter += spinLoopIterationsPerCall
         // if yield limit is approached,
         // then yield and give other threads the opportunity to run
         if (counter % yieldLimit == 0) {
@@ -148,6 +149,33 @@ class Spinner(val nThreads: Int = -1) {
         return true
     }
 
+    /**
+     * Auxiliary variable storing a pseudo-random value,
+     * which is used inside the [spinWait] to perform spin waiting.
+     */
+    private var sink: Long = System.nanoTime()
+
+    /**
+     * Implements a spin waiting procedure.
+     */
+    private fun spinWait() {
+        // Initialize with a pseudo-random number to prevent optimizations.
+        var x = sink
+        // We want to perform few spins while avoiding accesses to the shared memory.
+        // To achieve this, we do some arithmetic operations on a local variable
+        // and try to obfuscate the loop body so that the compiler
+        // would not be able to optimize it out.
+        for (i in spinLoopIterationsPerCall downTo 1) {
+            x += (31 * x + 0xBEEF + i) and (0xFFFFFFFFFFFFFFFL)
+        }
+        // This if statement ensures that the result of the computation
+        // will have a visible side effect and thus will not be optimized,
+        // but at the same time it avoids the actual store on a hot-path.
+        if (x == 0xDEADL) {
+            sink += x
+        }
+    }
+
     /**
      * Resets the state of the spinner.
      */
@@ -224,6 +252,6 @@ inline fun <T> Spinner.spinWaitBoundedFor(getter: () -> T?): T? {
     return null
 }
 
-private const val SPIN_LOOP_ITERATIONS_PER_CALL     : Int = 1 shl 5  // 32
-private const val SPIN_LOOP_ITERATIONS_BEFORE_YIELD : Int = 1 shl 14 // 16,384
-private const val SPIN_LOOP_ITERATIONS_BEFORE_EXIT  : Int = 1 shl 20 // 1,048,576
\ No newline at end of file
+private const val SPIN_LOOP_ITERATIONS_PER_CALL     : Int = 100
+private const val SPIN_LOOP_ITERATIONS_BEFORE_YIELD : Int = 10_000_000
+private const val SPIN_LOOP_ITERATIONS_BEFORE_EXIT  : Int = 100_000_000
\ No newline at end of file
diff --git a/src/jvm/test/org/jetbrains/kotlinx/lincheck_test/verifier/linearizability/LockFreeSetTest.kt b/src/jvm/test/org/jetbrains/kotlinx/lincheck_test/verifier/linearizability/LockFreeSetTest.kt
index cc1ca9b14..21a773c6a 100644
--- a/src/jvm/test/org/jetbrains/kotlinx/lincheck_test/verifier/linearizability/LockFreeSetTest.kt
+++ b/src/jvm/test/org/jetbrains/kotlinx/lincheck_test/verifier/linearizability/LockFreeSetTest.kt
@@ -38,7 +38,7 @@ class LockFreeSetTest {
 
         StressOptions()
             .addCustomScenario(scenario)
-            .invocationsPerIteration(1000000)
+            .invocationsPerIteration(10_000_000)
             .iterations(0)
             .check(LockFreeSet::class)
     }