From 5e52a1d13fd87efbad684ef12182d2ae710cb7f9 Mon Sep 17 00:00:00 2001
From: not-matthias <matthias@codspeed.io>
Date: Fri, 23 Oct 2026 18:06:15 +0200
Subject: [PATCH] fix(core): seed the V8 RNG and string hashes in walltime mode

Walltime runs left Math.random() and V8's hash seed random per process, so benchmarks using random inputs or hash-heavy structures varied between runs for reasons unrelated to the code under test. Pass the analysis-mode seed flags (--hash-seed=1, --random-seed=1, and --no-randomize-hashes on Node < 18) in walltime too, via one shared helper. Disabled mode stays unseeded. crypto randomness and key iteration order are unchanged.
---
 packages/core/src/introspection.ts | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/packages/core/src/introspection.ts b/packages/core/src/introspection.ts
index 18c29ba0..8906c1d9 100644
--- a/packages/core/src/introspection.ts
+++ b/packages/core/src/introspection.ts
@@ -7,6 +7,15 @@ const CUSTOM_INTROSPECTION_EXIT_CODE = 0;
 
 const V8_LOG_FILENAME_PATTERN = "codspeed-v8-%p.log";
 
+// Fixed seeds so Math.random() and hash table layout match on every run
+const getSeedFlags = (nodeVersionMajor: number) => {
+  const flags = ["--hash-seed=1", "--random-seed=1"];
+  if (nodeVersionMajor < 18) {
+    flags.push("--no-randomize-hashes");
+  }
+  return flags;
+};
+
 export const getV8Flags = () => {
   const nodeVersionMajor = parseInt(process.version.slice(1).split(".")[0]);
   const instrumentMode = getInstrumentMode();
@@ -17,8 +26,7 @@ export const getV8Flags = () => {
     case "analysis": {
       flags.push(
         ...[
-          "--hash-seed=1",
-          "--random-seed=1",
+          ...getSeedFlags(nodeVersionMajor),
           "--no-opt",
           "--always-sparkplug",
           "--predictable",
@@ -28,9 +36,6 @@ export const getV8Flags = () => {
           "--max-old-space-size=4096",
         ],
       );
-      if (nodeVersionMajor < 18) {
-        flags.push("--no-randomize-hashes");
-      }
       if (nodeVersionMajor < 20) {
         flags.push("--no-scavenge-task");
       } else {
@@ -48,6 +53,8 @@ export const getV8Flags = () => {
     }
 
     case "walltime": {
+      flags.push(...getSeedFlags(nodeVersionMajor));
+
       // Emit the V8 jitdump
       flags.push("--perf-prof");