From 5c37ebf855147997e7cbc84d8ef67ab142cc78b4 Mon Sep 17 00:00:00 2001
From: Michael Wilson <mjwilson@chromium.org>
Date: Wed, 15 Apr 2026 03:37:44 -0700
Subject: [PATCH] Disable denormals before isolate creation for AudioWorklet threads

This is to ensure consistency in the denormal flag state between the
isolate and the global scope.

Bug: 499565267
Change-Id: Iec7735bd6bc543da2ad08aea2f7bc716265dc54d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7744866
Reviewed-by: Hiroshige Hayashizaki <hiroshige@chromium.org>
Commit-Queue: Michael Wilson <mjwilson@chromium.org>
Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1615043}
---

diff --git a/third_party/blink/renderer/core/workers/worker_backing_thread.cc b/third_party/blink/renderer/core/workers/worker_backing_thread.cc
index 38d221a6..5997bed3 100644
--- a/third_party/blink/renderer/core/workers/worker_backing_thread.cc
+++ b/third_party/blink/renderer/core/workers/worker_backing_thread.cc
@@ -18,6 +18,7 @@
 #include "third_party/blink/renderer/bindings/core/v8/v8_initializer.h"
 #include "third_party/blink/renderer/core/inspector/worker_thread_debugger.h"
 #include "third_party/blink/renderer/core/workers/worker_backing_thread_startup_data.h"
+#include "third_party/blink/renderer/platform/audio/denormal_disabler.h"
 #include "third_party/blink/renderer/platform/heap/thread_state.h"
 #include "third_party/blink/renderer/platform/runtime_enabled_features.h"
 #include "third_party/blink/renderer/platform/scheduler/public/main_thread.h"
@@ -91,6 +92,14 @@
   ForegroundedIsolates().erase(isolate);
 }
 
+bool IsDenormalDisabledThreadType(ThreadType type) {
+  // Disable denormals on WebAudio threads for performance reasons.  See:
+  // https://esdiscuss.org/topic/float-denormal-issue-in-javascript-processor-node-in-web-audio-api
+  return type == ThreadType::kOfflineAudioWorkletThread ||
+         type == ThreadType::kRealtimeAudioWorkletThread ||
+         type == ThreadType::kSemiRealtimeAudioWorkletThread;
+}
+
 }  // namespace
 
 // Wrapper functions defined in third_party/blink/public/web/blink.h
@@ -119,7 +128,9 @@
 
 WorkerBackingThread::WorkerBackingThread(const ThreadCreationParams& params)
     : backing_thread_(blink::NonMainThread::CreateThread(
-          ThreadCreationParams(params).SetSupportsGC(true))) {}
+          ThreadCreationParams(params).SetSupportsGC(true))),
+      is_denormal_disabled_thread_(
+          IsDenormalDisabledThreadType(params.thread_type)) {}
 
 WorkerBackingThread::~WorkerBackingThread() = default;
 
@@ -127,6 +138,12 @@
     const WorkerBackingThreadStartupData& startup_data) {
   DCHECK(backing_thread_->IsCurrentThread());
 
+  // Denormals must be disabled before the V8 isolate is initialized so that the
+  // isolate's internal state and generated code respect the flag.
+  if (is_denormal_disabled_thread_) {
+    DenormalModifier::DisableDenormals();
+  }
+
   DCHECK(!isolate_);
   ThreadScheduler* scheduler = BackingThread().Scheduler();
   isolate_ = V8PerIsolateData::Initialize(
diff --git a/third_party/blink/renderer/core/workers/worker_backing_thread.h b/third_party/blink/renderer/core/workers/worker_backing_thread.h
index 445aad71..4c5f3cc7 100644
--- a/third_party/blink/renderer/core/workers/worker_backing_thread.h
+++ b/third_party/blink/renderer/core/workers/worker_backing_thread.h
@@ -56,6 +56,7 @@
  private:
   std::unique_ptr<blink::NonMainThread> backing_thread_;
   v8::Isolate* isolate_ = nullptr;
+  const bool is_denormal_disabled_thread_;
 };
 
 }  // namespace blink
diff --git a/third_party/blink/renderer/modules/webaudio/audio_worklet_global_scope.cc b/third_party/blink/renderer/modules/webaudio/audio_worklet_global_scope.cc
index b258751..e3c9d55 100644
--- a/third_party/blink/renderer/modules/webaudio/audio_worklet_global_scope.cc
+++ b/third_party/blink/renderer/modules/webaudio/audio_worklet_global_scope.cc
@@ -23,7 +23,6 @@
 #include "third_party/blink/renderer/modules/webaudio/audio_worklet_processor.h"
 #include "third_party/blink/renderer/modules/webaudio/audio_worklet_processor_definition.h"
 #include "third_party/blink/renderer/modules/webaudio/cross_thread_audio_worklet_processor_info.h"
-#include "third_party/blink/renderer/platform/audio/denormal_disabler.h"
 #include "third_party/blink/renderer/platform/bindings/callback_method_retriever.h"
 #include "third_party/blink/renderer/platform/heap/garbage_collected.h"
 #include "third_party/blink/renderer/platform/wtf/text/strcat.h"
@@ -36,9 +35,6 @@
     : WorkletGlobalScope(std::move(creation_params),
                          thread->GetWorkerReportingProxy(),
                          thread) {
-  // Disable denormals for performance.
-  DenormalModifier::DisableDenormals();
-
   // Audio is prone to jank introduced by e.g. the garbage collector. Workers
   // are generally put in a background mode (as they are non-visible). Audio is
   // an exception here, requiring low-latency behavior similar to any visible