From dbf748e3f44e17227cd1cc52a7ed87e6bc4b62b5 Mon Sep 17 00:00:00 2001
From: Paul Bone <pbone@mozilla.com>
Date: Tue, 1 Sep 2026 00:08:56 +0000
Subject: [PATCH] Bug 2055507 - pt 8. Disable the chunk cache on Windows
 r=glandium

Differential Revision: https://phabricator.services.mozilla.com/D313993
---
 memory/build/Chunk.cpp       | 30 +++++++++++++-----------------
 memory/build/Chunk.h         | 21 +++++++++------------
 memory/build/mozjemalloc.cpp |  3 +++
 3 files changed, 25 insertions(+), 29 deletions(-)

diff --git a/memory/build/Chunk.cpp b/memory/build/Chunk.cpp
index b78446472783..119fce7f840a 100644
--- a/memory/build/Chunk.cpp
+++ b/memory/build/Chunk.cpp
@@ -517,9 +517,11 @@ void base_chunk_dealloc(void* aChunk, size_t aSize, ChunkType aType) {
   MOZ_ASSERT((aSize & kChunkSizeMask) == 0);
   MOZ_ASSERT(!gChunkRTree.Get(aChunk));
 
+#ifndef XP_WIN
   if (gCache.TryRecord(aChunk, aSize, aType)) {
     return;
   }
+#endif
 
   pages_unmap(aChunk, aSize);
 }
@@ -575,10 +577,16 @@ void* arena_chunk_alloc(chunk_allocator_t* aChunkAllocator, size_t aSize,
 }
 
 static void* system_pages_map(size_t aSize, size_t aAlignment) {
-  void* ret = gCache.Recycle(aSize, aAlignment);
+  void* ret = nullptr;
+
+#ifndef XP_WIN
+  ret = gCache.Recycle(aSize, aAlignment);
   if (!ret) {
+#endif
     ret = pages_mmap_aligned(aSize, aAlignment, ReserveAndCommit);
+#ifndef XP_WIN
   }
+#endif
 
   return ret;
 }
@@ -602,11 +610,9 @@ bool arena_chunk_t::IsEmpty() {
           (~gPageSizeMask | CHUNK_MAP_ALLOCATED)) == gMaxLargeClass;
 }
 
-bool ChunkCache::TryRecord(void* aChunk, size_t aSize, ChunkType aType) {
-  if (!CanRecycle(aSize)) {
-    return false;
-  }
+#ifndef XP_WIN
 
+bool ChunkCache::TryRecord(void* aChunk, size_t aSize, ChunkType aType) {
   size_t recycled_so_far = mRecycledSize;
 
   // In case some race condition put us above the limit.
@@ -617,17 +623,9 @@ bool ChunkCache::TryRecord(void* aChunk, size_t aSize, ChunkType aType) {
   size_t recycle_remaining = gRecycleLimit - recycled_so_far;
   size_t to_recycle;
   if (aSize > recycle_remaining) {
-#ifndef XP_WIN
     to_recycle = recycle_remaining;
     // Drop pages that would overflow the recycle limit
     pages_trim(aChunk, aSize, 0, to_recycle, ReserveAndCommit);
-#else
-    // On windows pages_trim unallocates and reallocates the whole
-    // chunk, there's no point doing that during recycling so instead we
-    // fail.
-    pages_unmap(aChunk, aSize);
-    return;
-#endif
   } else {
     to_recycle = aSize;
   }
@@ -705,10 +703,6 @@ void ChunkCache::Record(void* aChunk, size_t aSize, ChunkType aType) {
 }
 
 void* ChunkCache::Recycle(size_t aSize, size_t aAlignment) {
-  if (!CanRecycle(aSize)) {
-    return nullptr;
-  }
-
   size_t alloc_size = aSize + aAlignment - kChunkSize;
   // Beware size_t wrap-around.
   if (alloc_size < aSize) {
@@ -780,3 +774,5 @@ void* ChunkCache::Recycle(size_t aSize, size_t aAlignment) {
 
 // The global chunk cache.
 ChunkCache gCache;
+
+#endif /* ! XP_WIN */
diff --git a/memory/build/Chunk.h b/memory/build/Chunk.h
index 894293095bed..c09dc01cb5f2 100644
--- a/memory/build/Chunk.h
+++ b/memory/build/Chunk.h
@@ -230,6 +230,13 @@ void* pages_mmap_aligned(size_t size, size_t alignment,
 
 void pages_unmap(void* aAddr, size_t aSize);
 
+// On Windows, calls to VirtualAlloc and VirtualFree must be matched, making
+// it awkward to recycle allocations of varying sizes.
+// Therefore the chunk cache is disabled on windows since arenas have a
+// chunk list for arena chunks, and non-arena chunks arbitrary sizes
+// don't cache well without splitting.
+#ifndef XP_WIN
+
 class ChunkCache {
  private:
   Mutex mMutex;
@@ -251,18 +258,6 @@ class ChunkCache {
 
   void Init() { mMutex.Init(); }
 
-  static constexpr bool CanRecycle(size_t aSize) {
-#ifdef XP_WIN
-    // On Windows, calls to VirtualAlloc and VirtualFree must be matched, making
-    // it awkward to recycle allocations of varying sizes. Therefore we only
-    // allow recycling when the size equals the chunksize, unless deallocation
-    // is entirely disabled.
-    return aSize == kChunkSize;
-#else
-    return true;
-#endif
-  }
-
   // Try to put this chunk in the cache, false if the cache is full.
   bool TryRecord(void* aChunk, size_t aSize, ChunkType aType);
 
@@ -277,4 +272,6 @@ class ChunkCache {
 
 extern ChunkCache gCache;
 
+#endif /* ! XP_WIN */
+
 #endif /* ! CHUNK_H */
diff --git a/memory/build/mozjemalloc.cpp b/memory/build/mozjemalloc.cpp
index ee1d5ab8d696..d07ff71c3e0e 100644
--- a/memory/build/mozjemalloc.cpp
+++ b/memory/build/mozjemalloc.cpp
@@ -3448,7 +3448,10 @@ static bool malloc_init_hard() {
   DefineGlobals();
 #endif
 
+#ifndef XP_WIN
   gCache.Init();
+#endif
+
   huge_init();
   sBaseAlloc.Init();