PersistentValueStorage: remember last free page instead of reshuffling

This unifies the operation when the gc is running with the normal mode,
avoiding a potentially costly iteration over all pages for allocations
during an ongoing gc cycle.

Change-Id: I39e8990c303149e3bc458b10522cc1487e62a401
Reviewed-by: Olivier De Cannière <olivier.decanniere@qt.io>
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

2 files changed, 27 insertions, 4 deletions

diff --git a/src/qml/jsruntime/qv4persistent.cpp b/src/qml/jsruntime/qv4persistent.cpp
index 3cbaec1dac..81153dd36f 100644
--- a/src/qml/jsruntime/qv4persistent.cpp
+++ b/src/qml/jsruntime/qv4persistent.cpp
@@ -142,6 +142,7 @@ PersistentValueStorage::PersistentValueStorage(ExecutionEngine *engine)
 
 PersistentValueStorage::~PersistentValueStorage()
 {
+    clearFreePageHint();
     Page *p = static_cast<Page *>(firstPage);
     while (p) {
         for (int i = 0; i < kEntriesPerPage; ++i) {
@@ -159,7 +160,9 @@ PersistentValueStorage::~PersistentValueStorage()
 
 Value *PersistentValueStorage::allocate()
 {
-    Page *p = static_cast<Page *>(firstPage);
+    Page *p = static_cast<Page *>(freePageHint);
+    if (p && p->header.freeList == -1)
+        p = static_cast<Page *>(firstPage);
     while (p) {
         if (p->header.freeList != -1)
             break;
@@ -171,9 +174,15 @@ Value *PersistentValueStorage::allocate()
     Value *v = p->values + p->header.freeList;
     p->header.freeList = v->int_32();
 
-    if (p->header.freeList != -1 && p != firstPage && !engine->isGCOngoing) {
-        unlink(p);
-        insertInFront(this, p);
+    if (p->header.freeList != -1 && p != freePageHint) {
+        if (auto oldHint = static_cast<Page *>(freePageHint)) {
+            oldHint->header.refCount--;
+            // no need to free - if the old page were unused,
+            // we would have used it to serve the allocation
+            Q_ASSERT(oldHint->header.refCount);
+        }
+        freePageHint = p;
+        p->header.refCount++;
     }
 
     ++p->header.refCount;
@@ -207,6 +216,17 @@ void PersistentValueStorage::mark(MarkStack *markStack)
     }
 }
 
+void PersistentValueStorage::clearFreePageHint()
+{
+    if (!freePageHint)
+        return;
+    auto page = static_cast<Page *>(freePageHint);
+    if (!--page->header.refCount)
+        freePage(page);
+    freePageHint = nullptr;
+
+}
+
 ExecutionEngine *PersistentValueStorage::getEngine(const Value *v)
 {
     return getPage(v)->header.engine;
diff --git a/src/qml/jsruntime/qv4persistent_p.h b/src/qml/jsruntime/qv4persistent_p.h
index 7e208bd4fd..c24d337b60 100644
--- a/src/qml/jsruntime/qv4persistent_p.h
+++ b/src/qml/jsruntime/qv4persistent_p.h
@@ -51,10 +51,13 @@ struct Q_QML_EXPORT PersistentValueStorage
     Iterator begin() { return Iterator(firstPage, 0); }
     Iterator end() { return Iterator(nullptr, 0); }
 
+    void clearFreePageHint();
+
     static ExecutionEngine *getEngine(const Value *v);
 
     ExecutionEngine *engine;
     void *firstPage;
+    void *freePageHint = nullptr;
 private:
     static void freeUnchecked(Value *v);
     static void freePage(void *page);