Use on-disk compilation unit if available

If we succeeded in saving the compilation unit to disk, then attempt to use it
right away. This replaces the C++ heap usage for the compilation unit data as
well as the anonymous allocated executable memory with file-backed mmap'ed
memory. That means the memory can be discarded when overall availability is low
and paged in on-demand.

Change-Id: Ide1b1e11752d861eb049a99a26ca12cec5e2502e
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

src/qml/compiler/qv4compileddata.cpp

@@ -424,6 +424,7 @@ bool CompilationUnit::loadFromDisk(const QUrl &url, EvalISelFactory *iselFactory
         return false;
     }
 
+    const Unit * const oldDataPtr = (data && !(data->flags & QV4::CompiledData::Unit::StaticData)) ? data : nullptr;
     QScopedValueRollback<const Unit *> dataPtrChange(data, reinterpret_cast<const Unit *>(cacheData));
 
     {
@@ -448,6 +449,7 @@ bool CompilationUnit::loadFromDisk(const QUrl &url, EvalISelFactory *iselFactory
         return false;
 
     dataPtrChange.commit();
+    free(const_cast<Unit*>(oldDataPtr));
     backingFile.reset(cacheFile.take());
     return true;
 }

src/qml/compiler/qv4compileddata_p.h

@@ -666,8 +666,10 @@ struct Unit
             return QString();
 #if Q_BYTE_ORDER == Q_LITTLE_ENDIAN
         const QChar *characters = reinterpret_cast<const QChar *>(str + 1);
-        if (flags & StaticData)
+        // Too risky to do this while we unmap disk backed compilation but keep pointers to string
-            return QString::fromRawData(characters, str->size);
+        // data in the identifier tables.
+        //        if (flags & StaticData)
+        //            return QString::fromRawData(characters, str->size);
         return QString(characters, str->size);
 #else
         const LEUInt16 *characters = reinterpret_cast<const LEUInt16 *>(str + 1);

src/qml/compiler/qv4isel_moth.cpp

@@ -1665,8 +1665,7 @@ bool CompilationUnit::saveCodeToDisk(QIODevice *device, const CompiledData::Unit
 bool CompilationUnit::memoryMapCode(QString *errorString)
 {
     Q_UNUSED(errorString);
-    Q_ASSERT(codeRefs.isEmpty());
+    codeRefs.resize(data->functionTableSize);
-    codeRefs.reserve(data->functionTableSize);
 
     const char *basePtr = reinterpret_cast<const char *>(data);
 
@@ -1680,7 +1679,7 @@ bool CompilationUnit::memoryMapCode(QString *errorString)
 #else
         QByteArray code = QByteArray::fromRawData(codePtr, compiledFunction->codeSize);
 #endif
-        codeRefs.append(code);
+        codeRefs[i] = code;
     }
 
     return true;

src/qml/jit/qv4assembler.cpp

@@ -129,8 +129,7 @@ bool CompilationUnit::saveCodeToDisk(QIODevice *device, const CompiledData::Unit
 bool CompilationUnit::memoryMapCode(QString *errorString)
 {
     Q_UNUSED(errorString);
-    Q_ASSERT(codeRefs.isEmpty());
+    codeRefs.resize(data->functionTableSize);
-    codeRefs.reserve(data->functionTableSize);
 
     const char *basePtr = reinterpret_cast<const char *>(data);
 
@@ -139,7 +138,7 @@ bool CompilationUnit::memoryMapCode(QString *errorString)
         void *codePtr = const_cast<void *>(reinterpret_cast<const void *>(basePtr + compiledFunction->codeOffset));
         JSC::MacroAssemblerCodeRef codeRef = JSC::MacroAssemblerCodeRef::createSelfManagedCodeRef(JSC::MacroAssemblerCodePtr(codePtr));
         JSC::ExecutableAllocator::makeExecutable(codePtr, compiledFunction->codeSize);
-        codeRefs.append(codeRef);
+        codeRefs[i] = codeRef;
     }
 
     return true;

src/qml/qml/qqmltypeloader.cpp

@@ -2502,7 +2502,8 @@ void QQmlTypeData::compile(const QQmlRefPointer<QQmlTypeNameCache> &importCache,
 {
     Q_ASSERT(m_compiledData.isNull());
 
-    QQmlTypeCompiler compiler(QQmlEnginePrivate::get(typeLoader()->engine()), this, m_document.data(), importCache, resolvedTypeCache);
+    QQmlEnginePrivate * const enginePrivate = QQmlEnginePrivate::get(typeLoader()->engine());
+    QQmlTypeCompiler compiler(enginePrivate, this, m_document.data(), importCache, resolvedTypeCache);
     m_compiledData = compiler.compile();
     if (!m_compiledData) {
         setError(compiler.compilationErrors());
@@ -2510,7 +2511,12 @@ void QQmlTypeData::compile(const QQmlRefPointer<QQmlTypeNameCache> &importCache,
     }
     if (diskCache() || forceDiskCacheRefresh()) {
         QString errorString;
-        if (!m_compiledData->saveToDisk(url(), &errorString)) {
+        if (m_compiledData->saveToDisk(url(), &errorString)) {
+            QString error;
+            if (!m_compiledData->loadFromDisk(url(), enginePrivate->v4engine()->iselFactory.data(), &error)) {
+                // ignore error, keep using the in-memory compilation unit.
+            }
+        } else {
             qCDebug(DBG_DISK_CACHE) << "Error saving cached version of" << m_compiledData->url().toString() << "to disk:" << errorString;
         }
     }

tests/auto/qml/qqmllanguage/tst_qqmllanguage.cpp

@@ -2118,8 +2118,13 @@ void tst_qqmllanguage::scriptStringWithoutSourceCode()
         QQmlTypeData *td = eng->typeLoader.getType(url);
         Q_ASSERT(td);
 
-        const QV4::CompiledData::Unit *qmlUnit = td->compilationUnit()->data;
+        const QV4::CompiledData::Unit *readOnlyQmlUnit = td->compilationUnit()->data;
-        Q_ASSERT(qmlUnit);
+        Q_ASSERT(readOnlyQmlUnit);
+        QV4::CompiledData::Unit *qmlUnit = reinterpret_cast<QV4::CompiledData::Unit *>(malloc(readOnlyQmlUnit->unitSize));
+        memcpy(qmlUnit, readOnlyQmlUnit, readOnlyQmlUnit->unitSize);
+        qmlUnit->flags &= ~QV4::CompiledData::Unit::StaticData;
+        td->compilationUnit()->data = qmlUnit;
+
         const QV4::CompiledData::Object *rootObject = qmlUnit->objectAt(qmlUnit->indexOfRootObject);
         QCOMPARE(qmlUnit->stringAt(rootObject->inheritedTypeNameIndex), QString("MyTypeObject"));
         quint32 i;