Clang: Little speedup for code completion

...by ensuring that the precompiled preamble is created.

Change-Id: I80784b6a79c40a7e452530e8288783f33f881efc
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>

src/tools/clangbackend/ipcsource/translationunit.cpp

@@ -163,10 +163,6 @@ void TranslationUnit::removeOutdatedTranslationUnit() const
 
 void TranslationUnit::createTranslationUnitIfNeeded() const
 {
-    const auto options = CXTranslationUnit_CacheCompletionResults
-            | CXTranslationUnit_PrecompiledPreamble
-            | CXTranslationUnit_SkipFunctionBodies;
-
     if (!d->translationUnit) {
         d->translationUnit = CXTranslationUnit();
         CXErrorCode errorCode = clang_parseTranslationUnit2(index(),
@@ -175,11 +171,15 @@ void TranslationUnit::createTranslationUnitIfNeeded() const
                                                             d->projectPart.argumentCount(),
                                                             d->unsavedFiles.cxUnsavedFiles(),
                                                             d->unsavedFiles.count(),
-                                                            options,
+                                                            defaultOptions(),
                                                             &d->translationUnit);
 
         checkTranslationUnitErrorCode(errorCode);
 
+        // We need to reparse to create the precompiled preamble, which will speed up further calls,
+        // e.g. clang_codeCompleteAt() dramatically.
+        reparseTranslationUnit();
+
         updateLastChangeTimePoint();
     }
 }
@@ -192,6 +192,21 @@ void TranslationUnit::checkTranslationUnitErrorCode(CXErrorCode errorCode) const
     }
 }
 
+void TranslationUnit::reparseTranslationUnit() const
+{
+    clang_reparseTranslationUnit(d->translationUnit,
+                                 d->unsavedFiles.count(),
+                                 d->unsavedFiles.cxUnsavedFiles(),
+                                 clang_defaultReparseOptions(d->translationUnit));
+}
+
+int TranslationUnit::defaultOptions()
+{
+    return CXTranslationUnit_CacheCompletionResults
+         | CXTranslationUnit_PrecompiledPreamble
+         | CXTranslationUnit_SkipFunctionBodies;
+}
+
 uint TranslationUnit::unsavedFilesCount() const
 {
     return d->unsavedFiles.count();

src/tools/clangbackend/ipcsource/translationunit.h

@@ -91,6 +91,8 @@ private:
     void removeOutdatedTranslationUnit() const;
     void createTranslationUnitIfNeeded() const;
     void checkTranslationUnitErrorCode(CXErrorCode errorCode) const;
+    void reparseTranslationUnit() const;
+    static int defaultOptions();
 
 private:
     mutable std::shared_ptr<TranslationUnitData> d;