Switch clang_createTranslationUnitFromSourceFile to use ASTUnit::LoadFromCommandLine.

 - This is much faster, as it avoids the overhead of dumping an AST file to disk and reloading it.
 - For debugging purposes, there is a clang_setUseExternalASTGeneration hook which can be used to disable this.

On the Sketch Cocoa app, the speedup is pretty nice, especially when using a PCH file while scanning the source:

Wall time to c-index-test all files (no PCH):
Old: 23.4221
New: 12.3884

Wall time to c-index-test all files (with a PCH, and "local" mode):
Old: 10.9233
New:  1.9038

llvm-svn: 90385

1 files changed, 5 insertions, 0 deletions

diff --git a/clang/tools/c-index-test/c-index-test.c b/clang/tools/c-index-test/c-index-test.c
index efff1ae3188..7300585a1d8 100644
--- a/clang/tools/c-index-test/c-index-test.c
+++ b/clang/tools/c-index-test/c-index-test.c
@@ -179,12 +179,17 @@ int perform_test_load_tu(const char *file, const char *filter) {
 }
 
 int perform_test_load_source(int argc, const char **argv, const char *filter) {
+  const char *UseExternalASTs =
+    getenv("CINDEXTEST_USE_EXTERNAL_AST_GENERATION");
   CXIndex Idx;
   CXTranslationUnit TU;
   Idx = clang_createIndex(/* excludeDeclsFromPCH */
                           !strcmp(filter, "local") ? 1 : 0,
                           /* displayDiagnostics */ 1);
 
+  if (UseExternalASTs && strlen(UseExternalASTs))
+    clang_setUseExternalASTGeneration(Idx, 1);
+
   TU = clang_createTranslationUnitFromSourceFile(Idx, 0, argc, argv);
   if (!TU) {
     fprintf(stderr, "Unable to load translation unit!\n");