make clang plugin check code in headers only once if in PCH

When using Clang PCHs, we know for certain that the content of a PCH
will be used once by the PCH's dedicated source file. So it is not
necessary to let clang plugin check locations coming from a PCH
every time, but just once when compiling that dedicated source.

For starmath's parse.cxx this reduces compilation time 0.94s->0.4s
(0.1s when not using plugins at all), for sc's document.cxx it is
5.9s->5.0s (4.0s without plugins). For reference, without PCHs
the numbers are (with/without plugins) 2.1s/1.9s for parse.cxx
and 11.2s/10.3s for document.cxx.

Change-Id: Ie39787e65d7951187941dcff4899d053da63cbdd
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/113817
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/compilerplugins/clang/pluginhandler.cxx b/compilerplugins/clang/pluginhandler.cxx
index 54dba3e3bcc0..622caf917b8e 100644
--- a/compilerplugins/clang/pluginhandler.cxx
+++ b/compilerplugins/clang/pluginhandler.cxx
@@ -231,6 +231,19 @@ bool PluginHandler::ignoreLocation(SourceLocation loc) {
 
 bool PluginHandler::checkIgnoreLocation(SourceLocation loc)
 {
+#if HAVE_BUILDING_PCH_WITH_OBJ
+    // If we're building a separate precompiled_foo.cxx file
+    // for a PCH, then it is not necessary to check contents of everything
+    // included by the PCH in every compilation, check only once for that
+    // precompiled_foo.cxx file.
+    if( !compiler.getSourceManager().isLocalSourceLocation( loc ))
+    {
+        if( !compiler.getLangOpts().BuildingPCHWithObjectFile )
+            return true;
+    }
+#elif CLANG_VERSION >= 110000
+#error Internal error, -building-pch-with-obj support not detected in Clang.
+#endif
     SourceLocation expansionLoc = compiler.getSourceManager().getExpansionLoc( loc );
     if( compiler.getSourceManager().isInSystemHeader( expansionLoc ))
         return true;