moc: Cache header lookups to reduce number of file stat's

This improves moc performance on Windows where file-stat'ing is slow
and where the number of project include paths to search is often high
because project third-party headers are installed in separate
directories rather than a shared include path such as /usr/include.

In a real project of non-trivial size it reduces the total
from-scratch build time of an optimized build using CMake+ninja with
32 cores by 11% from ~11m35s to ~10m15s.

Change-Id: Ieed59646927ed75c55ed9efa97600c328b2fed2c
Reviewed-by: Olivier Goffart (Woboq GmbH) <[email protected]>

1 files changed, 1 insertions, 0 deletions

diff --git a/src/tools/moc/preprocessor.h b/src/tools/moc/preprocessor.h
index a7eb1a19e19..39f56d6e923 100644
--- a/src/tools/moc/preprocessor.h
+++ b/src/tools/moc/preprocessor.h
@@ -61,6 +61,7 @@ public:
     static bool preprocessOnly;
     QList<QByteArray> frameworks;
     QSet<QByteArray> preprocessedIncludes;
+    QHash<QByteArray, QByteArray> nonlocalIncludePathResolutionCache;
     Macros macros;
     QByteArray resolveInclude(const QByteArray &filename, const QByteArray &relativeTo);
     Symbols preprocessed(const QByteArray &filename, QFile *device);