Make it configurable whether system headers become dependencies

We spent an inordinate amount of resources on scanning and collecting
system headers, of which there are a lot and which typically do not
change in a relevant way. Make this behavior opt-in.

========== Performance data for Rule Execution ==========
    Old instruction count: 4331820265
    New instruction count: 2557300533
    Relative change: -41 %
    Old peak memory usage: 18892592 Bytes
    New peak memory usage: 16887880 Bytes
    Relative change: -11 %
========== Performance data for Null Build ==========
    Old instruction count: 521956733
    New instruction count: 443021349
    Relative change: -16 %
    Old peak memory usage: 12498888 Bytes
    New peak memory usage: 11186984 Bytes
    Relative change: -11 %

[ChangeLog] Introduced cpp.treatSystemHeadersAsDependencies

Change-Id: Iae9d9ca63852fb38a68e8dd2cc10b512eafe15e1
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

1 files changed, 9 insertions, 5 deletions

diff --git a/src/lib/corelib/buildgraph/depscanner.cpp b/src/lib/corelib/buildgraph/depscanner.cpp
index 8378f9c7a..ab597da1a 100644
--- a/src/lib/corelib/buildgraph/depscanner.cpp
+++ b/src/lib/corelib/buildgraph/depscanner.cpp
@@ -72,11 +72,15 @@ static QStringList collectCppIncludePaths(const QVariantMap &modules)
     if (cpp.isEmpty())
         return result;
 
-    result
-        << cpp.value(QLatin1String("includePaths")).toStringList()
-        << cpp.value(QLatin1String("systemIncludePaths")).toStringList()
-        << cpp.value(QLatin1String("distributionIncludePaths")).toStringList()
-        << cpp.value(QLatin1String("compilerIncludePaths")).toStringList();
+    result << cpp.value(QLatin1String("includePaths")).toStringList();
+    const bool useSystemHeaders
+            = cpp.value(QLatin1String("treatSystemHeadersAsDependencies")).toBool();
+    if (useSystemHeaders) {
+        result
+            << cpp.value(QLatin1String("systemIncludePaths")).toStringList()
+            << cpp.value(QLatin1String("distributionIncludePaths")).toStringList()
+            << cpp.value(QLatin1String("compilerIncludePaths")).toStringList();
+    }
     result.removeDuplicates();
     return result;
 }