From 86c250d98f5842cb4bd971c95af301ebef2518b6 Mon Sep 17 00:00:00 2001
From: Joerg Bornemann <joerg.bornemann@qt.io>
Date: Sun, 6 Apr 2025 17:50:08 +0200
Subject: [PATCH] CMake: make ccache + pch + clang build more robust

The ccache man page says we have to compile with -fno-pch-timestamp when
using clang and precompiled headers.

"Professional CMake" also recommends this approach, and the ccache
statistics report a marginally better hit rate with this patch.

Task-number: QTBUG-131650
Change-Id: Ic1c17a3977771673ac256674143dbdf8d5ffa28c
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>
---
 cmake/QtInternalTargets.cmake | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/cmake/QtInternalTargets.cmake b/cmake/QtInternalTargets.cmake
index d7c823995c0..a1f4cb7281c 100644
--- a/cmake/QtInternalTargets.cmake
+++ b/cmake/QtInternalTargets.cmake
@@ -381,6 +381,15 @@ if(QT_FEATURE_stdlib_libcpp)
     target_compile_definitions(PlatformCommonInternal INTERFACE _LIBCPP_REMOVE_TRANSITIVE_INCLUDES)
 endif()
 
+if(QT_USE_CCACHE AND CLANG AND BUILD_WITH_PCH)
+    # The ccache man page says we must compile with -fno-pch-timestamp when using clang and pch.
+    foreach(language IN ITEMS C CXX OBJC OBJCXX)
+        target_compile_options(PlatformCommonInternal INTERFACE
+            "$<$<COMPILE_LANGUAGE:${language}>:SHELL:-Xclang -fno-pch-timestamp>"
+        )
+    endforeach()
+endif()
+
 # Hardening options
 
 qt_internal_apply_intel_cet_harderning(PlatformCommonInternal)