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Prevent static plugin triggering autogen dependency on reconfigure

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A call to file(WRITE) will unconditionally update the file's timestamp
even if the file's contents don't change. The *Plugin.cpp file was
being written using configure_file() which avoids that, but the .cpp.in
file it was configuring from was being written out using file(WRITE)
every time CMake ran. Autogen saw that file as a dependency and then
regenerated the mocs_compilation.cpp file, which in turn results in
unnecessary rebuilds and relinking when nothing is actually changing.

The file(WRITE) - configure_file() dance is no longer needed anyway,
since the generated *Plugin.cpp file is very simple with no
substitutions required. Therefore, we can simplify that file's
generation with a single file(WRITE) that only executes if the file
contents will change or the file is missing.

Pick-to: 6.1 6.0
Change-Id: I2b7d1ff678b85ea7811969d656555592c9b6865f
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
This commit is contained in:
Craig Scott 2021-03-16 12:27:53 +11:00
parent 2409e9b2c7
commit 63a0d263cf
1 changed files with 15 additions and 7 deletions
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20
cmake/QtPlugins.cmake.in
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@ -139,15 +139,23 @@ function(__qt_internal_add_static_plugins_once)
set(_generated_qt_plugin_file_name
"${CMAKE_CURRENT_BINARY_DIR}/qt_@QT_MODULE@_${target}.cpp")
set(_generated_qt_plugin_file_name_template "${_generated_qt_plugin_file_name}.in")
set(_generated_qt_plugin_file_content "#include <QtPlugin>\nQ_IMPORT_PLUGIN(${_classname})")
# Generate a source file to import that plug-in. Has to be done with configure_file,
# because file(GENERATE) and target_sources has issues with scopes.
file(WRITE "${_generated_qt_plugin_file_name_template}"
# Generate a source file to import that plug-in. Be careful not to
# update the timestamp of the generated file if we are not going to
# change anything. Otherwise we will trigger CMake's autogen to re-run
# and executables will then need to at least relink.
set(need_write TRUE)
if(EXISTS ${_generated_qt_plugin_file_name})
file(READ ${_generated_qt_plugin_file_name} old_contents)
if(old_contents STREQUAL "${_generated_qt_plugin_file_content}")
set(need_write FALSE)
endif()
endif()
if(need_write)
file(WRITE "${_generated_qt_plugin_file_name}"
"${_generated_qt_plugin_file_content}")
configure_file("${_generated_qt_plugin_file_name_template}"
"${_generated_qt_plugin_file_name}")
endif()
target_sources(${_module_target} INTERFACE
"$<${_plugin_condition}:${_generated_qt_plugin_file_name}>")