Qt Contributors Summit 2019 - lupdate

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Clang-based cpp parser for lupdate - Session Notes

The old C++ parser in lupdate has several problems, for example

  • It's lacking support for Objective C/C++ (QTBUG-44808)
  • It lacks support for a lot of modern C++ constructs. "Modern" is here C++11. See QTBUG-42744, QTBUG-53850 and QTBUG-63373.
  • It is deemed hard to maintain.

There is the ongoing effort to replace the old outdated parser with a clang, similar to what's been done in qdoc. The corresponding JIRA issue is QTBUG-71835.

Work is done in dev and therefore targeting Qt 6.

QDoc uses clang's C API, whereas lupdate uses clang's C++ API, which means library requirements and thus configuration are a bit more involved than qdoc's.

The old parser will still be available as fallback if build requirements are not met. The parser can also be switched on run time.

The new clang-based parser needs all compiler flags to "fake build" the project. A compilation database can be provided. Concerns were raised where this compilation db is supposed to come from. Luckily, CMake is able to create one.

Current Status: It works and produces seemingly the correct output, able to consume modern C++ constructs. But it's sloooooow. Running on Qt Creator takes 1 min with the old parser, and 50 min with the new one.

Proposed Solutions for the speed problems:

  • precompiled headers are under investigation
  • jumbo builds (include all sources from a central file)
  • running the AST creation in parallel
  • restrict the AST even more, e.g. tell clang to leave out information we don't need

Due to the usage of clang the source that's scanned for translations must be buildable with the provided options. That implies:

  • lupdate does not see #ifdef'ed code that's not compiled for the current platform
  • lupdate does not see source files that are not built for the current build configuration

How to overcome this? Unless we can convince clang to be more lenient towards non-buildable code, we could instead

  • Use the more lenient parser of clang-format?
  • Use Qt Creator's (non-clang) C++ parser instead of the lupdate one?
  • Rumors have it that work on clangd is done in that direction.