[QGIS-Developer] Progress on SIP incremental build grant

[David Koňařík]

Hi all,
I'd like to share with you a report of the work I did on QEP 338 (SIP 
incremental builds):

My original plan was to build each header file as a separate binding, 
then use SIP from a Python script, overriding a few methods to allow 
building just one binding out of a project.

After a lot of effort, this plan sadly doesn't seem workable. PyQt's 
bindings aren't modularised enough, so building a single binding still 
needs to parse almost all of PyQt. Furthermore, SIP has a multi-stage 
parse-resolve-generate design, but the "parser" does more than just 
parse the code into an AST, not all references are resolved in the 
resolve phase, and imports are currently basically done by textual 
inclusion.

I've tried making the necessary changes to SIP [1] and QGIS [2], but for 
the above reasons, I don't think the performance benefits for 
single-file builds are worth the added complexity and performance 
penalty for clean builds (which look to be over an hour currently).

The good news is that with the knowledge from working on SIP, I've been 
able to improve the performance of regular clean builds, and those 
improvements might soon be merged into SIP itself [3]. I've also made 
some changes on the QGIS side to not rebuild unchanged code generated by 
SIP [4]. With code compilation now taking longer than SIP code 
generation, this effectively gives us incremental builds, just at a 
larger granularity.

David Koňařík

[1]: https://github.com/dvdkon/sip/tree/qgis-gb
[2]: https://github.com/dvdkon/QGIS/tree/sip-incremental-build
[3]: https://github.com/Python-SIP/sip/pull/87
[4]: https://github.com/qgis/QGIS/pull/63160