twisted.web._flatten._getSlotValue copies slotData on every call.

[dreid]

Describe the incorrect behavior you saw

_getSlotValue attempts retrieve a value for a named slot from the slotData stack by using slice syntax to reverse it and then iterate over it.

This also causes it create a copy of the list.

In templates with large amounts of data such those used by pydoctor's summary pages. This copying ends up being a significant amount of time spent generating summary templates.

Describe how to cause this behavior

I discovered this issue while running pydoctor under the austin profiler.

I can reproduce it on twisted's own apidocs.

You can view the results of this profiling in speedscope: https://www.speedscope.app/#profileURL=https%3A%2F%2Fgist.githubusercontent.com%2Fdreid%2F197566471f39a96523f5065d19d0bf7f%2Fraw%2F3e7ec92a17bc82d40acceb1e2efcaa3ef7c8ef07%2Ftwisted-austin-trunk.speedscope

In the above profile _getSlotValue accounts for ~55s of the total runtime which was 509.42.

Describe the correct behavior you'd like to see
slotData should not need to be copied on every call to _getSlotValue simply to iterate over it in reverse.

Initial profiling using reversed instead resulted in reduced runtime to 357.36s

speedscope profile of using reversed is here: https://www.speedscope.app/#profileURL=https%3A%2F%2Fgist.githubusercontent.com%2Fdreid%2F197566471f39a96523f5065d19d0bf7f%2Fraw%2F3e7ec92a17bc82d40acceb1e2efcaa3ef7c8ef07%2Ftwisted-pydoctor-reversed.speedscope

Testing environment

• Operating System and Version; paste the output of these commands:
  • on Linux, uname -a ; cat /etc/lsb-release
  • on Windows, systeminfo | Findstr /i "OS"
  • on macOS, sw_vers
• Twisted version [e.g. 22.2.0]
  • please paste the output of twist --version and pip --freeze
• Reactor [e.g. select, iocp]

Additional context
Add any other context about the problem here.