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Dieter Mayer
de82b2738f
The child lookup iterated parent_element.childNodes() via item(i), and
QDomNodeList::item() walks the sibling chain from the start on every
call - making the loop quadratic in the number of children, with an
extra QList allocation and a second pass on top. This lookup runs
several times per element instance while loading a project, against the
"import" category that holds every embedded definition, so the cost
scales with (instances x embedded definitions).
Replace it with a firstChildElement()/nextSiblingElement() walk with an
early return. Same semantics (first tag+name match in document order).
Measured on the Kaefer_1303 reference project (3.9 MB, 23 folios,
432 instances, media of 6 runs, Windows/MinGW, same GCC for both):
before after
Qt5 5.116 s 5.068 s
Qt6 6.683 s 4.640 s (-31 %)
This removes the entire Qt6 load-time regression discussed in #553 -
Qt6 goes from +31 % slower to 8 % faster than Qt5 on the very project
that exposed it (Qt6''s QDom makes the quadratic pattern much more
expensive than Qt5''s did). Smaller projects gain too (3.4 MB example:
-9 % on Qt6).
(cherry picked from commit 0d4ef8eca27601c37ba2b75d7c058e9d8e2beea4)