The old code called QList::erase(it, it) in a loop; which causes quadratic behavior. Fix by writing a std::unique-inspired merge algorithm that works in linear time, at the cost of merging only pairs of consecutive STRING_LITERALs. The old code tried to merge maximal sequences of adjacent STRING_LITERALs. This should not make much of a difference, since QByteArray grows geometrically, so appends (and insert at end - 1 is almost append) take amortized linear time in the final size of the result. Symbol is a QByteArray version of QStringRef (i.e. a cheap substring: string, from, len), except with individual copies of the underlying string, so make sure to rid Symbol::lex of non-lexem() data before merging. Aurélien reports that `lex` may contain a whole file, so this is an important step. Aurélien reports a 6x speedup on one of his benchmark files. Amends 0c9d1f99dac5c118d49e7f2b04f70eae3ba7b837. Reported-by: Aurélien Brooke <aurelien@bahiasoft.fr> Done-with: Aurélien Brooke <aurelien@bahiasoft.fr> Pick-to: 6.10 6.9 6.8 6.5 Change-Id: I1093a43c3727fde1785795b1aac7417be011bad8 Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io> Reviewed-by: Aurélien Brooke <aurelien@bahiasoft.fr>