https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114480
--- Comment #32 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Alexander Monakov <amona...@gcc.gnu.org>: https://gcc.gnu.org/g:4b9e68a6f3b22800a7f12b58ef6b25e3b339bb3c commit r15-628-g4b9e68a6f3b22800a7f12b58ef6b25e3b339bb3c Author: Alexander Monakov <amona...@ispras.ru> Date: Wed May 15 16:23:17 2024 +0300 tree-into-ssa: speed up sorting in prune_unused_phi_nodes [PR114480] In PR 114480 we are hitting a case where tree-into-ssa scales quadratically due to prune_unused_phi_nodes doing O(N log N) work for N basic blocks, for each variable individually. Sorting the 'defs' array is especially costly. It is possible to assist gcc_qsort by laying out dfs_out entries in the reverse order in the 'defs' array, starting from its tail. This is not always a win (in fact it flips most of 7-element qsorts in this testcase from 9 comparisons (best case) to 15 (worst case)), but overall it helps on the testcase and on libstdc++ build. On the testcase we go from 1.28e9 comparator invocations to 1.05e9, on libstdc++ from 2.91e6 to 2.84e6. gcc/ChangeLog: PR c++/114480 * tree-into-ssa.cc (prune_unused_phi_nodes): Add dfs_out entries to the 'defs' array in the reverse order.
