From 4b9e68a6f3b22800a7f12b58ef6b25e3b339bb3c Mon Sep 17 00:00:00 2001 From: Alexander Monakov Date: Wed, 15 May 2024 16:23:17 +0300 Subject: [PATCH] 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. --- gcc/tree-into-ssa.cc | 17 +++++++++-------- 1 file changed, 9 insertions(+), 8 deletions(-) diff --git a/gcc/tree-into-ssa.cc b/gcc/tree-into-ssa.cc index 3732c269ca3..5b367c35812 100644 --- a/gcc/tree-into-ssa.cc +++ b/gcc/tree-into-ssa.cc @@ -805,21 +805,22 @@ prune_unused_phi_nodes (bitmap phis, bitmap kills, bitmap uses) locate the nearest dominating def in logarithmic time by binary search.*/ bitmap_ior (to_remove, kills, phis); n_defs = bitmap_count_bits (to_remove); - defs = XNEWVEC (struct dom_dfsnum, 2 * n_defs + 1); + adef = 2 * n_defs + 1; + defs = XNEWVEC (struct dom_dfsnum, adef); defs[0].bb_index = 1; defs[0].dfs_num = 0; - adef = 1; + struct dom_dfsnum *head = defs + 1, *tail = defs + adef; EXECUTE_IF_SET_IN_BITMAP (to_remove, 0, i, bi) { def_bb = BASIC_BLOCK_FOR_FN (cfun, i); - defs[adef].bb_index = i; - defs[adef].dfs_num = bb_dom_dfs_in (CDI_DOMINATORS, def_bb); - defs[adef + 1].bb_index = i; - defs[adef + 1].dfs_num = bb_dom_dfs_out (CDI_DOMINATORS, def_bb); - adef += 2; + head->bb_index = i; + head->dfs_num = bb_dom_dfs_in (CDI_DOMINATORS, def_bb); + head++, tail--; + tail->bb_index = i; + tail->dfs_num = bb_dom_dfs_out (CDI_DOMINATORS, def_bb); } + gcc_checking_assert (head == tail); BITMAP_FREE (to_remove); - gcc_assert (adef == 2 * n_defs + 1); qsort (defs, adef, sizeof (struct dom_dfsnum), cmp_dfsnum); gcc_assert (defs[0].bb_index == 1);