https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117091
--- Comment #21 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Filip Kastl <phe...@gcc.gnu.org>: https://gcc.gnu.org/g:56946c801a7cf3a831a11870b7e11ba08bf9bd87 commit r15-6120-g56946c801a7cf3a831a11870b7e11ba08bf9bd87 Author: Filip Kastl <fka...@suse.cz> Date: Wed Dec 11 19:57:04 2024 +0100 gimple: Add limit after which slower switchlower algs are used [PR117091] [PR117352] This patch adds a limit on the number of cases of a switch. When this limit is exceeded, switch lowering decides to use faster but less powerful algorithms. In particular this means that for finding bit tests switch lowering decides between the old dynamic programming O(n^2) algorithm and the new greedy algorithm that Andi Kleen recently added but then reverted due to PR117352. It also means that switch lowering may bail out on finding jump tables if the switch is too large (Btw it also may not bail! It can happen that the greedy algorithms finds some bit tests which then basically split the switch into multiple smaller switches and those may be small enough to fit under the limit.) The limit is implemented as --param switch-lower-slow-alg-max-cases. Exceeding the limit is reported through -Wdisabled-optimization. This patch fixes the issue with the greedy algorithm described in PR117352. The problem was incorrect usage of the is_beneficial() heuristic. gcc/ChangeLog: PR middle-end/117091 PR middle-end/117352 * doc/invoke.texi: Add switch-lower-slow-alg-max-cases. * params.opt: Add switch-lower-slow-alg-max-cases. * tree-switch-conversion.cc (jump_table_cluster::find_jump_tables): Note in a comment that we are looking for jump tables in case sequences delimited by the already found bit tests. (bit_test_cluster::find_bit_tests): Decide between find_bit_tests_fast() and find_bit_tests_slow(). (bit_test_cluster::find_bit_tests_fast): New function. (bit_test_cluster::find_bit_tests_slow): New function. (switch_decision_tree::analyze_switch_statement): Report exceeding the limit. * tree-switch-conversion.h: Add find_bit_tests_fast() and find_bit_tests_slow(). Co-Authored-By: Andi Kleen <a...@gcc.gnu.org> Signed-off-by: Filip Kastl <fka...@suse.cz>
