https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120629
--- Comment #29 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jakub Jelinek <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:2661d87d54ab7185e7115cbcfa47cb42565eb925 commit r16-1499-g2661d87d54ab7185e7115cbcfa47cb42565eb925 Author: Jakub Jelinek <ja...@redhat.com> Date: Fri Jun 13 14:01:18 2025 +0200 expand: Fix up edge splitting for ENTRY block during expansion if there are any PHIs [PR120629] Andrew ran some extra ranger checking during bootstrap and found one more case (though much rarer than the GIMPLE_COND case). Seems on fold-const.cc (native_encode_expr) we end up with bb 2, ENTRY bb successor, having PHI nodes (usually there is some bb in between, even if empty, in the native_encode_expr it is tail recursion but haven't managed to construct a test with such case by hand). So, we have in optimized dump <bb 2> [local count: 1089340384]: # expr_12 = PHI <expr_199(D)(0), part_93(51)> # ptr_13 = PHI <ptr_86(D)(0), ptr_13(51)> # len_14 = PHI <len_103(D)(0), _198(51)> # off_10 = PHI <off_102(D)(0), _207(51)> # add_acc_99 = PHI <0(0), add_acc_101(51)> where there are mostly default defs from the 0->2 edge (and one zero) and some other values from the other edge. construct_init_block inserts a BB_RTL basic block with the function start instructions and similarly to the GIMPLE_COND case it wants to insert that bb on the edge from ENTRY to its single successor. Now, without this patch redirect_edge_succ redirects the 0->2 edge to 0->52, so the 51->2 edge gets moved first by unordered_remove, and make_single_succ_edge adds a new 52->2 edge. So we end up with # expr_12 = PHI <expr_199(D)(51), part_93(52)> # ptr_13 = PHI <ptr_86(D)(51), ptr_13(52)> # len_14 = PHI <len_103(D)(51), _198(52)> # off_10 = PHI <off_102(D)(51), _207(52)> # add_acc_99 = PHI <0(51), add_acc_101(52)> which is not correct, the default definitions and zero are now from the edge from end of function and the other values from the edge from the new BB_RTL successor of ENTRY. With this patch we get # expr_12 = PHI <expr_199(D)(52), part_93(51)> # ptr_13 = PHI <ptr_86(D)(52), ptr_13(51)> # len_14 = PHI <len_103(D)(52), _198(51)> # off_10 = PHI <off_102(D)(52), _207(51)> # add_acc_99 = PHI <0(52), add_acc_101(51)> instead. 2025-06-13 Jakub Jelinek <ja...@redhat.com> PR middle-end/120629 * cfgexpand.cc (construct_init_block): If first_block isn't BB_RTL, has any PHI nodes and false_edge->dest_idx before redirection is different from make_single_succ_edge result's dest_idx, swap the latter with the former last pred edge and their dest_idx members.
