https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118790
--- Comment #35 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:7738c6286fba7ec2112823f53cc2cefac2c8d007 commit r15-7506-g7738c6286fba7ec2112823f53cc2cefac2c8d007 Author: Jakub Jelinek <ja...@redhat.com> Date: Thu Feb 13 14:14:50 2025 +0100 tree, gengtype: Fix up GC issue with DECL_VALUE_EXPR [PR118790] The following testcase ICEs, because we have multiple levels of DECL_VALUE_EXPR VAR_DECLs: character(kind=1) id_string[1:.id_string] [value-expr: *id_string.55]; character(kind=1)[1:.id_string] * id_string.55 [value-expr: FRAME.107.id_string.55]; integer(kind=8) .id_string [value-expr: FRAME.107..id_string]; id_string is the user variable mentioned in BLOCK_VARS, it has DECL_VALUE_EXPR because it is a VLA, id_string.55 is a temporary created by gimplify_vla_decl as the address that points to the start of the VLA, what is normally used in the IL to access it. But as this artificial var is then used inside of a nested function, tree-nested.cc adds DECL_VALUE_EXPR to it too and moves the actual value into the FRAME.107 object's member. Now, remove_unused_locals removes id_string.55 (and various other VAR_DECLs) from cfun->local_decls, simply because it is not mentioned in the IL at all (neither is id_string itself, but that is kept in BLOCK_VARS as it has DECL_VALUE_EXPR). So, after this point, id_string.55 tree isn't referenced from anywhere but id_string's DECL_VALUE_EXPR. Next GC collection is triggered, and we are unlucky enough that in the value_expr_for_decl hash table (underlying hash map for DECL_VALUE_EXPR) the id_string.55 entry comes before the id_string entry. id_string is ggc_marked_p because it is referenced from BLOCK_VARS, but id_string.55 is not, as we don't mark DECL_VALUE_EXPR anywhere but by gt_cleare_cache on value_expr_for_decl. But gt_cleare_cache does two things, it calls clear_slots on entries where the key is not ggc_marked_p (so the id_string.55 mapping to FRAME.107.id_string.55 is lost and DECL_VALUE_EXPR (id_string.55) becomes NULL) but then later we see id_string entry, which is ggc_marked_p, so mark the whole hash table entry, which sets ggc_set_mark on id_string.55. But at this point its DECL_VALUE_EXPR is lost. Later during dwarf2out.cc we want to emit DW_AT_location for id_string, see it has DECL_VALUE_EXPR, so emit it as indirection of id_string.55 for which we again lookup DECL_VALUE_EXPR as it has DECL_HAS_VALUE_EXPR_P, but as it is NULL, we ICE, instead of finding it is a subobject of FRAME.107 for which we can find its stack location. Now, as can be seen in the PR, I've tried to tweak tree-ssa-live.cc so that it would keep id_string.55 in cfun->local_decls; that prohibits it from the DECL_VALUE_EXPR of it being GC until expansion, but then we shrink and free cfun->local_decls completely and so GC at that point still can throw it away. The following patch adds an extension to the GTY ((cache)) option, before calling the gt_cleare_cache on some hash table by specifying GTY ((cache ("somefn"))) it calls somefn on that hash table as well. And this extra hook can do any additional ggc_set_mark needed so that gt_cleare_cache preserves everything that is actually needed and throws away the rest. In order to make it just 2 pass rather than up to n passes - (if we had say id1 -> something, id2 -> x(id1), id3 -> x(id2), id4 -> x(id3), id5 -> x(id4) in the value_expr_for_decl hash table in that order (where idN are VAR_DECLs with DECL_HAS_VALUE_EXPR_P, id5 is the only one mentioned from outside and idN -> X stands for idN having DECL_VALUE_EXPR X, something for some arbitrary tree and x(idN) for some arbitrary tree which mentions idN variable) and in each pass just marked the to part of entries with ggc_marked_p base.from we'd need to repeat until we don't mark anything) the patch calls walk_tree on DECL_VALUE_EXPR of the marked trees and if it finds yet unmarked tree, it marks it and walks its DECL_VALUE_EXPR as well the same way. 2025-02-13 Jakub Jelinek <ja...@redhat.com> PR debug/118790 * gengtype.cc (write_roots): Remove cache variable, instead break from the loop on match and test o for NULL. If the cache option has non-empty string argument, call the specified function with v->name as argument before calling gt_cleare_cache on it. * tree.cc (gt_value_expr_mark_2, gt_value_expr_mark_1, gt_value_expr_mark): New functions. (value_expr_for_decl): Use GTY ((cache ("gt_value_expr_mark"))) rather than just GTY ((cache)). * doc/gty.texi (cache): Document optional argument of cache option. * gfortran.dg/gomp/pr118790.f90: New test.
