https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117516
--- Comment #7 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:6141fd55181d4a4005ef499694ad6dcaeba6249b commit r15-7311-g6141fd55181d4a4005ef499694ad6dcaeba6249b Author: Jakub Jelinek <ja...@redhat.com> Date: Sat Feb 1 00:48:21 2025 +0100 c++: check_flexarray fixes [PR117516] On the pr117516.C testcase check_flexarrays and its helper functions have exponential complexity, plus it reports the same bug over and over again in some cases instead of reporting perhaps other bugs. The functions want to diagnose flexible array member (and strangely [0] arrays too) followed by some other non-empty or array members in the same strcuture, or followed by other non-empty or array members in a containing structure (any of them), or flexible array members/[0] arrays in structures with no other non-empty members, or those nested in other structures. Strangely it doesn't complain if flexible array member is in a structure used in an array. As can be seeen on e.g. the flexary41.C test, it keeps reporting the same bug over and over: flexary41.C:5:24: error: flexible array member âA::bâ not at end of âstruct Aâ flexary41.C:5:24: error: flexible array member âA::bâ not at end of âstruct Bâ flexary41.C:5:24: error: flexible array member âA::bâ not at end of âstruct Câ flexary41.C:5:24: error: flexible array member âA::bâ not at end of âstruct Dâ flexary41.C:13:39: error: flexible array member âE::<unnamed struct>::nâ not at end of âstruct Eâ flexary41.C:18:23: error: flexible array member âH::tâ not at end of âstruct Kâ flexary41.C:25:36: note: next member âint K::abâ declared here flexary41.C:25:8: note: in the definition of âstruct Kâ The bug that A::b is followed by A::c is one bug reported 4 times, while it doesn't report the other bugs, that B::e flexarray is followed by B::f and that C::h flexarray is followed by C::i. That is because it always walks all the structures/unions of all the members and just finds the first flexarray in there. Now, this has horrible complexity plus it doesn't seem really useful to users. So, for cases where a flexible array member is followed by a non-empty other member in the same structure, the following patch just reports it once when finalizing that structure, and otherwise just recurses in structures solely into the last member, so that it can report cases like struct X { int a; int b[]; }; struct Y { X c; int d; }; or struct Z { X c; }; i.e. correct use of flexarray in X but following it by another member in Y or just nesting it (the former is error, the latter pedwarn as before). By only looking at the last member for structures we get rid of the complexity. Note, the patch doesn't do anything about unions, I think we still could spend a lot of time compiling. struct S { char s; }; union U0 { S a, b; }; union U1 { union U0 a, b; }; union U2 { union U1 a, b; }; ... union U32 { union U31 a, b; }; struct T { union U32 a; int b; }; Not really sure what we could do about that, all the elements are "last" (but admittedly I haven't studied in detail how the original code worked in union, there is fmem->after[pun] where pun is whether it is somewhere inside of a union). Perhaps in a hash table marking unions which don't have any flexarrays at the end, nested or not, so that we don't walk them again? Plus if we find some with flexarray at the end, maybe there is no point to look other union members? In any case, I think that is less severe, because people usually don't nest unions deeply. 2025-02-01 Jakub Jelinek <ja...@redhat.com> PR c++/117516 * class.cc (field_nonempty_p): Formatting fixes. Use integer_zerop instead of tree_int_cst_equal with size_zero_node. (struct flexmems_t): Change type of first member from tree to bool. (find_flexarrays): Add nested_p argument. Change pun argument type from tree to bool, adjust uses. Formatting fixes. If BASE_P or NESTED_P and T is RECORD_TYPE, start looking only at the last non-empty or array FIELD_DECL. Adjust recursive call, set first if it was a nested call and found an array. (diagnose_invalid_flexarray, diagnose_flexarrays, check_flexarrays): Formatting fixes. * g++.dg/ext/flexary9.C: Expect different wording of one of the warnings and at a different line. * g++.dg/ext/flexary19.C: Likewise. * g++.dg/ext/flexary42.C: New test. * g++.dg/other/pr117516.C: New test.
