https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44677
--- Comment #25 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:0eac9cfee8cb0b21de866a04d5d59685ab35208f commit r16-2258-g0eac9cfee8cb0b21de866a04d5d59685ab35208f Author: Jakub Jelinek <ja...@redhat.com> Date: Tue Jul 15 15:00:33 2025 +0200 c, c++: Extend -Wunused-but-set-* warnings [PR44677] The -Wunused-but-set-* warnings work by using 2 bits on VAR_DECLs & PARM_DECLs, TREE_USED and DECL_READ_P. If neither is set, we typically emit -Wunused-variable or -Wunused-parameter warning, that is for variables which are just declared (including initializer) and completely unused. If TREE_USED is set and DECL_READ_P is unset, -Wunused-but-set-* warnings are emitted, i.e. for variables which can appear on the lhs of an assignment expression but aren't actually used elsewhere. The DECL_READ_P marking is done through mark_exp_read called from lots of places (e.g. lvalue to rvalue conversions etc.). LLVM has an extension on top of that in that it doesn't count pre/post inc/decrements as use (i.e. DECL_READ_P for GCC). The following patch does that too, though because we had the current behavior for 11+ years already and lot of people is -Wunused-but-set-* warning free in the current GCC behavior and not in the clang one (including GCC sources), it allows users to choose. Furthermore, it implements another level, where also var @= expr uses of var (except when it is also used in expr) aren't counted as DECL_READ_P. I think it would be nice to also handle var = var @ expr or var = expr @ var but unfortunately mark_exp_read is then done in both FEs during parsing of var @ expr or expr @ var and the code doesn't know it is rhs of an assignment with var as lhs. The patch works mostly by checking if DECL_READ_P is clear at some point and then clearing it again after some operation which might have set it. -Wunused or -Wall or -Wunused -Wextra or -Wall -Wextra turn on the 3 level of the new warning (i.e. the one which ignores also var++, ++var etc. as well as var @= expr), so does -Wunused-but-set-{variable,parameter}, but users can use explicit -Wunused-but-set-{variable,parameter}={1,2} to select a different level. 2025-07-15 Jakub Jelinek <ja...@redhat.com> Jason Merrill <ja...@redhat.com> PR c/44677 gcc/ * common.opt (Wunused-but-set-parameter=, Wunused-but-set-variable=): New options. (Wunused-but-set-parameter, Wunused-but-set-variable): Turn into aliases. * common.opt.urls: Regenerate. * diagnostic-spec.cc (nowarn_spec_t::nowarn_spec_t): Use OPT_Wunused_but_set_variable_ instead of OPT_Wunused_but_set_variable and OPT_Wunused_but_set_parameter_ instead of OPT_Wunused_but_set_parameter. * gimple-ssa-store-merging.cc (find_bswap_or_nop_1): Remove unused but set variable tmp. * ipa-strub.cc (pass_ipa_strub::execute): Cast named_args to (void) if ATTR_FNSPEC_DECONST_WATERMARK is not defined. * doc/invoke.texi (Wunused-but-set-parameter=, Wunused-but-set-variable=): Document new options. (Wunused-but-set-parameter, Wunused-but-set-variable): Adjust documentation now that they are just aliases. gcc/c-family/ * c-opts.cc (c_common_post_options): Change warn_unused_but_set_parameter and warn_unused_but_set_variable from 1 to 3 if they were set only implicitly. * c-attribs.cc (build_attr_access_from_parms): Remove unused but set variable nelts. gcc/c/ * c-parser.cc (c_parser_unary_expression): Clear DECL_READ_P after default_function_array_read_conversion for -Wunused-but-set-{parameter,variable}={2,3} on PRE{IN,DE}CREMENT_EXPR argument. (c_parser_postfix_expression_after_primary): Similarly for POST{IN,DE}CREMENT_EXPR. * c-decl.cc (pop_scope): Use OPT_Wunused_but_set_variable_ instead of OPT_Wunused_but_set_variable. (finish_function): Use OPT_Wunused_but_set_parameter_ instead of OPT_Wunused_but_set_parameter. * c-typeck.cc (mark_exp_read): Handle {PRE,POST}{IN,DE}CREMENT_EXPR and don't handle it when cast to void. (build_modify_expr): Clear DECL_READ_P after build_binary_op for -Wunused-but-set-{parameter,variable}=3. gcc/cp/ * cp-gimplify.cc (cp_fold): Clear DECL_READ_P on lhs of MODIFY_EXPR after cp_fold_rvalue if it wasn't set before. * decl.cc (poplevel): Use OPT_Wunused_but_set_variable_ instead of OPT_Wunused_but_set_variable. (finish_function): Use OPT_Wunused_but_set_parameter_ instead of OPT_Wunused_but_set_parameter. * expr.cc (mark_use): Clear read_p for {PRE,POST}{IN,DE}CREMENT_EXPR cast to void on {VAR,PARM}_DECL for -Wunused-but-set-{parameter,variable}={2,3}. (mark_exp_read): Handle {PRE,POST}{IN,DE}CREMENT_EXPR and don't handle it when cast to void. * module.cc (trees_in::fn_parms_fini): Remove unused but set variable ix. * semantics.cc (finish_unary_op_expr): Return early for PRE{IN,DE}CREMENT_EXPR. * typeck.cc (cp_build_unary_op): Clear DECL_READ_P after mark_lvalue_use for -Wunused-but-set-{parameter,variable}={2,3} on PRE{IN,DE}CREMENT_EXPR argument. (cp_build_modify_expr): Clear DECL_READ_P after cp_build_binary_op for -Wunused-but-set-{parameter,variable}=3. gcc/go/ * gofrontend/gogo.cc (Function::export_func_with_type): Remove unused but set variable i. gcc/cobol/ * gcobolspec.cc (lang_specific_driver): Remove unused but set variable n_cobol_files. gcc/testsuite/ * c-c++-common/Wunused-parm-1.c: New test. * c-c++-common/Wunused-parm-2.c: New test. * c-c++-common/Wunused-parm-3.c: New test. * c-c++-common/Wunused-parm-4.c: New test. * c-c++-common/Wunused-parm-5.c: New test. * c-c++-common/Wunused-parm-6.c: New test. * c-c++-common/Wunused-var-7.c (bar, baz): Expect warning on a. * c-c++-common/Wunused-var-19.c: New test. * c-c++-common/Wunused-var-20.c: New test. * c-c++-common/Wunused-var-21.c: New test. * c-c++-common/Wunused-var-22.c: New test. * c-c++-common/Wunused-var-23.c: New test. * c-c++-common/Wunused-var-24.c: New test. * g++.dg/cpp26/name-independent-decl1.C (foo): Expect one set but not used warning. * g++.dg/warn/Wunused-parm-12.C: New test. * g++.dg/warn/Wunused-parm-13.C: New test. * g++.dg/warn/Wunused-var-2.C (f2): Expect set but not used warning on parameter x and variable a. * g++.dg/warn/Wunused-var-40.C: New test. * g++.dg/warn/Wunused-var-41.C: New test. * gcc.dg/memchr-3.c (test_find): Change return type from void to int, and add return n; statement. * gcc.dg/unused-9.c (g): Move dg-bogus to the correct line and expect a warning on i.
