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Honza has fairly recently changed operand_equal_p to compare DECL_FIELD_OFFSET for COMPONENT_REFs when comparing addresses. As the first testcase in this patch shows, while that is very nice for optimizations, for the -Wduplicated-branches warning it causes regressions. Pedantically a union in both C and C++ has only one active member at a time, so using some other union member even if it has the same type is UB, so I think the warning shouldn't warn when it sees access to different fields that happen to have the same offset and should consider them different. In my first attempt to fix this I've keyed the old behavior on OEP_LEXICOGRAPHIC, but unfortunately that has various problems, the warning has a quick non-lexicographic compare in build_conditional_expr* and another lexicographic more expensive one later during genericization and turning the first one into lexicographic would mean wasting compile time on large conditionals. So, this patch instead introduces a new OEP_ flag and makes sure to pass it to operand_equal_p in all -Wduplicated-branches cases. The cvt.c changes are because on the other testcase we were warning with UNKNOWN_LOCATION, so the user wouldn't really know where the questionable code is. 2021-03-25 Jakub Jelinek <jakub@redhat.com> PR c++/99565 * tree-core.h (enum operand_equal_flag): Add OEP_ADDRESS_OF_SAME_FIELD. * fold-const.c (operand_compare::operand_equal_p): Don't compare field offsets if OEP_ADDRESS_OF_SAME_FIELD. * c-warn.c (do_warn_duplicated_branches): Pass also OEP_ADDRESS_OF_SAME_FIELD to operand_equal_p. * c-typeck.c (build_conditional_expr): Pass OEP_ADDRESS_OF_SAME_FIELD to operand_equal_p. * call.c (build_conditional_expr_1): Pass OEP_ADDRESS_OF_SAME_FIELD to operand_equal_p. * cvt.c (convert_to_void): Preserve location_t on COND_EXPR or or COMPOUND_EXPR. * g++.dg/warn/Wduplicated-branches6.C: New test. * g++.dg/warn/Wduplicated-branches7.C: New test.
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