[Bug c++/97553] [missed optimization] constexprness not noticed when UBsan enabled

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97553

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:5fea1be820508e1fbc610d1a54b61c1add33c36f

commit r13-6120-g5fea1be820508e1fbc610d1a54b61c1add33c36f
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Feb 17 15:18:10 2023 -0500

    c++: speculative constexpr and is_constant_evaluated [PR108243]

    This PR illustrates that __builtin_is_constant_evaluated currently acts
    as an optimization barrier for our speculative constexpr evaluation,
    since we don't want to prematurely fold the builtin to false before the
    expression in question undergoes manifestly constant evaluation if
    appropriate (in which case the builtin must instead be folded to true).

    This patch fixes this by permitting __builtin_is_constant_evaluated to
    get folded as false at appropiate points, namely during cp_fold_function
    and cp_fully_fold_init where we know we're done with manifestly constant
    evaluation.  The function cp_fold gets a flags parameter that controls
    whether we pass mce_false or mce_unknown to maybe_constant_value when
    folding a CALL_EXPR.

            PR c++/108243
            PR c++/97553

    gcc/cp/ChangeLog:

            * cp-gimplify.cc (enum fold_flags): Define.
            (fold_flags_t): Declare.
            (cp_fold_data::genericize): Replace this data member with ...
            (cp_fold_data::fold_flags): ... this.
            (cp_fold_r): Adjust use of cp_fold_data and calls to cp_fold.
            (cp_fold_function): Likewise.
            (cp_fold_maybe_rvalue): Add an internal overload that
            additionally takes and propagates a fold_flags_t parameter, and
            define the existing public overload in terms of it.
            (cp_fold_rvalue): Likewise.
            (cp_fully_fold_init): Adjust use of cp_fold_data.
            (fold_cache): Replace with ...
            (fold_caches): ... this 2-element array of caches.
            (get_fold_cache): Define.
            (clear_fold_cache): Adjust.
            (cp_fold): Add fold_flags_t parameter.  Use get_fold_cache.
            Pass flags to calls to cp_fold, cp_fold_rvalue and
            cp_fold_maybe_rvalue.
            <case CALL_EXPR>: If ff_mce_false is set, fold
            __builtin_is_constant_evaluated to false and pass mce_false to
            maybe_constant_value.

    gcc/testsuite/ChangeLog:

            * g++.dg/opt/is_constant_evaluated1.C: New test.
            * g++.dg/opt/is_constant_evaluated2.C: New test.