[Bug c++/96197] Excess memory consumption, positive correlation with the size of a constexpr array

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96197

--- Comment #7 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:8c00059ce058ea2aec2933319e270f5443b8b909

commit r11-2445-g8c00059ce058ea2aec2933319e270f5443b8b909
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Jul 30 22:21:41 2020 -0400

    c++: decl_constant_value and unsharing [PR96197]

    In the testcase from the PR we're seeing excessive memory use (> 5GB)
    during constexpr evaluation, almost all of which is due to the call to
    decl_constant_value in the VAR_DECL/CONST_DECL branch of
    cxx_eval_constant_expression.  We reach here every time we evaluate an
    ARRAY_REF of a constexpr VAR_DECL, and from there decl_constant_value
    makes an unshared copy of the VAR_DECL's initializer.  But unsharing
    here is unnecessary because callers of cxx_eval_constant_expression
    already unshare its result when necessary.

    To fix this excessive unsharing, this patch adds a new defaulted
    parameter unshare_p to decl_really_constant_value and
    decl_constant_value so that callers can control whether to unshare.

    As a simplification, we can also move the call to unshare_expr in
    constant_value_1 outside of the loop, since doing unshare_expr on a
    DECL_P is a no-op.

    Now that we no longer unshare the result of decl_constant_value and
    decl_really_constant_value from cxx_eval_constant_expression, memory use
    during constexpr evaluation for the testcase from the PR falls from ~5GB
    to 15MB according to -ftime-report.

    gcc/cp/ChangeLog:

            PR c++/96197
            * constexpr.c (cxx_eval_constant_expression) <case CONST_DECL>:
            Pass false to decl_constant_value and decl_really_constant_value
            so that they don't unshare their result.
            * cp-tree.h (decl_constant_value): New declaration with an added
            bool parameter.
            (decl_really_constant_value): Add bool parameter defaulting to
            true to existing declaration.
            * init.c (constant_value_1): Add bool parameter which controls
            whether to unshare the initializer before returning.  Call
            unshare_expr at most once.
            (scalar_constant_value): Pass true to constant_value_1's new
            bool parameter.
            (decl_really_constant_value): Add bool parameter and forward it
            to constant_value_1.
            (decl_constant_value): Likewise, but instead define a new
            overload with an added bool parameter.

    gcc/testsuite/ChangeLog:

            PR c++/96197
            * g++.dg/cpp1y/constexpr-array8.C: New test.