From: Jason Merrill <jason@redhat.com>
To: Patrick Palka <ppalka@redhat.com>, gcc-patches@gcc.gnu.org
Subject: Re: [PATCH] c++: 'mutable' within constexpr [PR92505]
Date: Fri, 16 Sep 2022 14:10:26 +0200 [thread overview]
Message-ID: <99dcf250-4d50-c64c-d607-d012208a8eec@redhat.com> (raw)
In-Reply-To: <20220915180312.1596193-1-ppalka@redhat.com>
On 9/15/22 14:03, Patrick Palka wrote:
> This patch permits accessing 'mutable' members of local objects during
> constexpr evaluation (which other compilers seem to accept in C++14
> mode, while we reject), while continuing to reject it for global objects
> (as in the last line of cpp0x/constexpr-mutable1.C, which other
> compilers also reject). To distinguish between the two cases, it looks
> like we just need to additionally check CONSTRUCTOR_MUTABLE_POISION
> alongside DECL_MUTABLE_P in cxx_eval_component_reference before
> rejecting the access.
>
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?
OK.
> PR c++/92505
>
> gcc/cp/ChangeLog:
>
> * constexpr.cc (cxx_eval_component_reference): Test non_constant_p
> earlier. In C++14 or later, reject DECL_MUTABLE_P member
> accesses only if CONSTRUCTOR_MUTABLE_POISION is also set.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/cpp0x/constexpr-mutable3.C: New test.
> * g++.dg/cpp1y/constexpr-mutable1.C: New test.
> ---
> gcc/cp/constexpr.cc | 11 +++++++----
> gcc/testsuite/g++.dg/cpp0x/constexpr-mutable3.C | 7 +++++++
> gcc/testsuite/g++.dg/cpp1y/constexpr-mutable1.C | 16 ++++++++++++++++
> 3 files changed, 30 insertions(+), 4 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-mutable3.C
> create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-mutable1.C
>
> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> index 57283eabf3c..10639876d9c 100644
> --- a/gcc/cp/constexpr.cc
> +++ b/gcc/cp/constexpr.cc
> @@ -4088,6 +4088,8 @@ cxx_eval_component_reference (const constexpr_ctx *ctx, tree t,
> tree whole = cxx_eval_constant_expression (ctx, orig_whole,
> lval,
> non_constant_p, overflow_p);
> + if (*non_constant_p)
> + return t;
> if (INDIRECT_REF_P (whole)
> && integer_zerop (TREE_OPERAND (whole, 0)))
> {
> @@ -4108,20 +4110,21 @@ cxx_eval_component_reference (const constexpr_ctx *ctx, tree t,
> whole, part, NULL_TREE);
> /* Don't VERIFY_CONSTANT here; we only want to check that we got a
> CONSTRUCTOR. */
> - if (!*non_constant_p && TREE_CODE (whole) != CONSTRUCTOR)
> + if (TREE_CODE (whole) != CONSTRUCTOR)
> {
> if (!ctx->quiet)
> error ("%qE is not a constant expression", orig_whole);
> *non_constant_p = true;
> + return t;
> }
> - if (DECL_MUTABLE_P (part))
> + if ((cxx_dialect < cxx14 || CONSTRUCTOR_MUTABLE_POISON (whole))
> + && DECL_MUTABLE_P (part))
> {
> if (!ctx->quiet)
> error ("mutable %qD is not usable in a constant expression", part);
> *non_constant_p = true;
> + return t;
> }
> - if (*non_constant_p)
> - return t;
> bool pmf = TYPE_PTRMEMFUNC_P (TREE_TYPE (whole));
> FOR_EACH_CONSTRUCTOR_ELT (CONSTRUCTOR_ELTS (whole), i, field, value)
> {
> diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-mutable3.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-mutable3.C
> new file mode 100644
> index 00000000000..46c9d8437be
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-mutable3.C
> @@ -0,0 +1,7 @@
> +// PR c++/92505
> +// { dg-do compile { target c++11 } }
> +
> +struct A { mutable int m; };
> +constexpr int f(A a) { return a.m; }
> +static_assert(f({42}) == 42, "");
> +// { dg-error "non-constant|mutable" "" { target c++11_only } .-1 }
> diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-mutable1.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-mutable1.C
> new file mode 100644
> index 00000000000..6c47988c01a
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-mutable1.C
> @@ -0,0 +1,16 @@
> +// PR c++/92505
> +// { dg-do compile { target c++14 } }
> +
> +struct S { mutable int m; };
> +
> +static_assert(S{42}.m == 42, "");
> +
> +constexpr int f() {
> + S s = {40};
> + s.m++;
> + const auto& cs = s;
> + ++cs.m;
> + return cs.m;
> +}
> +
> +static_assert(f() == 42, "");
prev parent reply other threads:[~2022-09-16 12:10 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-15 18:03 Patrick Palka
2022-09-16 12:10 ` Jason Merrill [this message]
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