* [PATCH] c++: 'mutable' within constexpr [PR92505]
@ 2022-09-15 18:03 Patrick Palka
2022-09-16 12:10 ` Jason Merrill
0 siblings, 1 reply; 2+ messages in thread
From: Patrick Palka @ 2022-09-15 18:03 UTC (permalink / raw)
To: gcc-patches; +Cc: jason, Patrick Palka
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?
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, "");
--
2.37.3.662.g36f8e7ed7d
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: [PATCH] c++: 'mutable' within constexpr [PR92505]
2022-09-15 18:03 [PATCH] c++: 'mutable' within constexpr [PR92505] Patrick Palka
@ 2022-09-16 12:10 ` Jason Merrill
0 siblings, 0 replies; 2+ messages in thread
From: Jason Merrill @ 2022-09-16 12:10 UTC (permalink / raw)
To: Patrick Palka, gcc-patches
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, "");
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