Bootstrapped and regtested on x86_64-pc-linux-gnu, does this
look OK for trunk and ideally backports? The parent libstdc++
regression PR114865 is present in all release branches, and
this would allow us to fix it everywhere.
Note that there is a potential library-only solution for
PR114865, namely adding an empty [[no_unique_address]] data member
to std::atomic and through its member-initializer sneaking in the
__builtin_clear_padding call, roughly
template<typename _Tp>
struct atomic
{
alignas(_S_alignment) _Tp _M_i;
[[no_unique_address]] struct __empty { } _M_empty;
constexpr atomic(_Tp __t)
: _M_i(__t),
_M_empty(((__atomic_impl::__maybe_has_padding<_Tp>()
&& !std::__is_constant_evaluated()
? __builtin_clear_padding(&_M_i)
: void()), __empty{}))
{ }
};
None of us is confident that adding such a member to atomic is safe
from an ABI perspective (and e.g. safe from triggering a
[[no_unique_address]] bug on the release branches). It seems this
frontend approach of first relaxing the C++11 constexpr rules is safer
to backport, all things considered.
-- >8 --
This patch extends our support for C++14 non-empty constexpr
constructor bodies to C++11, as an extension. This will make
it trivial to safely fix the C++11 library regression PR114865
which requires calling __builtin_clear_padding after initializing
_M_i in std::atomic's single-parameter constructor, and that's not
really possible with the C++11 constexpr restrictions.
Since we already desugar member initializers to statements internally
even in C++11 mode, and so their bodies are already effectively non-empty
internally, supporting non-empty bodies in user code is mostly a matter
of relaxing the parse-time error.
But constexpr-ex3.C revealed that by accepting the non-empty body of A's
constructor, build_data_member_initialization goes on to mistake the
'i = _i' assignment as a member-initializer, and we incorrectly accept
the constructor in C++11 mode (even though it's only valid in C++20).
Turns out this is caused by that function recognizing MODIFY_EXPR only
in C++11 mode, logic that was last changed by r5-5013 (presumably to
limit impact of the patch at the time) but I reckon could just be
removed outright. This should be safe because the result of
build_data_member_initialization is only used for rejecting invalid
constructors (e.g. missing initializers), and actual evaluation uses
the desugared constructor body.
With this patch we can fix PR114865 by defining the affected
constructor as (avoding if statements which are still disallowed
in C++11 constexpr):
constexpr atomic(_Tp __i) noexcept : _M_i(__i)
{
(__atomic_impl::__maybe_has_padding<_Tp>()
&& !std::__is_constant_evaluated()
? __builtin_clear_padding(std::__addressof(_M_i))
: void());
}
(Note that Clang accepts the code as an extension in C++11 mode as
well.)
PR c++/123845
PR libstdc++/114865
gcc/cp/ChangeLog:
* constexpr.cc (build_data_member_initialization): Remove
C++11-specific recognition of MODIFY_EXPR.
(check_constexpr_ctor_body): Relax error diagnostic to a
pedwarn and don't clear DECL_DECLARED_CONSTEXPR_P upon
error. Return true if complaining.
gcc/testsuite/ChangeLog:
* g++.dg/cpp0x/constexpr-ex3.C: Adjust C++11 non-empty
constexpr constructor dg-error to a dg-warning. Expect
a follow-up missing member initializer diagnostic in C++11 mode.
* g++.dg/cpp2a/constexpr-try1.C: Expect a follow-up
compound-statement in constexpr function diagnostic in C++11
mode.
* g++.dg/cpp2a/constexpr-try2.C: Likewise. Adjust C++11
non-empty constexpr constructor dg-error to a dg-warning.
* g++.dg/cpp2a/constexpr-try3.C: Adjust C++11 non-empty
constexpr constructor dg-error to a dg-warning.
* g++.dg/cpp0x/constexpr-ctor23.C: New test.
---
gcc/cp/constexpr.cc | 14 ++++------
gcc/testsuite/g++.dg/cpp0x/constexpr-ctor23.C | 26 +++++++++++++++++++
gcc/testsuite/g++.dg/cpp0x/constexpr-ex3.C | 3 ++-
gcc/testsuite/g++.dg/cpp2a/constexpr-try1.C | 1 +
gcc/testsuite/g++.dg/cpp2a/constexpr-try2.C | 3 ++-
gcc/testsuite/g++.dg/cpp2a/constexpr-try3.C | 2 +-
6 files changed, 37 insertions(+), 12 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-ctor23.C
diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 1527e9dcbac8..a31fe6b113a9 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -412,11 +412,7 @@ build_data_member_initialization (tree t,
vec<constructor_elt, va_gc> **vec)
}
if (TREE_CODE (t) == CONVERT_EXPR)
t = TREE_OPERAND (t, 0);
- if (TREE_CODE (t) == INIT_EXPR
- /* vptr initialization shows up as a MODIFY_EXPR. In C++14 we only
- use what this function builds for cx_check_missing_mem_inits, and
- assignment in the ctor body doesn't count. */
- || (cxx_dialect < cxx14 && TREE_CODE (t) == MODIFY_EXPR))
+ if (TREE_CODE (t) == INIT_EXPR)
{
member = TREE_OPERAND (t, 0);
init = break_out_target_exprs (TREE_OPERAND (t, 1));
@@ -578,11 +574,11 @@ check_constexpr_ctor_body (tree last, tree list, bool
complain)
else if (list != last
&& !check_constexpr_ctor_body_1 (last, list))
ok = false;
- if (!ok)
+ if (!ok && complain)
{
- if (complain)
- error ("%<constexpr%> constructor does not have empty body");
- DECL_DECLARED_CONSTEXPR_P (current_function_decl) = false;
+ pedwarn (input_location, OPT_Wc__14_extensions,
+ "%<constexpr%> constructor does not have empty body");
+ ok = true;
}
return ok;
}
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-ctor23.C
b/gcc/testsuite/g++.dg/cpp0x/constexpr-ctor23.C
new file mode 100644
index 000000000000..4019804ab166
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-ctor23.C
@@ -0,0 +1,26 @@
+// Verify we diagnose and accept, as an extension, a non-empty constexpr
+// constructor body in C++11 mode.
+// PR c++/123845
+// { dg-do compile { target c++11_only } }
+// { dg-options "" }
+
+constexpr int negate(int n) { return -n; }
+
+struct A {
+ int m;
+ constexpr A() : m(42) {
+ ++m;
+ m = negate(m);
+ } // { dg-warning "does not have empty body \[-Wc++14-extensions\]" }
+};
+static_assert(A().m == -43, "");
+
+template<class T>
+struct B {
+ int m;
+ constexpr B() : m(42) {
+ ++m;
+ m = negate(m);
+ } // { dg-warning "does not have empty body \[-Wc++14-extensions\]" }
+};
+static_assert(B<int>().m == -43, "");
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-ex3.C
b/gcc/testsuite/g++.dg/cpp0x/constexpr-ex3.C
index 9d6d5ff587ca..169976afbaf7 100644
--- a/gcc/testsuite/g++.dg/cpp0x/constexpr-ex3.C
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-ex3.C
@@ -6,7 +6,8 @@
struct A
{
int i;
- constexpr A(int _i) { i = _i; } // { dg-error "empty body|A::i" "" { target
c++17_down } }
+ constexpr A(int _i) { i = _i; } // { dg-warning "empty body" "" { target
c++11_only } }
+ // { dg-error "'A::i' must be init" "" {
target c++17_down } .-1 }
};
template <class T>
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-try1.C
b/gcc/testsuite/g++.dg/cpp2a/constexpr-try1.C
index 977eb86dd192..e5e70a62b50f 100644
--- a/gcc/testsuite/g++.dg/cpp2a/constexpr-try1.C
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-try1.C
@@ -32,6 +32,7 @@ struct S {
} catch (int) { // { dg-error "compound-statement in 'constexpr'
function" "" { target c++11_only } }
} // { dg-error "compound-statement in 'constexpr'
function" "" { target c++11_only } .-2 }
} catch (...) { // { dg-error "'constexpr' constructor does not have
empty body" "" { target c++11_only } }
+ // { dg-error "compound-statement in 'constexpr'
function" "" { target c++11_only } .-1 }
}
int m;
};
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-try2.C
b/gcc/testsuite/g++.dg/cpp2a/constexpr-try2.C
index 7ca7261a9e00..9504fdaa8696 100644
--- a/gcc/testsuite/g++.dg/cpp2a/constexpr-try2.C
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-try2.C
@@ -32,7 +32,8 @@ struct S {
try { // { dg-warning "'try' in 'constexpr' function only
available with" "" { target c++17_down } }
} catch (int) { // { dg-warning "compound-statement in 'constexpr'
function" "" { target c++11_only } }
} // { dg-warning "compound-statement in 'constexpr'
function" "" { target c++11_only } .-2 }
- } catch (...) { // { dg-error "'constexpr' constructor does not have
empty body" "" { target c++11_only } }
+ } catch (...) { // { dg-warning "'constexpr' constructor does not have
empty body" "" { target c++11_only } }
+ // { dg-warning "compound-statement in 'constexpr'
function" "" { target c++11_only } .-1 }
}
int m;
};
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-try3.C
b/gcc/testsuite/g++.dg/cpp2a/constexpr-try3.C
index ab7e8f6d4649..070040c5deef 100644
--- a/gcc/testsuite/g++.dg/cpp2a/constexpr-try3.C
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-try3.C
@@ -31,7 +31,7 @@ struct S {
try { // { dg-warning "'try' in 'constexpr' function only
available with" "" { target c++17_down } }
} catch (int) {
}
- } catch (...) { // { dg-error "'constexpr' constructor does not have
empty body" "" { target c++11_only } }
+ } catch (...) { // { dg-warning "'constexpr' constructor does not have
empty body" "" { target c++11_only } }
}
int m;
};
--
2.53.0.rc1.65.gea24e2c554
