From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 1888) id 0EC3A3858402; Tue, 14 Sep 2021 15:23:25 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 0EC3A3858402 MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="utf-8" From: Patrick Palka To: gcc-cvs@gcc.gnu.org Subject: [gcc r12-3527] c++: empty union member activation during constexpr [PR102163] X-Act-Checkin: gcc X-Git-Author: Patrick Palka X-Git-Refname: refs/heads/master X-Git-Oldrev: 818c505188ff5cd8eb048eb0e614c4ef732225bd X-Git-Newrev: de07cff96abd43f6f65dcf333958899c2ec42598 Message-Id: <20210914152325.0EC3A3858402@sourceware.org> Date: Tue, 14 Sep 2021 15:23:25 +0000 (GMT) X-BeenThere: gcc-cvs@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc-cvs mailing list List-Unsubscribe: , List-Archive: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 14 Sep 2021 15:23:25 -0000 https://gcc.gnu.org/g:de07cff96abd43f6f65dcf333958899c2ec42598 commit r12-3527-gde07cff96abd43f6f65dcf333958899c2ec42598 Author: Patrick Palka Date: Tue Sep 14 11:22:12 2021 -0400 c++: empty union member activation during constexpr [PR102163] Here, the union's constructor is defined to activate its empty data member _M_rest, but during constexpr evaluation of this constructor the subobject constructor call O::O(&_M_rest, 42) doesn't produce a side effect that actually activates the member, so the union still appears uninitialized after its constructor has run. This patch fixes this by using a dummy MODIFY_EXPR in this situation, whose evaluation ensures the member gets activated. PR c++/102163 gcc/cp/ChangeLog: * constexpr.c (cxx_eval_call_expression): After evaluating a subobject constructor call for an empty union member, produce a side effect that makes sure the member gets activated. gcc/testsuite/ChangeLog: * g++.dg/cpp0x/constexpr-empty17.C: New test. Diff: --- gcc/cp/constexpr.c | 34 +++++++++++++++++++++----- gcc/testsuite/g++.dg/cpp0x/constexpr-empty17.C | 21 ++++++++++++++++ 2 files changed, 49 insertions(+), 6 deletions(-) diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c index 0c2498aee22..d3c075c5ab8 100644 --- a/gcc/cp/constexpr.c +++ b/gcc/cp/constexpr.c @@ -2787,12 +2787,34 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t, &jump_target); if (DECL_CONSTRUCTOR_P (fun)) - /* This can be null for a subobject constructor call, in - which case what we care about is the initialization - side-effects rather than the value. We could get at the - value by evaluating *this, but we don't bother; there's - no need to put such a call in the hash table. */ - result = lval ? ctx->object : ctx->ctor; + { + /* This can be null for a subobject constructor call, in + which case what we care about is the initialization + side-effects rather than the value. We could get at the + value by evaluating *this, but we don't bother; there's + no need to put such a call in the hash table. */ + result = lval ? ctx->object : ctx->ctor; + + /* If we've just evaluated a subobject constructor call for an + empty union member, it might not have produced a side effect + that actually activated the union member. So produce such a + side effect now to ensure the union appears initialized. */ + if (!result && new_obj + && TREE_CODE (new_obj) == COMPONENT_REF + && TREE_CODE (TREE_TYPE + (TREE_OPERAND (new_obj, 0))) == UNION_TYPE + && is_really_empty_class (TREE_TYPE (new_obj), + /*ignore_vptr*/false)) + { + tree activate = build2 (MODIFY_EXPR, TREE_TYPE (new_obj), + new_obj, + build_constructor (TREE_TYPE (new_obj), + NULL)); + cxx_eval_constant_expression (ctx, activate, lval, + non_constant_p, overflow_p); + ggc_free (activate); + } + } else if (VOID_TYPE_P (TREE_TYPE (res))) result = void_node; else diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-empty17.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-empty17.C new file mode 100644 index 00000000000..86126dabdbb --- /dev/null +++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-empty17.C @@ -0,0 +1,21 @@ +// PR c++/102163 +// { dg-do compile { target c++11 } } + +struct O { + constexpr O(int) { } +}; + +union _Variadic_union { + constexpr _Variadic_union(int __arg) : _M_rest(__arg) { } + int _M_first; + O _M_rest; +}; + +constexpr _Variadic_union u(42); + +struct _Variant_storage { + constexpr _Variant_storage() : _M_u(42) {} + _Variadic_union _M_u; +}; + +constexpr _Variant_storage w;