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From: Giuliano Belinassi <giulianob@gcc.gnu.org> To: gcc-cvs@gcc.gnu.org Subject: [gcc/devel/autopar_devel] c++: Fix complex constexpr virtual cases [PR93310]. Date: Sat, 22 Aug 2020 22:03:02 +0000 (GMT) [thread overview] Message-ID: <20200822220302.AAB80393C8FC@sourceware.org> (raw) https://gcc.gnu.org/g:b3200082c0c4265669399980837ccd80869e1bef commit b3200082c0c4265669399980837ccd80869e1bef Author: Jason Merrill <jason@redhat.com> Date: Wed Jun 3 23:50:50 2020 -0400 c++: Fix complex constexpr virtual cases [PR93310]. The code in constexpr for looking up the actual type of the object and then getting the virtual function from there broke for both of these tests: for 16, it assumed incorrectly that the DECL_VINDEX would apply to the most derived type's vtable; for 17, it failed to consider that during construction the base subobject is treated as being of the base type. Fixed by just doing constant evaluation of the expression that looks up the function in the vtable. This means that a virtual call will involve loading the vptr, so we will reject some calls through non-constexpr variables that we previously accepted, but this seems appropriate to me. None of our testcases were affected. gcc/cp/ChangeLog: PR c++/93310 * constexpr.c (cxx_eval_constant_expression) [OBJ_TYPE_REF]: Evaluate OBJ_TYPE_REF_EXPR. gcc/testsuite/ChangeLog: PR c++/93310 * g++.dg/cpp2a/constexpr-virtual16.C: New test. * g++.dg/cpp2a/constexpr-virtual17.C: New test. * g++.dg/cpp2a/constexpr-new12.C: Adjust diagnostic. Diff: --- gcc/cp/constexpr.c | 44 ++---------------------- gcc/testsuite/g++.dg/cpp2a/constexpr-new12.C | 2 +- gcc/testsuite/g++.dg/cpp2a/constexpr-virtual16.C | 22 ++++++++++++ gcc/testsuite/g++.dg/cpp2a/constexpr-virtual17.C | 28 +++++++++++++++ 4 files changed, 54 insertions(+), 42 deletions(-) diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c index c7d17e9b86e..c01c42bf886 100644 --- a/gcc/cp/constexpr.c +++ b/gcc/cp/constexpr.c @@ -6373,47 +6373,9 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t, break; case OBJ_TYPE_REF: - { - /* Virtual function call. Let the constexpr machinery figure out - the dynamic type. */ - int token = tree_to_shwi (OBJ_TYPE_REF_TOKEN (t)); - tree obj = OBJ_TYPE_REF_OBJECT (t); - obj = cxx_eval_constant_expression (ctx, obj, lval, non_constant_p, - overflow_p); - STRIP_NOPS (obj); - /* We expect something in the form of &x.D.2103.D.2094; get x. */ - if (TREE_CODE (obj) != ADDR_EXPR - || !DECL_P (get_base_address (TREE_OPERAND (obj, 0)))) - { - if (!ctx->quiet) - error_at (loc, "expression %qE is not a constant expression", t); - *non_constant_p = true; - return t; - } - obj = TREE_OPERAND (obj, 0); - while (TREE_CODE (obj) == COMPONENT_REF - && DECL_FIELD_IS_BASE (TREE_OPERAND (obj, 1))) - obj = TREE_OPERAND (obj, 0); - tree objtype = TREE_TYPE (obj); - if (VAR_P (obj) - && DECL_NAME (obj) == heap_identifier - && TREE_CODE (objtype) == ARRAY_TYPE) - objtype = TREE_TYPE (objtype); - if (!CLASS_TYPE_P (objtype)) - { - if (!ctx->quiet) - error_at (loc, "expression %qE is not a constant expression", t); - *non_constant_p = true; - return t; - } - /* Find the function decl in the virtual functions list. TOKEN is - the DECL_VINDEX that says which function we're looking for. */ - tree virtuals = BINFO_VIRTUALS (TYPE_BINFO (objtype)); - if (TARGET_VTABLE_USES_DESCRIPTORS) - token /= MAX (TARGET_VTABLE_USES_DESCRIPTORS, 1); - r = TREE_VALUE (chain_index (token, virtuals)); - break; - } + /* Virtual function lookup. We don't need to do anything fancy. */ + return cxx_eval_constant_expression (ctx, OBJ_TYPE_REF_EXPR (t), + lval, non_constant_p, overflow_p); case PLACEHOLDER_EXPR: /* Use of the value or address of the current object. */ diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-new12.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-new12.C index fcf398f000e..5a3d06a5fab 100644 --- a/gcc/testsuite/g++.dg/cpp2a/constexpr-new12.C +++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-new12.C @@ -24,4 +24,4 @@ foo () return r; } -constexpr auto a = foo (); // { dg-error "is not a constant expression" } +constexpr auto a = foo (); // { dg-error "constant expression" } diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual16.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual16.C new file mode 100644 index 00000000000..8cca8a61723 --- /dev/null +++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual16.C @@ -0,0 +1,22 @@ +// Test constexpr virtual in non-primary vtable. +// { dg-do compile { target c++20 } } + +struct A +{ + virtual constexpr int f() const { return 1; }; +}; + +struct B +{ + virtual constexpr int g() const { return 2; }; +}; + +struct C: A, B +{ +}; + +constexpr C c; + +constexpr int g(const B& b) { return b.g(); } +static_assert (g(c) == 2); + diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual17.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual17.C new file mode 100644 index 00000000000..a8a89952626 --- /dev/null +++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual17.C @@ -0,0 +1,28 @@ +// PR c++/93310 +// { dg-do compile { target c++20 } } + +struct A +{ + virtual constexpr char f () const + { return 'A'; } +}; + +struct B : A +{ + char x; + + constexpr B () : x (0) + { x = ((A *)this)->f(); } + + virtual constexpr char f () const + { return 'B'; } +}; + +struct C : B +{ + virtual constexpr char f () const + { return 'C'; } +}; + +constexpr C c; +static_assert (c.x == 'B');
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