From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 61043 invoked by alias); 18 Sep 2018 03:28:31 -0000 Mailing-List: contact gcc-patches-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Id: List-Archive: List-Post: List-Help: Sender: gcc-patches-owner@gcc.gnu.org Received: (qmail 61034 invoked by uid 89); 18 Sep 2018 03:28:30 -0000 Authentication-Results: sourceware.org; auth=none X-Spam-SWARE-Status: No, score=-24.2 required=5.0 tests=AWL,BAYES_00,GIT_PATCH_0,GIT_PATCH_1,GIT_PATCH_2,GIT_PATCH_3 autolearn=ham version=3.3.2 spammy= X-HELO: mail-ot1-f66.google.com Received: from mail-ot1-f66.google.com (HELO mail-ot1-f66.google.com) (209.85.210.66) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with ESMTP; Tue, 18 Sep 2018 03:28:29 +0000 Received: by mail-ot1-f66.google.com with SMTP id i10-v6so525217oth.0 for ; Mon, 17 Sep 2018 20:28:28 -0700 (PDT) MIME-Version: 1.0 Received: by 2002:ac9:1522:0:0:0:0:0 with HTTP; Mon, 17 Sep 2018 20:28:06 -0700 (PDT) In-Reply-To: <20180917213938.GL5587@redhat.com> References: <20180914171950.GE5587@redhat.com> <20180914204521.GJ5587@redhat.com> <20180917213938.GL5587@redhat.com> From: Jason Merrill Date: Tue, 18 Sep 2018 03:48:00 -0000 Message-ID: Subject: Re: C++ PATCH to implement P1064R0, Virtual Function Calls in Constant Expressions To: Marek Polacek Cc: GCC Patches , Jakub Jelinek Content-Type: text/plain; charset="UTF-8" X-IsSubscribed: yes X-SW-Source: 2018-09/txt/msg00952.txt.bz2 On Mon, Sep 17, 2018 at 5:39 PM, Marek Polacek wrote: > On Fri, Sep 14, 2018 at 04:45:22PM -0400, Marek Polacek wrote: >> On Fri, Sep 14, 2018 at 04:30:46PM -0400, Jason Merrill wrote: >> > On Fri, Sep 14, 2018 at 1:19 PM, Marek Polacek wrote: >> > > This patch implements another bit of C++20, virtual calls in constant >> > > expression: >> > > >> > > The basic idea is that since in a constant expression we know the dynamic >> > > type (to detect invalid code etc.), the restriction that prohibits virtual >> > > calls is unnecessary. >> > > >> > > Handling virtual function calls turned out to be fairly easy (as anticipated); >> > > I simply let the constexpr machinery figure out the dynamic type and then >> > > OBJ_TYPE_REF_TOKEN gives us the index into the virtual function table. That >> > > way we get the function decl we're interested in, and cxx_eval_call_expression >> > > takes it from there. >> > > >> > > But handling pointer-to-virtual-member-functions doesn't work like that. >> > > get_member_function_from_ptrfunc creates a COND_EXPR which looks like >> > > if (pf.__pfn & 1) // is it a virtual function? >> > > // yes, find the pointer in the vtable >> > > else >> > > // no, just return the pointer >> > > so ideally we want to evaluate the then-branch. Eventually it'll evaluate it >> > > to something like _ZTV2X2[2], but the vtable isn't constexpr so we'd end up >> > > with "not a constant expression" error. >> > >> > Then let's mark the vtable as constexpr, there's no reason for it not to be. > > Done. But then I had to add indexes to the vtable's ctor (because find_array_ctor_elt > expects it), which broke an assert in gimple_get_virt_method_for_vtable. But I > don't need the array_ref hack anymore! > Also, I had to set DECL_DECLARED_CONSTEXPR_P after maybe_commonize_var, > otherwise we run into the sorry in that function with -fno-weak... Hmm, we shouldn't give that sorry for DECL_ARTIFICIAL variables. Looking more closely, it seems that the call to maybe_commonize_var from initialize_artificial_var did nothing before this change, since the vtable is DECL_ARTIFICIAL, so it didn't pass the condition at the top. I suppose we should extend the !DECL_ARTIFICIAL check in maybe_commonize_var to the inline variable case as well. >> Ok, unfortunately it wasn't as easy as merely marking it DECL_DECLARED_CONSTEXPR_P >> in initialize_artificial_var because then I saw "used in its own initializer" >> error. Which I don't know why, but now that I know you agree with this direction >> I can dig deeper. >> >> > > Since the vtable initializer is >> > > a compile-time constant, I thought we could make it work by a hack as the one >> > > in cxx_eval_array_reference. We'll then let cxx_eval_call_expression do its >> > > job and everything is hunky-dory. >> > > >> > > Except when it isn't: I noticed that the presence of _vptr doesn't make the >> > > class non-empty, and when cxx_eval_constant_expression saw a decl with an empty >> > > class type, it just evaluated it to { }. But such a class still had gotten an >> > > initializer that looks like {.D.2082 = {._vptr.X2 = &_ZTV2X2 + 16}}. So >> > > replacing it with { } will lose the proper initializer whereupon we fail. >> > > The check I've added to cxx_eval_constant_expression won't win any beauty >> > > contests but unfortunately EMPTY_CONSTRUCTOR_P doesn't work there. >> > >> > Perhaps we should check !TYPE_POLYMORPHIC_P as well as >> > is_really_empty_class. Perhaps there should be a predicate for that, >> > say, is_really_nearly_empty_class... > > For now I've only added the !TYPE_POLYMORPHIC_P check, which works just fine. > > Bootstrapped/regtested on x86_64-linux, ok for trunk? > > 2018-09-17 Marek Polacek > > P1064R0 - Allowing Virtual Function Calls in Constant Expressions > * call.c (build_over_call): Add FIXME. > * class.c (initialize_vtable): Mark the vtable as constexpr. > (build_vtbl_initializer): Build vtable's constructor with indexes. > * constexpr.c (cxx_eval_constant_expression): Don't ignore _vptr's > initializer. Handle OBJ_TYPE_REF. > (potential_constant_expression_1): Handle OBJ_TYPE_REF. > * decl.c (grokdeclarator): Change error to pedwarn. Only warn when > pedantic and not C++2a. > > * gimple-fold.c (gimple_get_virt_method_for_vtable): Remove assert. > > * g++.dg/cpp0x/constexpr-virtual5.C: Adjust dg-error. > * g++.dg/cpp2a/constexpr-virtual1.C: New test. > * g++.dg/cpp2a/constexpr-virtual2.C: New test. > * g++.dg/cpp2a/constexpr-virtual3.C: New test. > * g++.dg/cpp2a/constexpr-virtual4.C: New test. > * g++.dg/cpp2a/constexpr-virtual5.C: New test. > * g++.dg/cpp2a/constexpr-virtual6.C: New test. > * g++.dg/cpp2a/constexpr-virtual7.C: New test. > * g++.dg/cpp2a/constexpr-virtual8.C: New test. > * g++.dg/cpp2a/constexpr-virtual9.C: New test. > * g++.dg/diagnostic/virtual-constexpr.C: Skip for C++2a. Use > -pedantic-errors. Adjust dg-error. > > diff --git gcc/cp/call.c gcc/cp/call.c > index 69503ca7920..6c70874af40 100644 > --- gcc/cp/call.c > +++ gcc/cp/call.c > @@ -8401,7 +8401,8 @@ build_over_call (struct z_candidate *cand, int flags, tsubst_flags_t complain) > > if (DECL_VINDEX (fn) && (flags & LOOKUP_NONVIRTUAL) == 0 > /* Don't mess with virtual lookup in instantiate_non_dependent_expr; > - virtual functions can't be constexpr. */ > + virtual functions can't be constexpr. FIXME Actually, no longer > + true in C++2a. */ > && !in_template_function ()) I notice that removing the in_template_function check doesn't break template/virtual4.C nowadays. Does it break anything else? > + /* The C++ FE now produces indexed fields but we can index the array > + directly. */ > if (access_index < CONSTRUCTOR_NELTS (init)) > { > fn = CONSTRUCTOR_ELT (init, access_index)->value; > - gcc_checking_assert (!CONSTRUCTOR_ELT (init, access_index)->index); Rather than remove this assert, let's fix it to check that the ->index matches access_index. Jason