From: Jason Merrill <jason@redhat.com>
To: Marek Polacek <polacek@redhat.com>
Cc: GCC Patches <gcc-patches@gcc.gnu.org>, Jakub Jelinek <jakub@redhat.com>
Subject: Re: C++ PATCH to implement P1064R0, Virtual Function Calls in Constant Expressions
Date: Fri, 14 Sep 2018 20:32:00 -0000 [thread overview]
Message-ID: <CADzB+2=emyy+pM1GjOTrGV3A6K+Lt5nVW=aKPQXQnNLE8+FjmQ@mail.gmail.com> (raw)
In-Reply-To: <20180914171950.GE5587@redhat.com>
On Fri, Sep 14, 2018 at 1:19 PM, Marek Polacek <polacek@redhat.com> wrote:
> This patch implements another bit of C++20, virtual calls in constant
> expression:
> <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1064r0.html>
> 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.
> 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...
Jason
next prev parent reply other threads:[~2018-09-14 20:31 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-09-14 17:21 Marek Polacek
2018-09-14 17:41 ` Jakub Jelinek
2018-09-14 19:43 ` C++ PATCH to implement P1064R0, Virtual Function Calls in Constant Expressions (v2) Marek Polacek
2018-09-14 20:32 ` Jason Merrill [this message]
2018-09-14 20:46 ` C++ PATCH to implement P1064R0, Virtual Function Calls in Constant Expressions Marek Polacek
2018-09-17 21:51 ` Marek Polacek
2018-09-18 3:48 ` Jason Merrill
2018-09-18 15:37 ` C++ PATCH to implement P1064R0, Virtual Function Calls in Constant Expressions (v4) Marek Polacek
2018-09-18 18:36 ` Jason Merrill
2018-09-18 18:58 ` Marek Polacek
2018-09-19 13:27 ` Andreas Schwab
2018-09-19 14:19 ` Marek Polacek
2018-09-19 15:10 ` Andreas Schwab
2018-09-19 15:11 ` Marek Polacek
2018-09-19 17:35 ` Jason Merrill
2018-09-20 8:26 ` Andreas Schwab
2018-09-20 9:23 ` Jakub Jelinek
2018-09-27 7:16 ` Jason Merrill
2018-09-27 23:18 ` Marek Polacek
2018-09-28 5:44 ` Jason Merrill
2018-09-28 6:48 ` Jakub Jelinek
2018-12-08 9:07 ` [C++ PATCH] FIx constexpr virtual function call handling on ia64 (PR c++/87861) Jakub Jelinek
2018-12-11 18:53 ` Jason Merrill
2018-10-08 14:18 ` C++ PATCH to implement P1064R0, Virtual Function Calls in Constant Expressions (v4) Andreas Schwab
2018-10-10 11:53 ` Jakub Jelinek
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