This is the mail archive of the
gcc-patches@gcc.gnu.org
mailing list for the GCC project.
Re: [C++ patch] Set attributes for C++ runtime library calls
- From: Mike Stump <mikestump at comcast dot net>
- To: Gabriel Dos Reis <gdr at integrable-solutions dot net>
- Cc: Jan Hubicka <hubicka at ucw dot cz>, Alexander Monakov <amonakov at ispras dot ru>, "gcc-patches at gcc dot gnu dot org" <gcc-patches at gcc dot gnu dot org>, Jason Merrill <jason at redhat dot com>
- Date: Thu, 22 Aug 2013 18:51:18 -0700
- Subject: Re: [C++ patch] Set attributes for C++ runtime library calls
- References: <20130822131927 dot GA18084 at kam dot mff dot cuni dot cz> <CAAiZkiDRZj-Fzy2+zUo9Z2B5ShvJ6K_duNyX1SKfrEZeX1NNZQ at mail dot gmail dot com> <alpine dot LNX dot 2 dot 00 dot 1308221836460 dot 30125 at monopod dot intra dot ispras dot ru> <20130822152111 dot GB19256 at kam dot mff dot cuni dot cz> <CAAiZkiA5wyTn0A_eMZ8d-crnq0KY0ut6R1ffh+2xsTp51dkWEg at mail dot gmail dot com> <20130822153958 dot GE19256 at kam dot mff dot cuni dot cz> <CAAiZkiCWf43bVyuVJC+D1=ghSLGTXsi5FbWVdUwX1kq4cNvavg at mail dot gmail dot com> <20130822161644 dot GC24022 at kam dot mff dot cuni dot cz> <CAAiZkiC7tztfKcwET95cacX52=xdRf+QuuUEUfOXTF+9QD69DA at mail dot gmail dot com> <960F238C-2F13-43A6-BE3E-65A917278ADA at comcast dot net> <CAAiZkiA-ZtmBTtgiijXuXkvY0x+M=9nat1TXt=vv86gtBun46w at mail dot gmail dot com> <C8BBC63A-ACAA-41CF-8EDD-7C31BA153314 at comcast dot net> <CAAiZkiCD1Dd7b7brFFK55mB8UjHziyq=00d4BBAzhP2rA70vfQ at mail dot gmail dot com>
On Aug 22, 2013, at 6:10 PM, Gabriel Dos Reis <gdr@integrable-solutions.net> wrote:
> I think we must distinguish what is "wrong" according to the standards
> we are implementing from what is "wrong" from a QoI point of view.
Not if they match, we don't.
> My reasoning (for C++98, but the same is true for C++11) is based
> on 3.8/1:
> […]
> The lifetime of an object of type T ends when:
> -- if T is a class type with a non-trivial destructor (12.4),
> the destructor call starts, or
> -- the storage which the object occupies is reused or released.
>
> Doing a placement-new on the storage occupied by 'a' is reusing
> its storage, therefore ending its lifetime.
The problem is that reused is not well defined, so, we are into the weeds right there.
int i, j;
int *ip = &i;
i = 1;
j = 2;
*ip = j;
++i;
++j;
here, we sees the storage being reused in the *ip = j; statement, or, is it merely changing the value? And what if we do a memcpy (ip, &j, sizeof (int)); Is that reused, or merely changing the value. I think the most logical line of reasoning is that when the requirements of [basic.lval] are met, the, this is a change of value of an object, not a modification to it's lifetime. So, in the case quoted, since the type of the accesses are both int, we don't reuse the storage, since the requirements of [basic.lval] are met. Indeed, the programmer expects that they can access i after *ip = j; and that the _value_ that object, while changed from the original 1, will be 2 just after the *ip = j; statement.
Since we know that i must be 3 at the end, we then know what the wording, reused, must mean, cause other meanings that could possibly make it work for you in the case you are considering, would destroy this property of pointers, and everyone knows the semantics of pointers, they are undisputed. Or put another way, you cannot misread reused in this way.