From: Jason Merrill <jason@redhat.com>
To: Martin Sebor <msebor@gmail.com>,
Jonathan Wakely <jwakely@redhat.com>,
Richard Biener <richard.guenther@gmail.com>
Cc: gcc-patches <gcc-patches@gcc.gnu.org>
Subject: Re: [PING][PATCH] define auto_vec copy ctor and assignment (PR 90904)
Date: Tue, 13 Jul 2021 23:39:35 -0400 [thread overview]
Message-ID: <2c60a7d0-3f60-b72f-c0f2-6fc7a4900740@redhat.com> (raw)
In-Reply-To: <7ebb37f3-76c3-8e00-7852-c93bf142043a@gmail.com>
On 7/13/21 4:02 PM, Martin Sebor wrote:
> On 7/13/21 12:37 PM, Jason Merrill wrote:
>> On 7/13/21 10:08 AM, Jonathan Wakely wrote:
>>> On Mon, 12 Jul 2021 at 12:02, Richard Biener wrote:
>>>> Somebody with more C++ knowledge than me needs to approve the
>>>> vec.h changes - I don't feel competent to assess all effects of the
>>>> change.
>>>
>>> They look OK to me except for:
>>>
>>> -extern vnull vNULL;
>>> +static constexpr vnull vNULL{ };
>>>
>>> Making vNULL have static linkage can make it an ODR violation to use
>>> vNULL in templates and inline functions, because different
>>> instantiations will refer to a different "vNULL" in each translation
>>> unit.
>>
>> The ODR says this is OK because it's a literal constant with the same
>> value (6.2/12.2.1).
>>
>> But it would be better without the explicit 'static'; then in C++17
>> it's implicitly inline instead of static.
>
> I'll remove the static.
>
>>
>> But then, do we really want to keep vNULL at all? It's a weird
>> blurring of the object/pointer boundary that is also dependent on vec
>> being a thin wrapper around a pointer. In almost all cases it can be
>> replaced with {}; one exception is == comparison, where it seems to be
>> testing that the embedded pointer is null, which is a weird thing to
>> want to test.
>
> The one use case I know of for vNULL where I can't think of
> an equally good substitute is in passing a vec as an argument by
> value. The only way to do that that I can think of is to name
> the full vec type (i.e., the specialization) which is more typing
> and less generic than vNULL. I don't use vNULL myself so I wouldn't
> miss this trick if it were to be removed but others might feel
> differently.
In C++11, it can be replaced by {} in that context as well.
> If not, I'm all for getting rid of vNULL but with over 350 uses
> of it left, unless there's some clever trick to make the removal
> (mostly) effortless and seamless, I'd much rather do it independently
> of this initial change. I also don't know if I can commit to making
> all this cleanup.
I already have a patch to replace all but one use of vNULL, but I'll
hold off with it until after your patch.
>> Somewhat relatedly, use of vec<T> variables or fields seems almost
>> always a mistake, as they need explicit .release() that could be
>> automatic with auto_vec, and is easy to forget. For instance, the
>> recursive call in get_all_loop_exits returns a vec that is never
>> released. And I see a couple of leaks in the C++ front end as well.
>
> I agree. The challenge I ran into with changing vec fields is with
> code that uses the vec member as a reference to auto_vec. This is
> the case in gcc/ipa-prop.h, for instance. Those instances could
> be changed to auto_vec references or pointers but again it's a more
> intrusive change than the simple replacements I was planning to make
> in this first iteration.
>
> So in summary, I agree with the changes you suggest. Given their
> scope I'd prefer not to make them in the same patch, and rather make
> them at some point in the future when I or someone else has the time
> and energy. I'm running out.
Oh, absolutely.
Jason
next prev parent reply other threads:[~2021-07-14 3:39 UTC|newest]
Thread overview: 59+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-26 23:30 [PATCH] " Martin Sebor
2021-04-27 7:58 ` Richard Biener
2021-04-27 13:58 ` Martin Sebor
2021-04-27 14:04 ` Richard Biener
2021-04-27 15:52 ` Martin Sebor
2021-05-03 21:50 ` [PING][PATCH] " Martin Sebor
2021-05-11 20:02 ` [PING 2][PATCH] " Martin Sebor
2021-05-27 19:33 ` [PING 3][PATCH] " Martin Sebor
2021-05-27 20:53 ` [PATCH] " Jason Merrill
2021-06-01 19:56 ` Martin Sebor
2021-06-01 21:38 ` Jason Merrill
2021-06-25 20:51 ` Martin Sebor
2021-06-25 22:11 ` Jason Merrill
2021-06-25 22:36 ` Martin Sebor
2021-06-28 8:07 ` Richard Biener
2021-06-28 18:07 ` Martin Sebor
2021-06-29 10:58 ` Richard Biener
2021-06-29 11:34 ` Martin Jambor
2021-06-30 1:46 ` Martin Sebor
2021-06-30 8:48 ` Richard Biener
2021-06-30 9:29 ` Martin Jambor
2021-07-06 15:06 ` [PING][PATCH] " Martin Sebor
2021-07-07 7:28 ` Richard Biener
2021-07-07 14:37 ` Martin Sebor
2021-07-12 11:02 ` Richard Biener
2021-07-13 14:08 ` Jonathan Wakely
2021-07-13 18:37 ` Jason Merrill
2021-07-13 20:02 ` Martin Sebor
2021-07-14 3:39 ` Jason Merrill [this message]
2021-07-14 10:47 ` Jonathan Wakely
2021-07-14 14:46 ` Martin Sebor
2021-07-14 16:23 ` Jason Merrill
2021-07-20 18:34 ` Martin Sebor
2021-07-20 20:08 ` Jason Merrill
2021-07-20 21:52 ` Martin Sebor
2021-07-27 18:56 ` Martin Sebor
2021-07-30 15:06 ` Jason Merrill
2021-08-06 2:07 ` Martin Sebor
2021-08-06 7:52 ` Christophe Lyon
2021-08-06 12:17 ` Christophe Lyon
2021-07-14 14:44 ` Martin Sebor
2021-06-29 14:43 ` [PATCH] " Jason Merrill
2021-06-29 17:18 ` Martin Sebor
2021-06-30 8:40 ` Richard Biener
2021-06-30 9:00 ` Richard Sandiford
2021-06-30 12:01 ` Richard Biener
2021-06-28 8:05 ` Richard Biener
2021-06-29 12:30 ` Trevor Saunders
2021-06-02 6:55 ` Richard Biener
2021-06-02 16:04 ` Martin Sebor
2021-06-03 8:29 ` Trevor Saunders
2021-06-07 8:51 ` Richard Biener
2021-06-07 10:33 ` Trevor Saunders
2021-06-07 13:33 ` Richard Biener
2021-06-07 20:34 ` Martin Sebor
2021-06-08 3:26 ` Trevor Saunders
2021-06-08 7:19 ` Richard Biener
2021-06-07 22:17 ` Martin Sebor
2021-06-08 2:41 ` Trevor Saunders
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