[cxx-abi-dev] Mangling of reference temporaries

[David Majnemer]

On Tue, May 6, 2014 at 4:46 PM, David Majnemer <david.majnemer at gmail.com>
wrote:

> On Mon, May 5, 2014 at 1:36 PM, John McCall <rjmccall at apple.com> wrote:
>
>> On May 5, 2014, at 1:32 PM, Richard Smith <richardsmith at google.com>
>> wrote:
>>
>> On 5 May 2014 12:10, John McCall <rjmccall at apple.com> wrote:
>>
>>> On May 5, 2014, at 11:07 AM, Richard Smith <richardsmith at google.com>
>>> wrote:
>>>
>>> On 5 May 2014 10:14, John McCall <rjmccall at apple.com> wrote:
>>>
>>>> On May 5, 2014, at 10:02 AM, Richard Smith <richardsmith at google.com>
>>>> wrote:
>>>>
>>>> On 5 May 2014 09:13, John McCall <rjmccall at apple.com> wrote:
>>>>
>>>>> On May 4, 2014, at 8:00 PM, David Majnemer <david.majnemer at gmail.com>
>>>>> wrote:
>>>>> > The Itanium ABI does not seem to provide a mangling for reference
>>>>> temporaries.
>>>>> >
>>>>> > Consider the following:
>>>>> > struct A { const int (&x)[3]; };
>>>>> > struct B { const A (&x)[2]; };
>>>>> > template <typename T> B &&b = { { { { 1, 2, 3 } }, { { 4, 5, 6 } } }
>>>>> };
>>>>> > B &temp = b<void>;
>>>>> >
>>>>> > The temporaries created by instantiating b<void> must be the same in
>>>>> all translation units.
>>>>> >
>>>>> > To satisfy this requirement, I propose that we mangle the
>>>>> temporaries in lexical order using a mangling similar to what GCC 4.9 uses
>>>>> and identical to what trunk clang uses.
>>>>>
>>>>> What does GCC do?
>>>>
>>>>
>>>> GCC trunk seems to use
>>>>
>>>>   <special-name> ::= GR <object name> <nonnegative number>
>>>>
>>>> where the first reference temporary gets number 0, and so on. It
>>>> appears to number them through a post-order tree walk of the expression.
>>>> Older versions of GCC did not add a number, IIRC.
>>>>
>>>>
>>>> Okay.  So we have two different manglings out there that both look
>>>> basically the same except for an off-by-one and a major semantic ordering
>>>> difference.  I think we should either standardize on one or the other or
>>>> switch to a different prefix entirely.
>>>>
>>>
>>> Looking at the GCC output again, I see:
>>>   * GCC actually does seem to be using lexical order (of the start of
>>> the expression) after all (at least in the std::initializer_list array
>>> temporary case).
>>>   * GCC emits these symbols with internal linkage.
>>>
>>> So I don't think there's any compatibility problem with GCC.
>>>
>>>
>>> Okay.
>>>
>>> Has the clang mangling actually been used in a released compiler, or did
>>>> it just get implemented?
>>>>
>>>
>>> Sort of? Until very recently, Clang used the same mangling for all the
>>> temporaries, and added numbers to disambiguate, so we got the current
>>> proposal by accident (except the numbering starts from 1 instead of from 0)
>>> -- at least, in some cases: Clang would number the temporaries in a
>>> different order if they were initialized by constant expressions (because
>>> it happened to emit them in a different order).
>>>
>>>
>>> Yeah, we don’t need to work to maintain compatibility with that.
>>>
>>> Hmm.  Putting a <number> after a <name> requires demangler lookahead,
>>>> doesn’t it?
>>>>
>>>
>>> <name> is self-delimiting, so a demangler can walk over it, then read
>>> digits until it sees a non-digit or end-of-mangled-name. (<encoding>s are
>>> only nested if they appear within a <local-name>, which has a terminating
>>> E.) Not sure if that addresses your concern, though.
>>>
>>>
>>> Ah, right, I was thinking of <encoding>.
>>>
>>> Let’s just follow the example of <susbtitution>, which is basically what
>>> you’re proposing except a <seq-id> instead of a <number> and always
>>> followed by a _.
>>>
>>
>> Compared to the previous proposal (without the _), that's an ABI break
>> for Clang in the overwhelmingly common case where a declaration
>> lifetime-extends a single temporary, but I can live with it.
>>
>>
>> Yeah, I’m comfortable with this.
>>
>> Do you want someone to provide wording for the ABI document?
>>
>>
>> Sure, might as well re-submit the proposal.  It would be nice to get some
>> feedback from someone not working on Clang, however.
>>
>
> To implement support for mangling reference temporaries:
>
> 1. An additional <special-name> non-terminal production should be added:
>
> <special-name> ::= GR <object name> [ <seq-id> ] _   # Reference
> temporaries
>
> The <seq-id> is strictly the lexical order in which the reference
> temporary was written in the source.
>
> The following exists as a practical example:
>
> _ZGR1bIvE_ would be given to the 'B' object that 't' would refer to.
> _ZGR1bIvE0_ would be given to the array of 'A' object references
> _ZGR1bIvE1_ would be given to the object containing the first array of
> ints, {1, 2, 3}
> _ZGR1bIvE2_ would be given to the object containing the second array of
> ints, {4, 5, 6}
>
> 2. The text describing <seq-id> should probably refrain from
> mentioning substitutable entities.
>

Does anything else need to happen to get this added to the ABI document?


>
> --
> David Majnemer
>
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