[cxx-abi-dev] Mangling of reference temporaries

[Richard Smith]

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.

Do you want someone to provide wording for the ABI document?
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