> On Jan 9, 2015, at 5:14 PM, David Majnemer <[email protected]> wrote:
>
>
> On Tue, May 6, 2014 at 4:46 PM, David Majnemer <[email protected]
> <mailto:[email protected]>> wrote:
> On Mon, May 5, 2014 at 1:36 PM, John McCall <[email protected]
> <mailto:[email protected]>> wrote:
> On May 5, 2014, at 1:32 PM, Richard Smith <[email protected]
> <mailto:[email protected]>> wrote:
>> On 5 May 2014 12:10, John McCall <[email protected]
>> <mailto:[email protected]>> wrote:
>> On May 5, 2014, at 11:07 AM, Richard Smith <[email protected]
>> <mailto:[email protected]>> wrote:
>>> On 5 May 2014 10:14, John McCall <[email protected]
>>> <mailto:[email protected]>> wrote:
>>> On May 5, 2014, at 10:02 AM, Richard Smith <[email protected]
>>> <mailto:[email protected]>> wrote:
>>>> On 5 May 2014 09:13, John McCall <[email protected]
>>>> <mailto:[email protected]>> wrote:
>>>> On May 4, 2014, at 8:00 PM, David Majnemer <[email protected]
>>>> <mailto:[email protected]>> 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?
It should be there now. Sorry for the delay.
John.
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