On May 5, 2014, at 1:32 PM, Richard Smith <[email protected]> wrote:
> On 5 May 2014 12:10, John McCall <[email protected]> wrote:
> On May 5, 2014, at 11:07 AM, Richard Smith <[email protected]> wrote:
>> On 5 May 2014 10:14, John McCall <[email protected]> wrote:
>> On May 5, 2014, at 10:02 AM, Richard Smith <[email protected]> wrote:
>>> On 5 May 2014 09:13, John McCall <[email protected]> wrote:
>>> On May 4, 2014, at 8:00 PM, David Majnemer <[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.
John.
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