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 _.
John.
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