[cxx-abi-dev] Mangling of reference temporaries

[John McCall]

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.

Has the clang mangling actually been used in a released compiler, or did it just get implemented?

Hmm.  Putting a <number> after a <name> requires demangler lookahead, doesn’t it?

John.
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