On Tue, May 6, 2014 at 4:46 PM, David Majnemer <[email protected]> wrote:
> On Mon, May 5, 2014 at 1:36 PM, John McCall <[email protected]> wrote: > >> 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. >> > > 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? > > -- > David Majnemer >
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