[cxx-abi-dev] mangling of vendor extension, expression on function (not function type)

[Nick Lewycky]

On 27 January 2014 15:27, John McCall <rjmccall at apple.com> wrote:

> On Jan 27, 2014, at 3:22 PM, Richard Smith <richardsmith at google.com>
> wrote:
>
> On 27 January 2014 15:08, John McCall <rjmccall at apple.com> wrote:
>
>> On Jan 27, 2014, at 12:26 PM, Richard Smith <richardsmith at google.com>
>> wrote:
>>
>> On 27 January 2014 11:57, John McCall <rjmccall at apple.com> wrote:
>>
>>> On Jan 24, 2014, at 7:30 PM, Nick Lewycky <nlewycky at google.com> wrote:
>>>
>>> On 24 January 2014 17:54, John McCall <rjmccall at apple.com> wrote:
>>>
>>>> On Jan 24, 2014, at 5:36 PM, Nick Lewycky <nlewycky at google.com> wrote:
>>>>
>>>> On 23 January 2014 11:52, John McCall <rjmccall at apple.com> wrote:
>>>>
>>>>> On Jan 21, 2014, at 2:01 PM, Richard Smith <richardsmith at google.com>
>>>>> wrote:
>>>>>
>>>>> On 21 January 2014 09:36, John McCall <rjmccall at apple.com> wrote:
>>>>>
>>>>>> On Jan 20, 2014, at 6:13 PM, Nick Lewycky <nlewycky at google.com>
>>>>>> wrote:
>>>>>> > I'm trying to mangle a vendor extension that encodes an expression
>>>>>> which applies to function overload candidates, with the goal that a user
>>>>>> running the demangler would see their expression demangled. While there are
>>>>>> various vendor extension points, none of them allow me to go into encoding
>>>>>> an expression, unless I stick a stray "decltype" in there, or similar
>>>>>> (expression in a template argument that doesn't actually exist, etc.).
>>>>>> >
>>>>>> > The vendor extension is described in full here:
>>>>>> http://clang.llvm.org/docs/LanguageExtensions.html#controlling-overload-resolution.
>>>>>> >
>>>>>> > I don't think I can do it without an extension to the mangling
>>>>>> rules. As a strawman proposal, I suggest this:
>>>>>> >
>>>>>> > <type> ::= Ue <expression> E # vendor extended type qualifier
>>>>>>
>>>>>> I think you mean
>>>>>>
>>>>>>   <type> ::= Ue <source-name> <expression> E <type>
>>>>>>
>>>>>> And this would be valuable for mangling e.g. dependent address space
>>>>>> qualifiers, if anybody ever wants to do those.
>>>>>>
>>>>> Yep, that's what I meant. Thanks!
>>>>
>>>>>  We could generalize this slightly to
>>>>>
>>>>>   <type> ::= U <source-name> <template-args> <type>
>>>>>
>>>>> to allow multiple arguments (as types or expressions), dependent pack
>>>>> expansions, and so on.
>>>>>
>>>>>
>>>>> That’s a bit more future-proof, I suppose, although I think it might
>>>>> stretch the definition of a type-qualifier to embed anything other than a
>>>>> value, and value pack-expansions are already part of the <expression>
>>>>> grammar.  You’re really asking for a “allow an arbitrarily complex type to
>>>>> be manufactured here” mangling.
>>>>>
>>>>> However, it feels really forced to add your feature, specifically, to
>>>>>> <type>, since it can’t appear in an arbitrary position; it’s much closer to
>>>>>> a qualified method name.  But the ref/cv-qualifiers area is only allowed in
>>>>>> a <nested-name>, and you need to be able to do this on a top-level
>>>>>> function, so I suppose extending <type> in a known-useful direction and
>>>>>> then abusing <type> might be the best thing.
>>>>>>
>>>>>> On the other hand, isn’t this a proposed direction for
>>>>>> standardization?  I would have no problem with giving this a proper,
>>>>>> non-vendor mangling just in case.
>>>>>
>>>>>
>>>>> It's not proposed for standardization with this syntax, and it's
>>>>> likely that the final semantics will differ from the Clang extension in
>>>>> some ways (the proposed partial ordering rules for constraints are rather
>>>>> more complex, for instance), but it's close enough that it's an option
>>>>> worth considering.
>>>>>
>>>>>
>>>>> Unless the feature is likely to diverge so much that it won’t even be
>>>>> an expression anymore, I don’t think this poses any problem for the ABI.
>>>>>
>>>>
>>>> Vendor hat on, I reserve the right to make my extension behave
>>>> differently from anything that's been standardized. As long as I can slip a
>>>> vendor extension particle into the mangled name I'll be happy to use
>>>> otherwise normal mangling. If it turns out I don't have to, all the better,
>>>> but I'm not banking on it.
>>>>
>>>>
>>>> I completely agree that this is acceptable vendor-hat behavior and that
>>>> the fake-qualifier idea isn’t a bad approach for it.
>>>>
>>>>  Do you want me to try to prepare a patch for template constraints? I
>>>> think it would look similar to my strawman proposal, except that I'd pick
>>>> some other available letter?
>>>>
>>>>
>>>> Yes, except that grammatically you should make it part of the function
>>>> <encoding> instead of adding it to <type>.  It works out to the same basic
>>>> position.
>>>>
>>>
>>> Okay, first attempt at a patch attached. Please review.
>>>
>>> A couple things. I chose 'Q', short for 'requires' to indicate a
>>> constraint. I put the new part on all encodings, not just functions,
>>> because you may need to mangle a static data member inside a class that has
>>> a concept applied, and similarly for its vtable and special members.
>>>
>>>
>>> I’m confused about what you mean by ‘concept’ here.  If it's just jargon
>>> for the enable_if feature, it seems completely counterproductive.
>>>
>>
>> This is jargon for the proposed constraints feature, not for enable_if.
>>
>>  We do not need to mangle enable_if conditions on class template
>>> patterns because of the ODR (and related rules in the templates chapter).
>>>
>>
>> We don't even support enable_if conditions on class templates (+
>> specializations of them).
>>
>>
>> Any particular reason?
>>
>
> This is a feature for guiding overload resolution, not for anything
> directly template-related. We'd need to invent different semantics for it
> if we wanted to support it on class templates and partial specializations.
>
>
> Okay.
>
> If you’ve got partial-ordering rules already, it seems like a very nice
>> way to significantly improve the expressiveness of class template partial
>> specialization.  Right now, users who want to specialize a template on e.g.
>> POD types have to factor that into the original template signature.
>>
>> enable_if has to be mangled for a function template because it’s part of
>>> the template signature.  You can have two function template declarations in
>>> the scope scope with the same name and with identical template parameter
>>> signatures and type signatures, but if they have different enable_if
>>> conditions, then they’re considered different templates.  In typical use,
>>> this isn’t actually important: two overlapping function templates are
>>> probably defined in the same header file, and it’s likely that every call
>>> site will see both of them, and so any given list of template arguments
>>> that doesn’t lead to an ambiguity will probably pick the same template
>>> every time.  But that’s not actually *required*: it’s totally fine to have
>>> two different function templates in two different translation units that
>>> both accept the same template arguments, and we need to distinguish them.
>>>
>>
>> It's more complex than that. enable_if can look at the values of function
>> arguments at a call site (if they are constant expressions), not just at
>> template arguments, so we need to mangle it into the signature for all
>> functions and function templates, even in the single-TU case.
>>
>>
>> What a majestic way of adding complexity without really making a
>> significant improvement to expressiveness.  Why not instead add a feature
>> to generally infer a non-type template parameter from a constant argument?
>>
>
> Primarily because it is intended to work in C as well as C++.
>
>
> Ah, yes, I keep forgetting that, sorry.
>

Going back to adding a proposal for mangling attribute enable_if without
concerning ourselves with concepts, I initially proposed:

<type> ::= Ue <expression> E # vendor extended type qualifier

which you corrected to:

<type> ::= Ue <source-name> <expression> E <type>

Richard dislikes this because we may be painting ourselves into a corner --
what about the next vendor expression that takes two expressions and a
type? -- and suggests this:

<type> ::= U <source-name> <template-args> <type>

which captures the full generality of things we might want to mangle. I
dislike the use of template-args to name something that isn't template
arguments, but that's an issue of naming only, and I don't have a better
name to offer.

Finally, you suggested putting it on <encoding> which would make it one of:

<encoding> ::= <function name> <bare-function-type> [Ue <template-args>]
or
<encoding> ::= <function name> <bare-function-type> [Ue <expression>]
?

I'm leaning towards the fully general form of:

<type> ::= U <source-name> <template-args> <type>

What do you think?
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