[cfe-dev] Recovering the spelling of a typedef

[Keno Fischer via cfe-dev]

Yes, in the very simple cases, no patch is needed, but yes, ROOT needs
to be able to look through templates which is where the problem comes
in.

On Tue, Jul 26, 2016 at 1:40 PM, Reid Kleckner <rnk at google.com> wrote:
> In simple cases, this information is already available as type sugar nodes.
> Consider this AST dump:
>
> typedef double Double32_t;
> struct Foo { Double32_t f; };
>
> |-TypedefDecl 0xd3af50 <t.cpp:1:1, col:16> col:16 referenced Double32_t
> 'double'
> | `-BuiltinType 0xd09d50 'double'
> `-CXXRecordDecl 0xd3afa0 <line:2:1, col:28> col:8 struct Foo definition
>   |-CXXRecordDecl 0xd3b0c0 <col:1, col:8> col:8 implicit struct Foo
>   `-FieldDecl 0xd3b190 <col:14, col:25> col:25 f 'Double32_t':'double'
>
> Template instantiation uses the canonical, desugared types, though. You can
> see it from this dump:
>
> typedef double Double32_t;
> template <typename T> struct Bar { T f; };
> template struct Bar<Double32_t>;
>
> `-ClassTemplateSpecializationDecl 0xc3b490 <line:3:1, col:31> col:17 struct
> Bar definition
>   |-TemplateArgument type 'double'
>   |-CXXRecordDecl 0xc3b688 prev 0xc3b490 <line:2:23, col:30> col:30 implicit
> struct Bar
>   `-FieldDecl 0xc3b758 <col:36, col:38> col:38 f 'double':'double'
>
> Does ROOT need a way to push the type sugar nodes through template
> instantiation? I seem to recall that there are reasons why it's hard to do
> that from an implementation standpoint, but it would also help us get better
> diagnostics when rinsing "std::string" through a template type parameter,
> for example.
>
> On Tue, Jul 26, 2016 at 9:58 AM, Keno Fischer <kfischer at college.harvard.edu>
> wrote:
>>
>> Yes, precisely. I am not fully versed in the details (Axel, Philippe,
>> please correct any inaccuracies), but essentially you can request an object
>> to be written to/ read from disk and ROOT will look up the corresponding
>> class and compute the appropriate disk format (for which it needs to
>> distinguish between double/Double32_t for any members). ROOT use a C++
>> Interpreter/JIT (custom one for a very long time, transitioning to
>> LLVM/Clang) for interactivity and introspection, so it has the ASTs for all
>> classes in the system available.
>>
>> On Tue, Jul 26, 2016 at 12:36 PM, Reid Kleckner <rnk at google.com> wrote:
>>>
>>> Can you elaborate on how this typedef information is used for I/O? Do you
>>> mean that it is used by some clang plugin that examines the AST, or
>>> something else?
>>>
>>> On Mon, Jul 25, 2016 at 6:55 PM, Keno Fischer via cfe-dev
>>> <cfe-dev at lists.llvm.org> wrote:
>>>>
>>>> Hi Everyone,
>>>>
>>>> We're trying to integrate the CERN ROOT framework with Julia, both
>>>> of which use LLVM/Clang for C++ interoperability. As such, we're hoping
>>>> to harmonize the versions of clang used in both projects. One major
>>>> obstacle to this currently is a patch that the ROOT folks are carrying
>>>> to support their I/O system which uses the structure of C++ classes to
>>>> determine the on-disk format. The patch as is isn't really in a form
>>>> that could be submitted upstream, but we're hoping to solicit some
>>>> advice
>>>> to come up with a solution that would be acceptable to clang, and not
>>>> require any local code patches.
>>>>
>>>> With that in mind, let us describe the problem:
>>>>
>>>> As mentioned, ROOT uses the structure of C++ classes to determine it's
>>>> IO format. The one wrinkle to that is that sometimes the I/O storage
>>>> format and the in-memory format are not exactly the same. In particular,
>>>> ROOT has a
>>>>
>>>> typedef double Double32_t;
>>>>
>>>> where if this typedef appears in a struct that is serialized to disk,
>>>> it indicates that it should be stored with 32bit precision on disk, but
>>>> with 64bit precision in memory.
>>>>
>>>> That's *only* for I/O information; for anything regarding symbols we
>>>> need these two to share their instantiation data.
>>>>
>>>> I.e. we want to distinguish the types of D<double>::m and
>>>> D<Double32_t>::m (and also D<vector<Double32_t>>::m and
>>>> D<vector<double>>::m) in
>>>>
>>>> template <class T>
>>>> struct D {
>>>> using type = std::remove_reference<D>;
>>>> T m;
>>>> static int s;
>>>> };
>>>>
>>>> But &D<double>::s must the the same as D<Double32_t>::s; more
>>>> importantly:
>>>>
>>>> void f(D<double>);
>>>>
>>>> must be called by f(D<Double32_t>{}). That is (IIRC) in contrast of what
>>>> the C++ committee discussed for strong typedefs.
>>>>
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>>>>
>>>
>>
>