[cfe-dev] Recovering the spelling of a typedef

Reid Kleckner via cfe-dev cfe-dev at lists.llvm.org
Tue Jul 26 10:40:51 PDT 2016

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
| `-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>

> 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.
>>> _______________________________________________
>>> cfe-dev mailing list
>>> cfe-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
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