On Thu, Sep 18, 2014 at 1:05 PM, David Blaikie <dblai...@gmail.com> wrote:
> > > On Thu, Sep 18, 2014 at 1:00 PM, Robinson, Paul < > paul_robin...@playstation.sony.com> wrote: > >> >From: David Blaikie [mailto:dblai...@gmail.com] >> >On Wed, Sep 17, 2014 at 11:54 AM, Robinson, Paul < >> paul_robin...@playstation.sony.com> wrote: >> >> From: David Blaikie [mailto:dblai...@gmail.com] >> >> > On Wed, Sep 17, 2014 at 7:45 AM, Robinson, Paul < >> paul_robin...@playstation.sony.com> wrote: >> >> > > From: David Blaikie [mailto:dblai...@gmail.com] >> >> > > On Sat, Sep 13, 2014 at 6:54 PM, Robinson, Paul < >> paul_robin...@playstation.sony.com> wrote: >> >> > > > > > On 09 Sep 2014, at 00:01, jing...@apple.com wrote: >> >> > > > > > > >> >> > > > > > > From the debugger's standpoint, the functional concern is >> that if you do >> >> > > > > > something more real, like: >> >> > > > > > > >> >> > > > > > > typedef int A; >> >> > > > > > > template <typename T> >> >> > > > > > > struct S >> >> > > > > > > { >> >> > > > > > > T my_t; >> >> > > > > > > }; >> >> > > > > > > >> >> > > > > > > I want to make sure that the type of my_t is given as "A" >> not as "int". >> >> > > > > > The reason for that is that it is not uncommon to have data >> formatters >> >> > > > > > that trigger off the typedef name. This happens when you >> use some common >> >> > > > > > underlying type like "int" but the value has some special >> meaning when it >> >> > > > > > is formally an "A", and you want to use the data formatters >> to give it an >> >> > > > > > appropriate presentation. Since the data formatters work by >> matching type >> >> > > > > > name, starting from the most specific on down, it is >> important that the >> >> > > > > > typedef name be preserved. >> >> > > > > > > >> >> > > > > > > However, it would be really odd to see: >> >> > > > > > > >> >> > > > > > > (lldb) expr -T -- my_s >> >> > > > > > > (S<int>) $1 = { >> >> > > > > > > (A) my_t = 5 >> >> > > > > > > } >> >> > > > > > > >> >> > > > > > > instead of: >> >> > > > > > > >> >> > > > > > > (lldb) expr -T -- my_s >> >> > > > > > > (S<A>) $1 = { >> >> > > > > > > (A) my_t = 5 >> >> > > > > > > } >> >> > > > > > > >> >> > > > > > > so I am in favor of presenting the template parameter type >> with the most >> >> > > > > > specific name it was given in the overall template type name. >> >> > > > > > >> >> > > > > > OK, we get this wrong today. I’ll try to look into it. >> >> > > > > > >> >> > > > > > What’s your take on the debug info representation for the >> templated class >> >> > > > > > type? The tentative patch introduces a typedef that declares >> S<A> as a >> >> > > > > > typedef for S<int>. The typedef doesn’t exist in the code, >> thus I find it >> >> > > > > > a bit of a lie to the debugger. I was more in favour of >> something like : >> >> > > > > > >> >> > > > > > DW_TAG_variable >> >> > > > > > DW_AT_type: -> DW_TAG_structure_type >> >> > > > > > DW_AT_name: S<A> >> >> > > > > > DW_AT_specification: -> DW_TAG_structure_type >> >> > > > > > DW_AT_name: S<int> >> >> > > > > > >> >> > > > > > This way the canonical type is kept in the debug >> information, and the >> >> > > > > > declaration type is a real class type aliasing the canonical >> type. But I’m >> >> > > > > > not sure debuggers can digest this kind of aliasing. >> >> > > > > > >> >> > > > > > Fred >> >> > > > > >> >> > > > > Why introduce the extra typedef? S<A> should have a template >> parameter >> >> > > > > entry pointing to A which points to int. The info should all >> be there >> >> > > > > without any extra stuff. Or if you think something is >> missing, please >> >> > > > > provide a more complete example. >> >> > > > My immediate concern here would be either loss of information or >> bloat >> >> > > > when using that with type units (either bloat because each >> instantiation >> >> > > > with differently spelled (but identical) parameters is treated >> as a separate >> >> > > > type - or loss when the types are considered the same and all >> but one are >> >> > > > dropped at link time) >> >> > > You'll need to unpack that more because I'm not following the >> concern. >> >> > > If the typedefs are spelled differently, don't they count as >> different types? >> >> > > DWARF wants to describe the program as-written, and there's no >> S<int> written >> >> > > in the program. >> >> > > >> >> > > Maybe not in this TU, but possibly in another TU? Or by the user. >> >> > > >> >> > > void func(S<int>); >> >> > > ... >> >> > > typedef int A; >> >> > > S<A> s; >> >> > > func(s); // calls the same function >> >> > > >> >> > > The user probably wants to be able to call void func with S<int> >> or S<A> >> >> > Sure. >> >> > >> >> > > (and, actually, in theory, with S<B> where B is another typedef of >> int, but >> >> > > that'll /really/ require DWARF consumer support and/or new DWARF >> wording). >> >> > >> >> > Not DWARF wording. DWARF doesn't say when you can and can't call >> something; >> >> > that's a debugger feature and therefore a debugger decision. >> >> > >> >> What I mean is we'd need some new DWARF to help explain which types are >> >> equivalent (or the debugger would have to do a lot of spelunking to try >> >> to find structurally equivalent types - "S<B>" and "S<A>", go look >> through >> >> their DW_TAG_template_type_params, see if they are typedefs to the same >> >> underlying type, etc... ) >> >> > >> >> > >> >> > > We can't emit these as completely independent types - it would be >> verbose >> >> > > (every instantiation with different typedefs would be a whole >> separate type >> >> > > in the DWARF, not deduplicated by type units, etc) and wrong >> >> > >> >> > Yes, "typedef int A;" creates a synonym/alias not a new type, so >> S<A> and S<int> >> >> > describe the same type from the C++ perspective, so you don't want >> two complete >> >> > descriptions with different names, because that really would be >> describing them >> >> > as separate types. What wrinkles my brow is having S<int> be the >> "real" >> >> > description even though it isn't instantiated that way in the >> program. I wonder >> >> > if it should be marked artificial... but if you do instantiate >> S<int> in another >> >> > TU then you don't want that. Huh. It also seems weird to have this: >> >> > DW_TAG_typedef >> >> > DW_AT_name "S<A>" >> >> > DW_AT_type -> S<int> >> >> > but I seem to be coming around to thinking that's the most viable >> way to have >> >> > a single actual instantiated type, and still have the correct names >> of things >> >*mostly* correct; this still loses "A" as the type of the data member. >> > >> >For the DW_TAG_template_type_parameter, you mean? No, it wouldn't. >> > >> > (as a side note, if you do actually have a data member (or any other >> mention) of >> >the template parameter type, neither Clang nor GCC really get that >> 'right' - >> >"template<typename T> struct foo { T t; }; foo<int> f;" - in both Clang >> and GCC, >> >the type of the 't' member of foo<int> is a direct reference to the >> "int" DIE, not >> >to the DW_TAG_template_type_parameter for "T" -> int) >> >> Huh. And DWARF doesn't say you should point to the >> template_type_parameter... >> I thought it did, but no. Okay, so nothing is lost, but it feels >> desirable >> to me, that uses of the template parameter should cite it in the DWARF as >> well. >> But I guess we can leave that part of the debate for another time. >> >> > >> >Crud. >> >But I haven't come up with a way to get that back without basically >> instantiating >> >S<A> and S<int> separately. >> > >> >> > >> >> Yep - it's the only way I can think of giving this information in a >> way that's >> >> likely to work with existing consumers. It would probably be harmless >> to add >> >> DW_AT_artificial to the DW_TAG_typedef, if that's any help to any >> debug info >> >> consumer. >> > >> >Hmmm no, S<A> is not the artificial name; >> > >> >It's not the artificial name, but it is an artificial typedef. >> >> If the source only says S<A>, then the entire S<int> description is >> artificial, >> because *that's not what the user wrote*. So both the typedef and the >> class type >> are artificial. Gah. Let's forget artificial here. >> >> > >> >some debuggers treat DW_AT_artificial >> >as meaning "don't show this to the user." >> > >> >In some sense that's what I want - we never wrote the typedef in the >> source >> >so I wouldn't want to see it rendered in the "list of typedefs" (or even >> >probably in the list of types, maybe). >> > >> >But S<A> is the name we *do* want to >> >show to the user. >> > >> >Maybe. Sometimes. But there could be many such aliases for the type. (& >> many >> >more that were never written in the source code, but are still valid in >> the >> >source language (every other typedef of int, every other way to name the >> int >> >type (decltype, etc))) >> >> But you *lose* cases where the typedef is the *same* *everywhere*. And in >> many cases that typedef is a valuable thing, not the trivial rename we've >> been bandying about. This is a more real example: >> >> typedef int int4 __attribute__((ext_vector_type(4))); >> template<typename T> struct TypeTraits {}; >> template<> >> struct TypeTraits<int4> { >> static unsigned MysteryNumber; >> }; >> unsigned TypeTraits<int4>::MysteryNumber = 3U; >> >> Displaying "TypeTraits<int __attribute__((ext_vector_type(4)))>" is much >> worse than "TypeTraits<int4>" (and not just because it's shorter). >> More to the point, having the debugger *complain* when the user says >> something like "ptype TypeTraits<int4>" is a problem. >> >> Reducing debug-info size is a worthy goal, but don't degrade the debugging >> experience to get there. >> > > I'm not sure which part of what I've said seemed like a suggestion to > degrade the debugging experience to minimize debug info size (the > proposition that we should use a typedef or other alias on top of the > canonical type? It wouldn't cause "ptype TypeTraits<int4>" to complain - > indeed for GDB ptyping a typedef gives /exactly/ the same output as if you > ptype the underlying type - it doesn't even mention that there's a typedef > involved: > > typedef fooA foo<int>; > (keyboard shortcuts are hard - accidentally sent before I finished) (gdb) ptype fooA type = struct foo<int> [with T = int] { <no data fields> } But in any case, I think what I'm saying boils down to: Short of changing debug info consumers, I think the only thing we can do is DW_TAG_typedef. That'll work for existing consumers. Anything else will need possibly new DWARF wording, or at least an agreement between a variety of debug info consumers and producers that some new cliche/use of existing DWARF be used to describe these situations. I could be wrong - if someone wants to try prototyping the DW_TAG_structure_type proposal Fred had and see if existing debuggers work with that, sure. I'm not opposed to someone coming up with a standardizable more descriptive form than DW_TAG_typedef, but that conversation probably needs to happen with the DWARF Committee more than the LLVM community. - David > > > > >> --paulr >> >> > >> > >> >> That said, I'm not opposed to proposing something to DWARF to define >> some more >> >> 'proper' way to describe this. >> > >> >Yah. I've been thinking about the DW_AT_specification idea too, which >> would be >> >something like this: >> > DW_TAG_class_type >> > DW_AT_name "S<A>" >> > DW_AT_specification -> S<int> >> > >> > DW_TAG_template_type_parameter >> > DW_AT_name "T" >> > DW_AT_type -> A >> > >> >The problem with this is you don't know where T is used in the template, >> so >> >you *still* don't know when to use A as the type of "field". Also it's >> kind >> >of an abuse of DW_AT_specification. If we can't get A as the type of >> "field" >> >then the typedef is more straightforward and understandable. >> > >> >It's still a lot of DWARF to emit for every way the user has named the >> template >> >& I'm not sure how much value it provides - are there use cases you have >> in mind >> >that would benefit from the increased fidelity of knowing which template >> argument >> >corresponds to the way the user wrote the type. >> > >> > (& what would we emit if the user named the type in some other more >> exotic way: >> >int func(); template<typename T> struct S { }; ... S<decltype(func())> >> s; ) >> > >> > >> >Maybe I'll pop a note to the DWARF committee for a broader set of >> opinions. >> > >> >> >> >> One other open question is then, when, if ever, to reference the >> DW_TAG_typedef >> >> rather than the underlying type? Do we just reference it whenever the >> user >> >> writes using that name? >> >> >> >> void f(S<A>); >> >> ... >> >> void f(S<B>) { ... } >> >> >> >> etc... (this would be just as possible/we could maybe treat it the >> same as if >> >> the user wrote "void f(A); ... void f(B) { ... }") >> > >> >That's what I would do, and I think is more conformant to the DWARF spec. >> >--paulr >> > >> >> >> >> > (because DWARF is all about the name "as it appears in the source >> program.") >> >> > >> >> > > (the debugger wouldn't know these are actually the same type so >> wouldn't >> >> > > allow function calls, etc). >> >> > > >> >> > > - David >> >> > > >> >> > > > >> >> > > > >> >> > > > > Jim >> >> > > > > >> >> > > >> On Sep 8, 2014, at 12:38 PM, Frédéric Riss <fr...@apple.com> >> wrote: >> >> > > >> >> >> > > >> >> >> > > >>> On 08 Sep 2014, at 19:31, Greg Clayton <gclay...@apple.com> >> wrote: >> >> > > >>> >> >> > > >>> This means you will see "S<A>" as the type for your variables >> in the >> >> > > debugger when you view variables or children of >> structs/unions/classes. I >> >> > > think this is not what the user would want to see. I would rather >> see >> >> > > "S<int>" as the type for my variable than see "S<A>”. >> >> > > >> >> >> > > >> I find it more accurate for the debugger to report what has >> actually >> >> > > been put in the code. Moreover when a typedef is used, it’s >> usually to >> >> > > make things more readable not to hide information, thus I guess it >> would >> >> > > usually be as informative while being more compact. The debugger >> needs to >> >> > > have a way to describe the real type behind the abbreviated name >> though, >> >> > > we must not have less information compared to what we have today. >> >> > > >> >> >> > > >> Another point: this allows the debugger to know what S<A> >> actually is. >> >> > > Without it, the debugger only knows the canonical type. This means >> that >> >> > > currently you can’t copy/paste a piece of code that references >> that kind >> >> > > of template names and have it parse correctly. I /think/ that >> having this >> >> > > information in the debug info will allow more of this to work. >> >> > > >> >> >> > > >> But we can agree to disagree :-) It would be great to have more >> people >> >> > > chime and give their opinion. >> >> > > >> >> >> > > >> Fred >> >> > > >> >> >> > > >>>> On Sep 5, 2014, at 4:00 PM, Adrian Prantl <apra...@apple.com> >> wrote: >> >> > > >>>> >> >> > > >>>> >> >> > > >>>>> On Sep 5, 2014, at 3:49 PM, Eric Christopher < >> echri...@gmail.com> >> >> > > wrote: >> >> > > >>>>> >> >> > > >>>>> >> >> > > >>>>> >> >> > > >>>>> >> >> > > >>>>> On Fri, Sep 5, 2014 at 3:43 PM, Duncan P. N. Exon Smith >> >> > > <dexonsm...@apple.com> wrote: >> >> > > >>>>> >> >> > > >>>>>> On 2014 Sep 5, at 16:01, Frédéric Riss <fr...@apple.com> >> wrote: >> >> > > >>>>>> >> >> > > >>>>>> I couldn’t even find a subject expressing exactly what this >> patch >> >> > > is about… First of all, it’s meant to start a discussion, and I’m >> not >> >> > > proposing it for inclusion right now. >> >> > > >>>>>> >> >> > > >>>>>> The issue I’m trying to address is that template types are >> always >> >> > > canonicalized when emitted in the debug information (this is the >> desugar() >> >> > > call in UnwrapTypeForDebugInformation). >> >> > > >>>>>> >> >> > > >>>>>> This means that if the developer writes: >> >> > > >>>>>> >> >> > > >>>>>> typedef int A; >> >> > > >>>>>> template <typename T> >> >> > > >>>>>> struct S {}; >> >> > > >>>>>> >> >> > > >>>>>> S<A> var; >> >> > > >>>>>> >> >> > > >>>>>> The variable var will have type S<int> and not S<A>. In >> this simple >> >> > > example, it’s not that much of an issue, but for heavily templated >> code, >> >> > > the full expansion might be really different from the original >> >> > > declaration. >> >> > > >>>>>> >> >> > > >>>>>> The attached patch makes us emit an intermediate typedef >> for the >> >> > > variable’s type: >> >> > > >>>>>> >> >> > > >>>>>> 0x0000002a: DW_TAG_variable [2] >> >> > > >>>>>> DW_AT_name [DW_FORM_strp] ( >> >> > > .debug_str[0x00000032] = “var") >> >> > > >>>>>> DW_AT_type [DW_FORM_ref4] (cu + 0x0040 => >> >> > > {0x00000040}) >> >> > > >>>>>> DW_AT_external [DW_FORM_flag] (0x01) >> >> > > >>>>>> DW_AT_decl_file [DW_FORM_data1] (0x01) >> >> > > >>>>>> DW_AT_decl_line [DW_FORM_data1] (8) >> >> > > >>>>>> DW_AT_location [DW_FORM_block1] (<0x09> 03 70 >> 6c 00 00 >> >> > > 00 00 00 00 ) >> >> > > >>>>>> >> >> > > >>>>>> 0x00000040: DW_TAG_typedef [3] >> >> > > >>>>>> DW_AT_type [DW_FORM_ref4] (cu + 0x004b => >> >> > > {0x0000004b}) >> >> > > >>>>>> DW_AT_name [DW_FORM_strp] ( >> >> > >.debug_str[0x00000035] = “S<A>") >> >> > > >>>>>> DW_AT_decl_file [DW_FORM_data1] (0x01) >> >> > > >>>>>> DW_AT_decl_line [DW_FORM_data1] (6) >> >> > > >>>>>> >> >> > > >>>>>> 0x0000004b: DW_TAG_structure_type [4] * >> >> > > >>>>>> DW_AT_name [DW_FORM_strp] ( >> >> > >.debug_str[0x0000003e] = “S<int>") >> >> > > >>>>>> DW_AT_byte_size [DW_FORM_data1] (0x01) >> >> > > >>>>>> DW_AT_decl_file [DW_FORM_data1] (0x01) >> >> > > >>>>>> DW_AT_decl_line [DW_FORM_data1] (6) >> >> > > >>>>>> >> >> > > >>>>>> >> >> > > >>>>>> Which basically is what I want, although I don’t like that >> it >> >> > > introduces a typedef where there is none in the code. I’d prefer >> that to >> >> > > be: >> >> > > >>>>>> >> >> > > >>>>>> DW_TAG_variable >> >> > > >>>>>> DW_AT_type: -> DW_TAG_structure_type >> >> > > >>>>>> DW_AT_name: S<A> >> >> > > >>>>>> DW_AT_specification: -> DW_TAG_structure_type >> >> > > >>>>>> DW_AT_name: S<int> >> >> > > >>>>>> … >> >> > > >>>>>> >> >> > > >>>>>> The patch also has the nice property of omitting the >> defaulted >> >> > > template arguments in the first level typedef. For example you get >> >> > > vector<A> instead of vector<int, std::__1::allocator<int> >. >> >> > > >>>>> >> >> > > >>>>> If you specify `vector<int>` in C++ do you get that instead >> of >> >> > > >>>>> `vector<int, std::__1::allocator<int>>`? >> >> > > >>>>> >> >> > > >>>>> Yeah, I mentioned this as possibly causing problems with >> debuggers >> >> > > or other consumers, but I don't have any proof past "ooooo scary!”. >> >> > > >>>> >> >> > > >>>> Well, [+lldb-dev], could this confuse debuggers? :-) >> >> > > >>>> >> >> > > >>>> -- adrian >> >> > > >>>>> >> >> > > >>>>> That said, I like the idea personally :) >> >> > > >>>>> >> >> > > >>>>> -eric >> >> > > >>>>> >> >> > > >>>>> >> >> > > >>>>>> Now there is one thing I really don’t like about the patch. >> In >> >> > > order not to emit typedefs for types that don’t need it, I use >> string >> >> > > comparison between the desugared and the original type. I haven’t >> >> > > quantified anything, but doing the construction of the type name >> for every >> >> > > template type and then comparing it to decide to use it or not >> seems like >> >> > > a big waste. >> >> > > >>>>> >> >> > > >>>>> Maybe someone on cfe-dev knows a better way. >> >> > > >>>>> >> >> > > >>>>>> >> >> > > >>>>>> Thoughts? >> >> > > >>>>>> >> >> > > >>>>>> <template-arg-typedefs.diff> >> >> > > >>>>>> >> >> > > >>>>>> Fred >> >> > > >>>>>> >> >> > > >>>>>> >> >> > > >>>>>> >> >> > > >>>>>> >> >> > > >>>>>> >> >> > > >>>>>> _______________________________________________ >> >> > > >>>>>> llvm-commits mailing list >> >> > > >>>>>> llvm-comm...@cs.uiuc.edu >> >> > > >>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits >> >> > > >>>>> >> >> > > >>>>> >> >> > > >>>> >> >> > > >>>> _______________________________________________ >> >> > > >>>> lldb-dev mailing list >> >> > > >>>> lldb-dev@cs.uiuc.edu >> >> > > >>>> http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev >> >> > > >>> >> >> > > >> >> >> > > >> >> >> > > >> _______________________________________________ >> >> > > >> lldb-dev mailing list >> >> > > >> lldb-dev@cs.uiuc.edu >> >> > > >> http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev >> >> > > > >> >> > > >> >> > > >> >> > > _______________________________________________ >> >> > > llvm-commits mailing list >> >> > > llvm-comm...@cs.uiuc.edu >> >> > > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits >> >> > >> >> > _______________________________________________ >> >> > llvm-commits mailing list >> >> > llvm-comm...@cs.uiuc.edu >> >> > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits >> >> >
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