[lldb-dev] [RFC][PATCH] Keep un-canonicalized template types in the debug information
Eric Christopher
echristo at gmail.com
Tue Sep 23 15:20:30 PDT 2014
>>
>> In a way it feels somewhat analogous to choices in supporting
>> extensions/dialects of C++. For practical purposes it's very worthwhile to
>> the community to support things that GCC supports, but that doesn't mean
>> that GCC defines the standard. In the case at hand, Clang has strayed from
>> the letter of the DWARF spec, and we'd really like to see a way back toward
>> it.
>
>
> The DWARF spec doesn't really describe the world of templates in a complete
> and useful manner. I think it's problematic to try to wedge the wording into
> saying "DWARF says this is the one way to encode this info" - DWARF makes
> some general suggestions about how certain constructs could be mapped, but
> until there's a document like the C++ ABI that says "this is the required
> lowering from C++ to DWARF" (and there's buy-in to conform to this from both
> DWARF producers and consumers) a lot of this is going to come down to "what
> do consumers and producers agree to".
>
Not that Dave needs me to echo/upvote his comments, but this.
-eric
>>
>> We're entirely willing to do work toward getting things realigned
>> (admittedly I personally have been mostly MIA for the past year, but I am
>> seeing an occasional photon from down the far end of my current tunnel)
>> given that the primary contributor and code owner are willing to go along
>> with it.
>>
>> Thanks,
>>
>> --paulr
>>
>>
>>
>> From: David Blaikie [mailto:dblaikie at gmail.com]
>> Sent: Monday, September 22, 2014 3:47 PM
>> To: Nick Lewycky
>> Cc: Robinson, Paul; lldb-dev at cs.uiuc.edu; llvm-commits at cs.uiuc.edu
>> Subject: Re: [lldb-dev] [RFC][PATCH] Keep un-canonicalized template types
>> in the debug information
>>
>>
>>
>>
>>
>>
>>
>> On Mon, Sep 22, 2014 at 3:40 PM, Nick Lewycky <nicholas at mxc.ca> wrote:
>>
>> Robinson, Paul wrote:
>>
>> I think it comes down to how the information is planning to be used. A
>> consumer with the dwarf information today could, in fact, get to the
>> S<int> type from a user who types S<A> pretty easily right?
>>
>> If the typedef actually appears in the DWARF, the consumer could figure
>> out what the user meant by typing S<A>, yes. In my experiments the
>> typedef is not always present, which leaves the user up a creek with no
>> paddle.
>>
>> How the debugger presents the types of things is also a consideration,
>> however. This is more evident with a less trivial example, such as the
>> vector typedef I described previously. It is clearly a step backward in
>> the end-user debugging experience if people are used to seeing
>>
>> S<int4>
>>
>> which the debugger has been displaying all along, but suddenly they
>> start seeing instead
>>
>> S<int __attribute__((ext_vector_type(4)))>
>>
>> which is what has started happening. Especially if 'int4' no longer
>> appears as a typedef at all, this is Just Wrong.
>>
>>
>> In clang, ConvertTypeToDiagnosticString deals with vectors specially. The
>> rationale, I think, is to prevent the compiler from showing the internal
>> implementation detail of how float4 and friends are defined. I think that
>> this is the wrong approach and would have preferred a second attribute. Does
>> attribute nodebug on a typedef have any meaning yet? Could we repurpose it
>> to mean that you shouldn't look through this typedef for compiler
>> diagnostics nor debug info? Any any case, our behaviour on diagnostics and
>> debug info should probably match here.
>>
>>
>>
>> One of the issues is that there's only /so/ different we can be from GCC
>> here before types/declarations/definitions won't match up in GDB. I believe
>> GCC has some smarts to tolerate differences like S<0> versus S<0u> or
>> S<'\0'> I think... at least some of those, but I don't know how it'll go
>> with:
>>
>> S<__attribute__((__vector_size__(4 * sizeof(int)))) int>
>>
>> V
>>
>> S<__vector(4) int>
>>
>> (using a GCC-compatible syntax, vector_size(sizeof(int) * 4) rather than
>> the ext_vector_type which isn't supported by GCC)
>>
>> Huh... apparently GDB ignores the entire adornment and allows
>> func(S<__vector(4) int>) to be called with a variable of type
>> S<__attribute__((__vector_size__(5 * sizeof(int)))) int> even... not sure
>> what to make of any of that.
>>
>>
>>
>>
>> Nick
>>
>> Wolfgang did some bisection and traced this change to r205447, and the
>> intent of that change was centered on default template arguments. This
>> de-referencing of typedefs appears to have been an *unintended side
>> effect* of that patch.
>>
>> I want my typedef'd template parameters back please…
>>
>> --paulr
>>
>> *From:*Eric Christopher [mailto:echristo at gmail.com]
>> *Sent:* Thursday, September 18, 2014 10:07 PM
>> *To:* Robinson, Paul
>> *Cc:* David Blaikie; lldb-dev at cs.uiuc.edu; llvm-commits at cs.uiuc.edu;
>> Frédéric Riss
>> *Subject:* Re: [lldb-dev] [RFC][PATCH] Keep un-canonicalized template
>> types in the debug information
>>
>> On Thu, Sep 18, 2014 at 5:57 PM, Robinson, Paul
>> <Paul_Robinson at playstation.sony.com
>>
>> <mailto:Paul_Robinson at playstation.sony.com>> wrote:
>>
>> The (limited) feedback I've had from the committee is along these lines.
>>
>> If the program uses the type name "S<A>" for something, the DWARF
>> should fully describe the type named "S<A>" because that's the name
>> as-in-the-source-program. If you use both S<A> and S<int> in the
>> program in different places, then you need to describe both in the
>> DWARF. There is sadly no standard way to associate the two as
>> aliases. Yes in C++ they are the same; in standard DWARF they are not.
>>
>> Yeah, I'm not sure I agree with this. I've seen the thread and I'm not
>> sure I like the logic.
>>
>> The typedef S<A> => S<int> hack might work [if the debugger can
>> tolerate that]. It is obviously not a real typedef. You could mark
>> it artificial as an indication that something funny is going on
>> (artificial typedefs being highly atypical).
>>
>> The DW_AT_specification hack is just wrong, because neither S<A> nor
>> S<int> is completing the other.
>>
>> I need to step back from the typedef hack. I believe our debugger
>> throws away the <brackets> on the theory that it can reconstruct
>> them from template-parameter children; that is, the <bracket> part
>> of the name is redundant. The typedef hack does not provide those
>> children, and the <brackets> are not redundant, so this is likely to
>> be a problem for us. Feh. I'd forgotten about that detail when I
>> started liking the typedef hack. Yes, this means I don't have a
>> suggestion, apart from emitting things redundantly as needed to
>> preserve as-in-the-source-program.
>>
>> Here's a bizarre data point. Going back to at least 3.2, Clang has
>> emitted S<int> instead of S<A>. But with my vector example, it used
>> to use the typedef name up through 3.4. That changed in 3.5, where
>> the type name 'int4' has entirely disappeared from the DWARF.
>> Clearly that's a bug; the type name needs to be in there somewhere.
>>
>> One more thing:
>>
>> it'd be good to figure out how to deal with all possible names for
>> the type, even the ones the user hasn't written (eg: typedef int A;
>> typedef int B; and make sure that the debugger can handle S<int>,
>> S<A> and S<B> in their code, even though the user only wrote one of
>> those in the source).
>>
>> The answer to this "how to deal" question is with debugger smarts,
>> not more complicated DWARF. DWARF is about the program as-written
>> and as-compiled, not about
>> anything-the-user-might-conceivably-try-to-write-in-the-debugger.
>> Handling this in DWARF is a combinatorial nightmare, for completely
>> speculative purposes. Not gonna happen.
>>
>> I think it comes down to how the information is planning to be used. A
>> consumer with the dwarf information today could, in fact, get to the
>> S<int> type from a user who types S<A> pretty easily right? Now if you'd
>> like a way to print out the textual representation of every type as it
>> was used in the program that's likely to be less possible without some
>> serious duplication of dwarf. You could use an unnamed type for the base
>> and then use DW_AT_specification with just a bare DW_AT_name to avoid
>> some of the unpleasantness of the specification hack, but then you come
>> to the problem of template arguments etc. It's fairly crazy to consider,
>> but a user could quite easily write:
>>
>> new std::vector<int, allocator>()
>>
>> with some allocator that was never used in the program with vector and
>> expect the code to be generated at run time and the rest of the type to
>> be found.
>>
>> Anyhow, I think the best bet is for the most general type to be left in
>> the debug information and then the typedefs etc to be their own DIEs.
>> Unless we have some use that we're not talking about here?
>>
>> -eric
>>
>> --paulr
>>
>> *From:*David Blaikie [mailto:dblaikie at gmail.com
>> <mailto:dblaikie at gmail.com>]
>> *Sent:* Thursday, September 18, 2014 4:03 PM
>>
>>
>> *To:* Robinson, Paul
>> *Cc:* llvm-commits at cs.uiuc.edu <mailto:llvm-commits at cs.uiuc.edu>;
>> Greg Clayton; Frédéric Riss; lldb-dev at cs.uiuc.edu
>> <mailto:lldb-dev at cs.uiuc.edu>; jingham at apple.com
>> <mailto:jingham at apple.com>
>> *Subject:* Re: [lldb-dev] [RFC][PATCH] Keep un-canonicalized
>> template types in the debug information
>>
>> On Thu, Sep 18, 2014 at 2:40 PM, Robinson, Paul
>> <Paul_Robinson at playstation.sony.com
>>
>> <mailto:Paul_Robinson at playstation.sony.com>> wrote:
>>
>> David,
>>
>> Sorry, thought you were protesting the typedef idea as interfering
>> with deduplication or type-unit commonality.
>>
>> So to recap, if we have source like this:
>>
>> typedef int A;
>>
>> template<typename T> struct S { T member; };
>>
>> S<A> s_a;
>>
>> then we'll get
>>
>> DW_TAG_typedef
>>
>> DW_AT_name "A"
>>
>> DW_AT_type -> int
>>
>> DW_TAG_structure_type
>>
>> DW_AT_name "S<A>"
>>
>> DW_TAG_member
>>
>> DW_AT_name "member"
>>
>> DW_AT_type -> int // or the typedef for "A" ?
>>
>> DW_TAG_template_type_parameter
>>
>> DW_AT_name "T"
>>
>> DW_AT_type -> (the typedef for "A")
>>
>> Are you suggesting putting the rest of S<int> here too? Or how would
>> S<A> refer to S<int> for the rest of the implementation?
>>
>> DW_TAG_variable
>>
>> DW_AT_name "s_a"
>>
>> DW_AT_type -> (the above structure_type)
>>
>> Ah, no - just a typedef of the template:
>>
>> 1: DW_TAG_structure_type // the debug info we already produce today
>> (S<int>)
>> ...
>>
>> 2: DW_TAG_typedef
>> DW_AT_name "S<A>"
>> DW_AT_type (1)
>>
>> And honestly, the variable would still be of type (1).
>>
>> Duplicating the entire type for each way of naming the same type is,
>> I'm fairly sure, not going to work for debuggers today. If someone
>> wants to propose a way of encoding this that will need new
>> code/support from debuggers, etc, then I feel the right venue to
>> discuss that is the DWARF committee - because you'll need buy-in
>> from producers and consumers. Without having that discussion, I
>> believe just providing a typedef of the template specialization is
>> probably a benefit to users.
>>
>> If we want to talk about a 'right' representation of this for DWARF
>> that would necessitate more substantial changes to both DWARF
>> producers and consumers... I think it'll be a bit more involved than
>> even what you're proposing. If we're going to deal with that, it'd
>> be good to figure out how to deal with all possible names for the
>> type, even the ones the user hasn't written (eg: typedef int A;
>> typedef int B; and make sure that the debugger can handle S<int>,
>> S<A> and S<B> in their code, even though the user only wrote one of
>> those in the source).
>>
>> Yes?
>>
>> --paulr
>>
>> *From:*David Blaikie [mailto:dblaikie at gmail.com
>> <mailto:dblaikie at gmail.com>]
>> *Sent:* Thursday, September 18, 2014 1:09 PM
>> *To:* Robinson, Paul
>> *Cc:* llvm-commits at cs.uiuc.edu
>> <mailto:llvm-commits at cs.uiuc.edu>; Greg Clayton; Frédéric Riss;
>> lldb-dev at cs.uiuc.edu <mailto:lldb-dev at cs.uiuc.edu>;
>> jingham at apple.com <mailto:jingham at apple.com>
>> *Subject:* Re: [lldb-dev] [RFC][PATCH] Keep un-canonicalized
>> template types in the debug information
>>
>> On Thu, Sep 18, 2014 at 1:05 PM, David Blaikie
>> <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>>
>> On Thu, Sep 18, 2014 at 1:00 PM, Robinson, Paul
>> <Paul_Robinson at playstation.sony.com
>> <mailto:Paul_Robinson at playstation.sony.com>> wrote:
>>
>> >From: David Blaikie [mailto:dblaikie at gmail.com
>> <mailto:dblaikie at gmail.com>]
>> >On Wed, Sep 17, 2014 at 11:54 AM, Robinson, Paul
>> <Paul_Robinson at playstation.sony.com
>> <mailto:Paul_Robinson at playstation.sony.com>> wrote:
>> >> From: David Blaikie [mailto:dblaikie at gmail.com
>> <mailto:dblaikie at gmail.com>]
>> >> > On Wed, Sep 17, 2014 at 7:45 AM, Robinson, Paul
>> <Paul_Robinson at playstation.sony.com
>> <mailto:Paul_Robinson at playstation.sony.com>> wrote:
>> >> > > From: David Blaikie [mailto:dblaikie at gmail.com
>> <mailto:dblaikie at gmail.com>]
>> >> > > On Sat, Sep 13, 2014 at 6:54 PM, Robinson, Paul
>> <Paul_Robinson at playstation.sony.com
>> <mailto:Paul_Robinson at playstation.sony.com>> wrote:
>> >> > > > > > On 09 Sep 2014, at 00:01, jingham at apple.com
>>
>> <mailto:jingham at 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
>>
>> <friss at apple.com <mailto:friss at apple.com>> wrote:
>> >> > > >>
>> >> > > >>
>> >> > > >>> On 08 Sep 2014, at 19:31, Greg Clayton
>>
>> <gclayton at apple.com <mailto:gclayton at 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
>>
>> <aprantl at apple.com <mailto:aprantl at apple.com>> wrote:
>> >> > > >>>>
>> >> > > >>>>
>> >> > > >>>>> On Sep 5, 2014, at 3:49 PM, Eric
>> Christopher <echristo at gmail.com
>> <mailto:echristo at gmail.com>>
>> >> > > wrote:
>> >> > > >>>>>
>> >> > > >>>>>
>> >> > > >>>>>
>> >> > > >>>>>
>> >> > > >>>>> On Fri, Sep 5, 2014 at 3:43 PM, Duncan P.
>> N. Exon Smith
>> >> > > <dexonsmith at apple.com
>> <mailto:dexonsmith at apple.com>> wrote:
>> >> > > >>>>>
>> >> > > >>>>>> On 2014 Sep 5, at 16:01, Frédéric Riss
>>
>> <friss at apple.com <mailto:friss at 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
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>>
>> <mailto:llvm-commits at cs.uiuc.edu>
>> >> > > >>>>>>
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
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>> >> > > >>>>>
>> >> > > >>>>
>> >> > > >>>>
>> _______________________________________________
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>> >> > > >>>>
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>> >> > > >>
>> >> > > >>
>> >> > > >> _______________________________________________
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>> >> > > >>
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