[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
>>                  >> > > >>>>>> llvm-commits at cs.uiuc.edu
>>
>>                 <mailto:llvm-commits at cs.uiuc.edu>
>>                  >> > > >>>>>>
>>                 http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>>                  >> > > >>>>>
>>                  >> > > >>>>>
>>                  >> > > >>>>
>>                  >> > > >>>>
>> _______________________________________________
>>                  >> > > >>>> lldb-dev mailing list
>>                  >> > > >>>> lldb-dev at cs.uiuc.edu
>>                 <mailto:lldb-dev at cs.uiuc.edu>
>>                  >> > > >>>>
>>                 http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
>>                  >> > > >>>
>>                  >> > > >>
>>                  >> > > >>
>>                  >> > > >> _______________________________________________
>>                  >> > > >> lldb-dev mailing list
>>                  >> > > >> lldb-dev at cs.uiuc.edu
>>                 <mailto:lldb-dev at cs.uiuc.edu>
>>                  >> > > >>
>>                 http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
>>                  >> > > >
>>                  >> > >
>>                  >> > >
>>                  >> > > _______________________________________________
>>                  >> > > llvm-commits mailing list
>>                  >> > > llvm-commits at cs.uiuc.edu
>>                 <mailto:llvm-commits at cs.uiuc.edu>
>>                  >> > >
>>                 http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>>                  >> >
>>                  >> > _______________________________________________
>>                  >> > llvm-commits mailing list
>>                  >> > llvm-commits at cs.uiuc.edu
>>                 <mailto:llvm-commits at cs.uiuc.edu>
>>                  >> >
>>                 http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>>
>>
>>     _______________________________________________
>>     lldb-dev mailing list
>>     lldb-dev at cs.uiuc.edu <mailto:lldb-dev at cs.uiuc.edu>
>>     http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
>>
>>
>>
>> _______________________________________________
>> llvm-commits mailing list
>> llvm-commits at cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>>
>>
>> _______________________________________________
>> llvm-commits mailing list
>> llvm-commits at cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>>
>>
>
>
>
> _______________________________________________
> llvm-commits mailing list
> llvm-commits at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>




More information about the lldb-dev mailing list