[lldb-dev] [RFC][PATCH] Keep un-canonicalized template types in the debug information
Robinson, Paul
Paul_Robinson at playstation.sony.com
Fri Sep 12 13:44:30 PDT 2014
Sorry for being late to the party. We've had complaints about seeing things
like S<int>, would rather see S<A>, extremely happy to see this direction.
Thanks,
--paulr
> -----Original Message-----
> From: llvm-commits-bounces at cs.uiuc.edu [mailto:llvm-commits-
> bounces at cs.uiuc.edu] On Behalf Of Frédéric Riss
> Sent: Tuesday, September 09, 2014 6:19 AM
> To: jingham at apple.com
> Cc: lldb-dev at cs.uiuc.edu; llvm-commits; Greg Clayton
> Subject: Re: [lldb-dev] [RFC][PATCH] Keep un-canonicalized template types
> in the debug information
>
>
> > On 09 Sep 2014, at 00:01, 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
>
>
> > Jim
> >
> >> On Sep 8, 2014, at 12:38 PM, Frédéric Riss <friss at apple.com> wrote:
> >>
> >>
> >>> On 08 Sep 2014, at 19:31, Greg Clayton <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> wrote:
> >>>>
> >>>>
> >>>>> On Sep 5, 2014, at 3:49 PM, Eric Christopher <echristo at gmail.com>
> wrote:
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>> On Fri, Sep 5, 2014 at 3:43 PM, Duncan P. N. Exon Smith
> <dexonsmith at apple.com> wrote:
> >>>>>
> >>>>>> On 2014 Sep 5, at 16:01, Frédéric Riss <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
> >>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
> >>>>>
> >>>>>
> >>>>
> >>>> _______________________________________________
> >>>> lldb-dev mailing list
> >>>> lldb-dev at cs.uiuc.edu
> >>>> http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
> >>>
> >>
> >>
> >> _______________________________________________
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> >
>
>
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