[lldb-dev] [RFC][PATCH] Keep un-canonicalized template types in the debug information
David Blaikie
dblaikie at gmail.com
Fri Sep 12 15:10:06 PDT 2014
On Sep 12, 2014 2:41 PM, "Robinson, Paul" <
Paul_Robinson at playstation.sony.com> wrote:
>
>
>
> > -----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
>
> 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)
> --paulr
>
> >
> >
> > > 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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> > >> lldb-dev at cs.uiuc.edu
> > >> http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
> > >
> >
> >
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>
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