[RFC][PATCH] Keep un-canonicalized template types in the debug information

[Eric Christopher]

On Fri, 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? :-)
>

*bugs random gdb developer as well*

-eric



>
> -- 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
>>
>>
>
>
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