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

[Greg Clayton]

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>".

> 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
>> >
>> >
>> >
>> >
>> >
>> > _______________________________________________
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>> 
>> 
> 
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