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

[Duncan P. N. Exon Smith]

> 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>>`?

> 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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