[cfe-dev] output metadata for extern declared functions?

[Lewis Burns]

I'm fixing an existing source-to-source translator, and it was 
unfortunately written by someone using LLVM as the intermediate step 
before Clang was mature. I don't need to generate object code at all, 
and performance is not a concern here.

Sounds like that the only way for me to save this translator is to patch 
Clang by generating debug info metadata for declarations whose 
definitions are not available. Is this a feasible approach? I mean how 
difficult it would be comparing with hacking into Clang AST and start 
from there? I guessed that it would be much easier than starting over 
with Clang AST from scratch. Since I haven't worked on Clang before, any 
suggestion and help would be greatly appreciated.



On 11/09/2013 04:14 PM, David Blaikie wrote:
>
> For those following this thread a critical detail would be that you 
> want debug info metadata.
>
> There's no simple flag for this as we don't attach the function debug 
> info metadata to every declaration, just to definitions (there's no 
> filtering step)
>
> But why do you want this anyway? If you're performing 
> optimizations/transformations based on debug info metadata, that's not 
> really the desired approach. Debug info is not meant to affect code 
> generation.
>
> On Nov 9, 2013 7:59 AM, "Lewis Burns" <lewisurn at gmail.com 
> <mailto:lewisurn at gmail.com>> wrote:
>
>     Hi,
>
>     I haven't worked on Clang before and have a simple question
>     (supposedly) for those who are familiar with metadata and LLVM
>     bitcode generation. Assume that I have a function which is
>     declared as extern as the following:
>
>     extern int convert(unsigned u);
>
>     I want to have Clang generate metadata nodes for it by adding a
>     metadata node of subprogram into the list of subprograms defined
>     in the current compilation unit. The subprogram metadata node and
>     its associated nodes should have the info of the type signature.
>     For example, I get the following set of metadata nodes for function
>
>     int convert(unsigned u) {return 0;}
>
>     !3 = metadata !{metadata !4, metadata !10, metadata !33}
>     !4 = metadata !{i32 786478, metadata !1, metadata !5, metadata
>     !"convert", metadata !"convert", metadata !"", i32 23, metadata
>     !6, i1 false, i1 true, i32 0, i32 0, null, i32 256, i1 false, i32
>     (i32)* @convert, null, null, metadata !2, i32 23} ; [
>     DW_TAG_subprogram ] [line 23] [def] [convert]
>     !7 = metadata !{metadata !8, metadata !9}
>     !8 = metadata !{i32 786468, null, null, metadata !"int", i32 0,
>     i64 32, i64 32, i64 0, i32 0, i32 5} ; [ DW_TAG_base_type ] [int]
>     [line 0, size 32, align 32, offset 0, enc DW_ATE_signed]
>     !9 = metadata !{i32 786468, null, null, metadata !"unsigned int",
>     i32 0, i64 32, i64 32, i64 0, i32 0, i32 7} ; [ DW_TAG_base_type ]
>     [unsigned int] [line 0, size 32, align 32, offset 0, enc
>     DW_ATE_unsigned]
>
>     which allows me to extract the source-level type signature for the
>     function by using LLVM debug info APIs. I'd like to get the
>     source-level type signature of the extern declared function, but
>     Clang does not produce metadata for it.
>
>     By looking at the Clang AST for the extern declared function
>
>     |-FunctionDecl 0x70598c0 <line:23:1, col:30> convert 'int
>     (unsigned int)' extern
>     | |-ParmVarDecl 0x7059800 <col:20, col:29> u 'unsigned int'
>
>     I know that Clang has the information I need, and I just need to
>     turn off or remove the filter that ignores functions whose bodies
>     are not available during metadata node or/and code generation. Are
>     there simple switches that do this? If not, can anyone please
>     explain how to do it by pointing me to the right code snippets?
>
>     Thanks very much,
>
>     -- 
>     Lewis
>
>
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>


-- 
Lewis

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