[lldb-dev] Extern globals showing up as locals

[Sam McCall via lldb-dev]

I sent my best attempt in https://reviews.llvm.org/D26908, would appreciate
any advice there :-)

On Fri, Nov 18, 2016 at 7:22 PM, Sam McCall <sammccall at google.com> wrote:

> I'm seeing particular globals spuriously being reported as local
> variables: they're returned by SBFrame::GetVariables(true, true, false,
> true) because their scope is eValueTypeVariableLocal.
>
> I've tracked down the problem but don't know DWARF well enough to
> determine the right fix. Anyone want to help me out?
>
> --
>
> I can reproduce with the following program:
>
> struct Obj{};
> namespace N { extern Obj& o; }
> using N::o;
> int main(){}
>
>
> When building with clang -g, lldb gives
>
> (lldb) frame v -g -s
>
> LOCAL: (Obj &) N::o = <no location, value may have been optimized out>
>
> (lldb) target v -s
>
> LOCAL: (Obj &) N::o = <no location, value may have been optimized out>
>
> It seems confused about whether it's local or global.
>
> readelf --dump-debug has the following info for the variable:
> <2><31>: Abbrev Number: 3 (DW_TAG_variable)
>     <32>   DW_AT_name        : (indirect string, offset: 0x51): o
>     <36>   DW_AT_type        : <0x42>
>     <3a>   DW_AT_external    : 1
>     <3a>   DW_AT_decl_file   : 1
>     <3b>   DW_AT_decl_line   : 2
>     <3c>   DW_AT_declaration : 1
>     <3c>   DW_AT_location    : 0 byte block:    ()
>     <3d>   DW_AT_linkage_name: (indirect string, offset: 0x57): _ZN1N1oE
> It seems that the zero-length location is a problem: SymbolFileDWARF.cpp
> thinks a variable without a valid location must be static or local.
>
> FWIW, gdb on the clang-built binary shows N::o as an optimized-out global,
> which seems fine. gcc generates debug info that omits the DW_AT_location,
> which causes gdb to hide it completely I think.
>
> Cheers, Sam
>
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