[lldb-dev] Hiding local variables not defined yet

[Emre Kultursay via lldb-dev]

>
> LLDB does only show variables that are in lexical scope in ...

Ah, yes, I got confused because I thought this filter was implemented
inside LLDB, yet the `frame variable` command was returning me all local
variables; I now notice that it's a filter that's implemented inside the
IDE (I'm looking at Android Studio).

Thanks for the explanation and also the details about the compiler provided
info.


On Fri, Apr 9, 2021 at 10:15 PM Greg Clayton <clayborg at gmail.com> wrote:

>
>
> On Apr 9, 2021, at 11:39 AM, Emre Kultursay via lldb-dev <
> lldb-dev at lists.llvm.org> wrote:
>
> When debugging C/C++ (statically scoped languages), does LLDB recognize
> (or does it have a setting for it) that a local variable is not defined yet
> at the current program address (i.e., DW_AT_decl_line is less than the
> source line for the address), and thus, not include it in the list of
> locals (i.e., frame variable)?
>
> Does it make sense to have such a setting?  The goal is to reduce the
> clutter in locals list.
>
>
> LLDB does not. We show exactly what the compiler emits. DWARF, the debug
> information, is powerful enough to say from [0x1000-0x1010) the variable is
> here, and from [0x1020-0x1100) the variable is there, these are called
> location expressions. But the compiler, for non optimized code, always just
> emits the variable's location on the stack and doesn't correctly limit it
> to when the variable has been initialized.
>
> So this could easily be fixed in the compiler. LLDB really needs to listen
> to what the compiler says because once you enable optimizations, the
> compiler can end up moving all sorts of code around and the variable
> _could_ become initialized before the DW_AT_decl_line.
>
> So we don't want to pretend we know better than the compiler when
> displaying debug information. But even if the compiler does emit better
> debug information that does give correct location expressions, we would
> still show the variable because it is in scope. LLDB does only show
> variables that are in lexical scope currently in Xcode, lldb-vscode, lldb,
> and Android Studio AFAIK. What debugger are you using?
>
> Greg
>
>
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