[lldb-dev] Hiding local variables not defined yet

Emre Kultursay via lldb-dev lldb-dev at lists.llvm.org
Mon Apr 12 12:31:46 PDT 2021

> 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

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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