[lldb-dev] Hiding local variables not defined yet

[Greg Clayton via lldb-dev]

> On Apr 12, 2021, at 4:44 PM, Emre Kultursay <emrekultursay at google.com> wrote:
> 
> Looking at Android Studio implementation a little deeper, it actually does the filtering the way I described in my first email, by comparing line numbers.  It does not use the location expressions.

Interesting if they are doing more to hide variables. Most people debug with debug builds without optimizations, so most of the time this will be ok. If you end up debugging optimized code, I guess your variables could end up being hidden even though they are available and could have valid values.

lldb command line, Xcode and lldb-vscode will do what I describe below when showing variables.

> 
> Do you have a pointer to another implementation (e.g., lldb-vscode) that filters based on location expressions, for comparison?

We always show variables that are in scope. The debug information contains descriptions of functions and lexical blocks within functions. So when we have a PC address, we find the deepest lexical block and show its variables and all variables from all parent lexical blocks all the way back to the function. When there is a location expression for a variable, you will still see the variable, but the value might show an error saying that the variable is not available.

> 
>          
> 
> On Mon, Apr 12, 2021 at 12:31 PM Emre Kultursay <emrekultursay at google.com <mailto:emrekultursay at google.com>> wrote:
> 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 <mailto:clayborg at gmail.com>> wrote:
> 
> 
>> On Apr 9, 2021, at 11:39 AM, Emre Kultursay via lldb-dev <lldb-dev at lists.llvm.org <mailto: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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