[lldb-dev] Hiding local variables not defined yet

Greg Clayton via lldb-dev lldb-dev at lists.llvm.org
Fri Apr 9 22:15:12 PDT 2021

> 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?

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