Re: Filtering shadowed variables in info local

[Kevin Buettner <kevinb@redhat.com>]

On Fri, 10 Nov 2023 17:54:53 +0000
"Sargsyan, Eduard" <eduard.sargsyan@intel.com> wrote:

> After https://sourceware.org/pipermail/gdb-patches/2021-June/180478.html
> and https://sourceware.org/pipermail/gdb-patches/2022-January/184819.html
> GDB prints "<shadowed: decl at line 5>" if variable has been marked as shadowed.
> But user might not be interested in seeing any shadowed variables.
> 
> For example, this code:
> 
>      1 int num = 1;
>      2 int
>      3 main ()
>      4  {
>      5    const char *str = "main";
>      6    int num = 3;
>      7    {
>      8       const char *str = "nested";
>      9       int num = 5;
>     10      num = 0; //  break here
>     11    }
>     12    return num;
>     13  }
> 
> Brings GDB to (when stopped at line 10):
> 
>   (gdb) info locals
>   str = 0x555555556009 "nested"
>   num = 5
>   str = 0x555555556004 "main"     <shadowed: decl at line 5>
>   num = 3 <shadowed: decl at line 6>
> 
> Prints both shadowed and local variables "str" and "num", but does not print
> global accessible variable "num".
> 
>   (gdb) print num
>   $1 = 5
>   (gdb) print ::num
>   $2 = 1
> 
> Setting value to local variable "num" works as expected:
>   (gdb) set num = 7
>   (gdb) info locals
>   str = 0x555555556009 "nested"
>   num = 7
>   str = 0x555555556004 "main"     <shadowed: decl at line 5>
>   num = 3 <shadowed: decl at line 6>
> 
> In C++ shadowed variable is not accessible, to there is no way to change value
> of variable "num" declared at line 5.  So only local and global variables can
> be modified:
> 
>   (gdb) set ::num = 9
>   (gdb) print num
>   $3 = 7
>   (gdb) print ::num
>   $4 = 9
> 
> I'm not sure if printing shadowed variables "num" and "str" are useful in the
> use case described above.  Maybe adding a setting to filter out shadowed
> variables makes sense (by default the setting will be disabled).
> 
> Example:
> 
>   (gdb) info locals
>   str = 0x555555556009 "nested"
>   num = 7
>   str = 0x555555556004 "main"     <shadowed: decl at line 5>
>   num = 3 <shadowed: decl at line 6>
>   (gdb) set print shadowed off
>   (gdb) info locals
>   str = 0x555555556009 "nested"
>   num = 7
>   (gdb) set print shadowed on
>   (gdb) info locals
>   str = 0x555555556009 "nested"
>   num = 7
>   str = 0x555555556004 "main"     <shadowed: decl at line 5>
>   num = 3 <shadowed: decl at line 6>
> 
> What are your thoughts?

Nice write-up!

I think it's useful to know the values of shadowed variables even if
they're not settable.  I think it might be useful to also know about the
shadowed global, which is currently not displayed by "info locals". 
OTOH, the command suggests that only locals should be displayed, but
the fact that a global is shadowed might be the source of a bug; IMO, it
wouldn't hurt to tell the user about it.

I have no objection to the addition of a "set print shadowed" command
to control output of shadowed variables, but I'd prefer that the
default be to preserve current behavior.

Kevin