[PATCH] D153536: [Clang] Implement P2169 A nice placeholder with no name

[Corentin Jabot via Phabricator via cfe-commits]

cor3ntin added a comment.

In D153536#4475240 <https://reviews.llvm.org/D153536#4475240>, @dblaikie wrote:

> In D153536#4475199 <https://reviews.llvm.org/D153536#4475199>, @cor3ntin wrote:
>
>> In D153536#4474733 <https://reviews.llvm.org/D153536#4474733>, @dblaikie wrote:
>>
>>> Maybe try testing with more different values, to demonstrate which entities the debugger is finding?
>>>
>>>   int _ = 1;
>>>   int _ = 2;
>>>   {
>>>     int _ = 3;
>>>     int _ = 7;
>>>   }
>>>
>>> Or something like that. But, honestly, if the point is that these variables should be unnameable - perhaps we shouldn't generate any DWARF for them?
>>
>> The variables should still be inspectable, ideally. It's true it makes no sense to be able to use them in expressions, and maybe if lldb use clang to evaluate expressions that would just work?
>
> I think that's going to be tricky - because we don't emit DWARF to say where one variable's lifetime starts compared to another in the same scope - so likely a debugger would just always show the first or last variable with the same name in a given scope (so in the above example you could probably only print {1,3} or {2,7}) - and getting it wrong is more likely and more problematic than existing cases of shadowed variables. If we can't get this right, which I don't think we can, practically speaking, at the moment (the DWARF feature we'd have to support to do this better would probably be `DW_AT_start_scope` to say at what instruction the lifetime of a variable starts) - it may be best not to include it at all?

It doesn't seems to be confused on simple examples

  cpp
  * thread #1, name = 'placeholder', stop reason = step in
      frame #0: 0x0000555555555157 placeholder`main at debug_placeholder.cpp:8:13
     5        {
     6            int _ = 4;
     7            int _ = 5;
  -> 8            int _ = 6;
     9        }
     10   }
  (lldb) frame variable
  (int) _ = 1
  (int) _ = 2
  (int) _ = 3
  (int) _ = 4
  (int) _ = 5
  (int) _ = -9432

That being said, if you think it's more prudent, I would not mind


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