r236059 - Debug Info: Represent local anonymous unions as anonymous unions

[David Blaikie]

On Wed, Apr 29, 2015 at 10:48 AM, Adrian Prantl <aprantl at apple.com> wrote:

>
> On Apr 29, 2015, at 10:04 AM, Eric Christopher <echristo at gmail.com> wrote:
>
>
>
> On Wed, Apr 29, 2015 at 9:58 AM Eric Christopher <echristo at gmail.com>
> wrote:
>
>> On Wed, Apr 29, 2015 at 8:20 AM Adrian Prantl <aprantl at apple.com> wrote:
>>
>>>
>>> > On Apr 28, 2015, at 6:26 PM, David Blaikie <dblaikie at gmail.com> wrote:
>>>
>>> > Could you revert & I'll look into getting you a reduced test
>>> case/demonstration of the issue? (can you run GDB 7.5? Perhaps a simple
>>> test case would demonstrate the issue if you're lucky, otherwise I can
>>> reduce one from the failing test case)
>>>
>>> I reverted the commit in r236110. I probably won’t need a reduction — my
>>> guess from the log is that gdb expects a local variable to be present.
>>>
>>> My suggestion is to emit local artificial shadow variables and then
>>> weaken the Verifier to not verify artificial variables. In a next step, we
>>> could use the new debugger tuning target feature to make the artificial
>>> local variables and the weakened verifier a gdb-specific behavior, file a
>>> bug against gdb, and eventually remove it altogether.
>>>
>>>
>> FWIW I don't want to use the "tuning" parameters to also affect
>> correctness.
>>
>
> OK. Dave and I debated this a little in person, here's a proposal:
>
> by default it will have the gdb specific behavior, but if you're tuning
> for lldb (or any other debugger I guess?) it won't be there.
>
>
> Thoughts?
>
>
> I think it’s a fair assumption that many debuggers won’t deal very well
> with anonymous local variables, from that perspective this is the right
> decision.
> (Nitpick: If it is only needed for GDB then it doesn’t seem right to make
> it the global default, because tuning for gdb will be the default on all
> non-lldb/sce platforms anyway.)
>
> I really want to push towards making the debug info handling stricter, so
> I’m unhappy about having the frontend willfully emit broken IR at the
> moment. But, I also haven’t found a way to emit the local shadow variables
> in a way that makes the verifier happy and isn’t entirely gross in other
> respects. I’ll keep looking for a better way to emit this without having to
> relax the verifier.
>

Im curious though - why is this harder to support than, say, ASan's habit
of smooshing all the variables into a single alloca? They seem rather
similar to me, but I'm probably missing something.

- David


>
> -- adrian
>
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