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

[David Blaikie]

On Wed, Apr 29, 2015 at 11:35 AM, Adrian Prantl <aprantl at apple.com> wrote:

>
> On Apr 29, 2015, at 11:15 AM, David Blaikie <dblaikie at gmail.com> wrote:
>
>
>
> 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.
>
>
> When ASan puts variables int the middle of a large alloca it creates a
> pointer to the variable’s storage deep inside the alloca. It then emits a
> dbg.declare describing the pointer (with the correct type) rather than the
> alloca so it does never emit illegal IR.
>

If this is about the IR type of the pointer passed to the dbg.declare...
those are going away (see: the copious changes I've been making to LLVM
over the last couple of months). So if the verifier is relying on that...
it shouldn't.


>
> -- adrian
>
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