> On Apr 29, 2015, at 11:15 AM, David Blaikie <[email protected]> wrote:
>
>
>
> On Wed, Apr 29, 2015 at 10:48 AM, Adrian Prantl <[email protected]
> <mailto:[email protected]>> wrote:
>
>> On Apr 29, 2015, at 10:04 AM, Eric Christopher <[email protected]
>> <mailto:[email protected]>> wrote:
>>
>>
>>
>> On Wed, Apr 29, 2015 at 9:58 AM Eric Christopher <[email protected]
>> <mailto:[email protected]>> wrote:
>> On Wed, Apr 29, 2015 at 8:20 AM Adrian Prantl <[email protected]
>> <mailto:[email protected]>> wrote:
>>
>> > On Apr 28, 2015, at 6:26 PM, David Blaikie <[email protected]
>> > <mailto:[email protected]>> 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.
-- adrian
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