[llvm-dev] [DebugInfo] RFC: Introduce LLVM DI Checker utility

[Djordje via llvm-dev]

Hi Vedant,

Thanks a lot for your comments!

 >It looks like a lot of the new infrastructure introduced here 
<https://github.com/djolertrk/llvm-di-checker/commit/9d26ac2557c584f6cf82ac5535fc47f8bd267a27> consists 
of logic copied from the debugify implementation. Why is introducing a 
new pair of passes better than extending the ones we have? The core 
infrastructure needed to track location loss for real (non-synthetic) 
source variables is is in place already.

Since it is a proposal, I thought it'd easier to understand the idea if 
I duplicate things. Ideally, we can make an API that could be used from 
both tools. Initially, I made a few patches locally turning the real 
debug info into debugify ones, but I realized it breaks the original 
idea/design of the debugify & that is why I decided to make a separate 
pass(es). This cannot stay as is with the respect to the implementation, 
it should be either merged into debugify file(s) or refactored using the 
API mentioned above. Another reason for implementing it as a different 
pass was the fact the debugify is meant to be used from 'opt' level 
only, but if we want to invoke the option from front end level, we need 
to merge it into the IR library.


 >Stepping back a bit, I’m also surprised by the decision to move away 
from synthetic testing when there’s still so much low-hanging fruit to 
pick using that technique. The example from 
https://reviews.llvm.org/D81939 illustrates this perfectly: in this case 
it’s not necessary to invent a new testing technique to uncover the bug, 
because simply running `./bin/llvm-lit -Dopt="opt -debugify-each" 
test/Transforms/DeadArgElim` finds the same issue.

As I mentioned in the previous mail, I do really think the debugify 
technique is great & I use it. But, in order to detect that variable "x" 
was optimized-out starting from pass Y, I only run the di-checker option 
(that performs analysis only) & find the variable in the final html 
report. I think that is very user friendly concept. At the end, when we 
detected what was the spot of loosing the location, we can run debugify 
on the pass-directory-tests (but there is a concern the tests does not 
cover all the possible cases; and the case found from the high level 
could be new to the pass). In addition, the di-checker detects issues 
for metadata other than locations (currently, the preservation map keeps 
the disubprograms only, but it should keep other kinds too).


 >In D81939, you discuss finding the new tool useful when responding to 
bug reports about optimized-out variables or missing locations. We 
sorely do need something better than -opt-bisect-limit, but why not 
start with something simple? -check-debugify already knows how to report 
when & where a location is dropped, it would be simple to teach it to 
emit a report when a variable is fully optimized-out.

I agree. We can do that and that could be used from both utilities.


Best regards,

Djordje


On 17.6.20. 21:14, Vedant Kumar wrote:
> Hey Djordje,
>
> It looks like a lot of the new infrastructure introduced here 
> <https://github.com/djolertrk/llvm-di-checker/commit/9d26ac2557c584f6cf82ac5535fc47f8bd267a27> consists 
> of logic copied from the debugify implementation. Why is introducing a 
> new pair of passes better than extending the ones we have? The core 
> infrastructure needed to track location loss for real (non-synthetic) 
> source variables is is in place already.
>
> Stepping back a bit, I’m also surprised by the decision to move away 
> from synthetic testing when there’s still so much low-hanging fruit to 
> pick using that technique. The example from 
> https://reviews.llvm.org/D81939 illustrates this perfectly: in this 
> case it’s not necessary to invent a new testing technique to uncover 
> the bug, because simply running `./bin/llvm-lit -Dopt="opt 
> -debugify-each" test/Transforms/DeadArgElim` finds the same issue.
>
> In D81939, you discuss finding the new tool useful when responding to 
> bug reports about optimized-out variables or missing locations. We 
> sorely do need something better than -opt-bisect-limit, but why not 
> start with something simple? -check-debugify already knows how to 
> report when & where a location is dropped, it would be simple to teach 
> it to emit a report when a variable is fully optimized-out.
>
>
>> On Jun 17, 2020, at 2:10 AM, Djordje <djordje.todorovic at syrmia.com 
>> <mailto:djordje.todorovic at syrmia.com>> wrote:
>>
>> I am sharing the proposal [0] which gives a brief introduction for 
>> the implementation of the LLVM DI Checker utility. On a very high 
>> level, it is a pair of LLVM (IR) Passes that check the preservation 
>> of the original debug info in the optimizations. There are options 
>> controlling the passes, that could be invoked from ``clang`` as well 
>> as from ``opt`` level.
>>
>> By testing the utility on the GDB 7.11 project (using it as a 
>> testbed), it has found a certain number of potential issues regarding 
>> the DILocations (using it on LLVM project build itself, it has found 
>> one bug regarding DISubprogram metadata). Please take a look into the 
>> final report (on the GDB 7.11 testbed) generated from the script that 
>> collects the data at [1]. By looking at these data, it looks that the 
>> utility like this could be useful when trying to detect the real 
>> issues related to debug info production by the compiler.
>
> Thanks for sharing these results. The data here is older (from the 
> 2018 debug info BoF) and from a different project (sqlite3), but we 
> saw some similar patterns: 
> https://llvm.org/devmtg/2018-10/slides/Prantl-Kumar-debug-info-bof-2018.pdf
>
> best
> vedant
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