[llvm-dev] [IDF][analyzer] Generalizing IDFCalculator to be used for Clang's CFG

[Kristóf Umann via llvm-dev]

Hi Jakub!

On Mon, 17 Jun 2019 at 17:01, Jakub (Kuba) Kuderski <kubakuderski at gmail.com>
wrote:

> Hi Kristóf,
>
>
>> 1. I read the article IDFCalculator is based on[1], but I found no
>> references to IDFCalculator::setLiveInBlocks, and the file header seems to
>> confirm that it's an implementation specific thing. Could I get away
>> restricting the clang::CFG tailored version to not contain this function?
>>
> What's stopping you from implementing it for clang CFG? I looked at the
> code and it seems like your new implementation handles it.
>

The link I sent is a diff of two branches, and I changed a lot of code
yesterday. Initally, I "bruteforced" my way through generalizing GraphDiff
and related classes, and that patch looked anything but appealing.

Thanks for your comments by the way, to this and the related patches!


> 2. I didn't really manage to understand the logic for GraphDiff. What does
>> it really do in IDFCalculator's context? I dag out the patch[2] that added
>> an extra constructor to use a snapshot of the CFG, but it seems to be
>> unused. Is it also unlikely that we find any use for this? Mind you, the
>> patch size grew a lot because I also "templatized" GraphDiff and all
>> related classes to handle clang's CFG.
>>
>
> GraphDiff is just a wrapper that was created to represent intermediate
> states of CFG updates based on fully updated IR. For example, you can have
> a transformation that replaces all uses of a basic block with another basic
> block in one operation, and yet you want to have a way to perform some
> analysis as if the transformation happened one change at a time. This was,
> at least originally, used for the incremental domtree updater and for
> memssa. (+cc Alina, as she is most familiar with GraphDiff)
>

This is used because the CFG used in LLVM changes all the time, right? If
so, I don't see this being useful for us, since we don't mess with Clang's
CFG much once it's built.


> I'd separate out a base of IDFCalculator, to a new class called
>> IDFCalculatorBase that wouldn't contain either of said functions.
>>
> If #2 if a lot of work/code, I'd say that separating it doesn't sound that
> bad. It's not that complicated, then I wouldn't split it.
>
>
> On Sun, Jun 16, 2019 at 10:56 AM Kristóf Umann <dkszelethus at gmail.com>
> wrote:
>
>> A polite ping, could someone please share a thought about this?
>>
>> On Sat, 8 Jun 2019 at 21:21, Kristóf Umann <dkszelethus at gmail.com> wrote:
>>
>>> A polite ping on this matter :)
>>>
>>> On Tue, 4 Jun 2019 at 01:51, Kristóf Umann <dkszelethus at gmail.com>
>>> wrote:
>>>
>>>> Hi!
>>>>
>>>> As the title suggests, I'd like to generalize llvm::IDFCalculator to be
>>>> able to calculate control dependencies on clang's CFG. The issue is
>>>> however, that many data structures it uses are "hardcoded" to use
>>>> llvm::BasicBlock, and requires a lot of code to turn it into template
>>>> arguments.
>>>>
>>>> I managed to pull this off by hammering the code until it compiled, and
>>>> it works perfectly, but I'm not really happy with how the patch came out:
>>>>
>>>> https://github.com/Szelethus/llvm-project/compare/domtree_unittest...Szelethus:control_dependency?expand=1
>>>>
>>>> Sadly, my knowledge on LLVM IR and phi nodes in general is very
>>>> limited. My questions are:
>>>>
>>>> 1. I read the article IDFCalculator is based on[1], but I found no
>>>> references to IDFCalculator::setLiveInBlocks, and the file header seems to
>>>> confirm that it's an implementation specific thing. Could I get away
>>>> restricting the clang::CFG tailored version to not contain this function?
>>>>
>>>> 2. I didn't really manage to understand the logic for GraphDiff. What
>>>> does it really do in IDFCalculator's context? I dag out the patch[2] that
>>>> added an extra constructor to use a snapshot of the CFG, but it seems to be
>>>> unused. Is it also unlikely that we find any use for this? Mind you, the
>>>> patch size grew a lot because I also "templatized" GraphDiff and all
>>>> related classes to handle clang's CFG.
>>>>
>>>> If the answer to both of these questions is "no", I'd separate out a
>>>> base of IDFCalculator, to a new class called IDFCalculatorBase that
>>>> wouldn't contain either of said functions.
>>>>
>>>> Cheers!
>>>> Kristóf
>>>>
>>>>
>>>> [1] Sreedhar, Vugranam C., and Guang R. Gao. "A linear time algorithm
>>>> for placing ϕ-nodes." Proceedings of the 22nd ACM SIGPLAN-SIGACT symposium
>>>> on Principles of programming languages. ACM, 1995.
>>>> [2] [IDF] Teach Iterated Dominance Frontier to use a snapshot CFG based
>>>> on a GraphDiff. https://reviews.llvm.org/D50675
>>>>
>>>
>
> --
> Jakub Kuderski
>
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