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

Kristóf Umann via llvm-dev llvm-dev at lists.llvm.org
Sun Jun 16 07:55:44 PDT 2019

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
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