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

Kristóf Umann via llvm-dev llvm-dev at lists.llvm.org
Sat Jun 8 12:21:10 PDT 2019

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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