[PATCH] D32614: [GVNHoist] Fix: PR32821, add check for anticipability in case of infinite loops

[Aditya Kumar via Phabricator via llvm-commits]

hiraditya added a comment.

In https://reviews.llvm.org/D32614#740129, @dberlin wrote:

> In https://reviews.llvm.org/D32614#740119, @hiraditya wrote:
>
> > In https://reviews.llvm.org/D32614#739947, @dberlin wrote:
> >
> > > I'm at a loss to understand why you believe you need to compute joint post-dominance (which is what this is) to compute ANTIC.
> >
> >
> > If the set of BasicBlocks (WL) do not joint-post-dominate the hoisting point (HoistBB), then the expression to be hoisted (from WL) cannot be ANTIC in HoistBB.
>
>
> Sure.
>  That is definitely a way of computing it.
>  It is a generally slow and unused way of computing it, because there are only certain points in the SSA graph where ANTIC can actually change.
>
> > It is a necessary condition what I'm checking here.
>
> It is not necessary to perform graph reachability to do this.
>
> https://pdfs.semanticscholar.org/6d0f/07ff330402b46e83d46142e202069d9aeceb.pdf
>
> Stare at the down-safety step.
>  With a few bits, it is only actually necessary to compute and check antic at the possible phis you would insert to do your hoisting.


>From the paper and the book (http://ssabook.gforge.inria.fr/latest/book.pdf page: 151), it seems DownSafety algorithm does exactly what I'm doing in the function (anticReachable), IIUC.

DownSafety:
...
10: for each f ∈ {Φ’s in the program} do
11:   if ∃ path P to program exit or alteration of expression along which f is not used
12:      downsafe (f ) ← false
...

It checks for each path from point P to the program exit. They have simplified the problem by assuming that end of the function is reachable from every node which is not the
case with LLVM representation of CFG (and that caused the bug).
If I implement the ANTIC the way they have done, it would require iterating through all dominance frontiers to figure out PHI insertion points.
Also, just ensuring downSafety at the PHI is not sufficient to guarantee downsafety at HoistPt because there might be safety issues between a definition of instruction and its dominance frontier.

e.g.,
B -> D -> E

|              |


C             |

|              |


v             v
F <-------/

F will have the PHI for instructions in C and D, but E might throw etc.
Please correct me if I'm wrong because all this is based on my limited understanding of paper and the book chapter.

Thanks,


https://reviews.llvm.org/D32614