[llvm-dev] An update on the DominatorTree and incremental dominators

[Jakub (Kuba) Kuderski via llvm-dev]

Hi folks,

Today is the last day of my internship, so here is a short list of changes
to dominators that I committed since the last email:

   - Infinite loops are now included in the PostDominatorTree.
   - Incrementally updated DominatorTree and PostDominatorTree are now
   guaranteed to be the same as freshly recalculated ones.
   - The low-level incremental API (insertEdge/deleteEdge) is used to
   update dominators in LoopDeletion and AgressiveDeadCodeElimination.
   - Support for batch updates (applyUpdates) was introduced to make
   updating dominators *much* easier. The batch updater landed upstream
   earlier this week.
   - The batch updater is now used to preserve dominators in
   LoopUnswitching, LoopRotation, and BreakCriticalEdges.

Thank you all for giving me feedback and reviewing my patches, especially
to Danny, Sanjoy, Davide, Tobias, and Brian. I cannot imagine getting so
far without your support :).

Best,
Kuba

On Mon, Jul 17, 2017 at 11:24 AM, Jakub (Kuba) Kuderski <
kubakuderski at gmail.com> wrote:

> Hi folks,
>
> For the past month I’ve been working on improving the DominatorTree and
> PostDominatorTree in LLVM. The RFC that explains the motivations and plans
> can be found here: http://lists.llvm.org/pipermail/llvm-dev/2017-June/
> 114045.html .
>
> Here’s a short summary of what changed upstream since posting it:
>
>    -
>
>    We switched from the Simple Lengauer-Tarjan algorithm for computing
>    dominators to Semi-NCA, which is a bit faster in most cases. Although it is
>    possible to construct a CFG that triggers the pathological quadratic
>    complexity of Semi-NCA, we weren’t able to observe it in practice.
>    -
>
>    DomTreeNodes now automatically store their level (depth) in the tree,
>    which is always up-to-date. This is used for fast nearest common dominator
>    computation and for building iterated dominance frontier. You can get it by
>    calling .getLevel() on a DomTreeNode.
>    -
>
>    We have a new set of verifiers that are able to prove or disprove
>    correctness of a DomTree -- you can explicitly do it by calling
>    DT.verify(). The check has a disadvantage of being quite slow (O(n^3)), so
>    the legacy DT.verifyDominatorTree() uses it only when
>    ENABLE_EXPENSIVE_CHECK is enabled, or when the -verify-dom-info command
>    line option is set to 1. Some of the transforms assume that calling
>    DT.verifyDomTree() is fast, so we cannot turn it on by default.
>    -
>
>    Dominators and Postdominators are now different types, i.e. IsPostDom
>    is a template argument and not a runtime value. It is no longer possible to
>    assign a PostDomTree to a DomTree (or the other way round).
>
>
> And now the big thing: support for incremental updates (insertEdge and
> deleteEdge) has just landed upstream. This should make updating
> DominatorTree much easier and less error-prone.
> Although he API is quite basic at the moment, it should be sufficient to
> replace manual DomTree updates in many places. Please give the new
> incremental API a try and share your feedback :).
>
> During the remaining 5 weeks of my internship at Google, I plan to make a
> few transforms use the new incremental API, further improve
> PostDominatorTree by making it always have a virtual root, and to
> investigate batch updates. After that I would like to continue maintaining
> dominators, but I won’t be able spend as much time on it as I do now.
>
> Best,
> Kuba
>
>


-- 
Jakub Kuderski
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