An update on the DominatorTree and incremental dominators

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1

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

2

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

3

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 :).

Dear Kuba,

thanks a lot for all of your hard work! I am very impressive by your
output. The incremental dominator construction in LLVM is indeed a large
step forward. We may not always have agreed on everything, but
independently it was a pleasure to work with you.
As you likely soon fly back to Europe, feel invited to pass by at ETH at
any point.

Best,
Tobias