[llvm-dev] RFC Storing BB order in llvm::Instruction for faster local dominance (original) (raw)

Nicolai Hähnle via llvm-dev llvm-dev at lists.llvm.org
Sun Feb 16 03:29:56 PST 2020

+1, this is a really good idea.

Cheers, Nicolai

On Fri, Feb 14, 2020 at 10:50 PM Reid Kleckner via llvm-dev <llvm-dev at lists.llvm.org> wrote:

Hello again. :) There has been renewed interest in having instructions track their own order in basic blocks to help make dominance queries fast. I have a very simple naive implementation of this here: https://reviews.llvm.org/D51664 Essentially, every instruction will carry an integer order number, and inserting new instructions invalidates the ordering. I know there are better algorithms for maintaining the ordering in the face of random insertions, but I wanted to focus on the simple case of making dominance queries amortized O(1) when no insertion is occuring. My personal belief is that most transforms are 80% analysis, 20% transformation, so most of the benefits can be delivered with simple heuristics. MLIR already uses this technique: https://github.com/llvm/llvm-project/blob/master/mlir/include/mlir/IR/Operation.h#L615 With the renewed interest in the patch, I believe there is broad consensus that we should do this, but I wanted to re-open this RFC that I started back in 2018 to give anyone a chance to say "no". http://lists.llvm.org/pipermail/llvm-dev/2018-September/126249.html Thanks, Reid

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