Fix correctness of InvarianceCheck and other SSA slice walks. - #61
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Provides a backport of llvm-project/pull/188758 which is used to implement generic SSA slice analyses. In particular, provides a cached control-flow aware `BackwardSliceAnalysis`, the `ForwardSlice` helper and the `LoopSliceAnalysis`. The `LoopSliceAnalysis` uses the `mlir::LoopLikeOpInterface` to determine all fundamental loop-variant SSA values and constructs a forward slice from them. This analysis is intended to replace the crude invariance check currently used in the scheduler. Signed-off-by: Karl F. A. Friebel <karl.friebel@ibm.com>
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This is a continuation of the pre-open-source issue with the following description:
The previous implementation of the InvarianceCheck did not properly examine the required MLIR interfaces, and did not consider dependencies carried through block arguments. In particular, it was not conservative enough. This can be done using built-in functionality provided in mlir/Analysis/SliceWalk.h.
This commit adds a new SliceAnalysis and LoopSliceAnalysis types that implement the correct behavior and a reusable cache as a managed MLIR analysis.
Unfortunately, there is a bug upstream that prevents the aforementioned MLIR utility from working correctly. Until the associated PR is merged in, the new SliceAnalysis is overly conservative.
There are now more uses for these analyses in the scheduler, and therefore we should hurry to centralize this key feature of IR analysis.