This is a friendly PSA to inform everyone that the semantics of program points in MLIR dataflow analysis will be modified by this PR([mlir] [dataflow] unify semantics of program point by cxy-1993 · Pull Request #110344 · llvm/llvm-project · GitHub).
The concept of a ‘program point’ in the original data flow framework is ambiguous.It can refer to either an operation or a block itself. This representation has different interpretations in forward and backward data-flow analysis. In forward data-flow analysis, the program point of an operation represents the state after the operation, while in backward data flow analysis, it represents the state before the operation. When using forward or backward data-flow analysis, it is crucial to carefully handle this distinction to ensure correctness.
This patch refactors the definition of program point, unifying the interpretation of program points in both forward and backward data-flow analysis.
How to integrate this patch?
For dense forward data-flow analysis and other analysis (except dense backward data-flow analysis), the program point corresponding to the original operation can be obtained by getProgramPointAfter(op), and the program point corresponding to the original block can be obtained by getProgramPointBefore(block).
For dense backward data-flow analysis, the program point corresponding to the original operation can be obtained by getProgramPointBefore(op), and the program point corresponding to the original block can be obtained by getProgramPointAfter(block).
NOTE: If you need to get the lattice of other data-flow analyses in dense backward data-flow analysis, you should still use the dense forward data-flow approach. For example, to get the Executable state of a block in dense backward data-flow analysis and add the dependency of the current operation, you should write:
getOrCreateFor<Executable>(getProgramPointBefore(op), getProgramPointBefore(block))
In case above, we use getProgramPointBefore(op) because the analysis we rely on is dense backward data-flow, and we use getProgramPointBefore(block) because the lattice we query is the result of a non-dense backward data flow computation.
related dsscussion: [RFC] Unify the semantics of program points - #8 by cxy
All relevant discussions are welcome.