[all-commits] [llvm/llvm-project] de0ebc: [mlir][dataflow] Consolidate AbstractSparseLattice...

[Zhixun Tan via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: de0ebc5263f391519f3909c4f436afdf8adbc1ad
      https://github.com/llvm/llvm-project/commit/de0ebc5263f391519f3909c4f436afdf8adbc1ad
  Author: Zhixun Tan <phisiart at gmail.com>
  Date:   2022-08-29 (Mon, 29 Aug 2022)

  Changed paths:
    M mlir/include/mlir/Analysis/DataFlow/ConstantPropagationAnalysis.h
    M mlir/include/mlir/Analysis/DataFlow/DenseAnalysis.h
    M mlir/include/mlir/Analysis/DataFlow/IntegerRangeAnalysis.h
    M mlir/include/mlir/Analysis/DataFlow/SparseAnalysis.h
    M mlir/include/mlir/Analysis/DataFlowFramework.h
    M mlir/lib/Analysis/DataFlow/ConstantPropagationAnalysis.cpp
    M mlir/lib/Analysis/DataFlow/DenseAnalysis.cpp
    M mlir/lib/Analysis/DataFlow/IntegerRangeAnalysis.cpp
    M mlir/lib/Analysis/DataFlow/SparseAnalysis.cpp
    M mlir/test/lib/Analysis/DataFlow/TestDeadCodeAnalysis.cpp
    M mlir/test/lib/Analysis/DataFlow/TestDenseDataFlowAnalysis.cpp

  Log Message:
  -----------
  [mlir][dataflow] Consolidate AbstractSparseLattice::markPessimisticFixpoint() and AbstractDenseLattice::reset() into Abstract{Sparse,Dense}DataFlowAnalysis::setToEntryState().

### Rationale

For a program point where we cannot reason about incoming dataflow (e.g. an argument of an entry block), the framework needs to initialize the state.

Currently, `AbstractSparseDataFlowAnalysis` initializes such state to the "pessimistic fixpoint", and `AbstractDenseDataFlowAnalysis` calls the state's `reset()` function.

However, entry states aren't necessarily the pessimistic fixpoint. Example: in reaching definition, the pessimistic fixpoint is `{all definitions}`, but the entry state is `{}`.

This awkwardness might be why the dense analysis API currently uses `reset()` instead of `markPessimisticFixpoint()`.

This patch consolidates entry point initialization into a single function `setToEntryState()`.

### API Location

Note that `setToEntryState()` is defined in the analysis rather than the lattice, so that we allow different analyses to use the same lattice but different entry states.

### Removal of the concept of optimistic/known value

The concept of optimistic/known value is too specific to SCCP.

Furthermore, the known value is not really used: In the current SCCP implementation, the known value (pessimistic fixpoint) is always `Attribute{}` (non-constant). This means there's no point storing a `knownValue` in each state.

If we do need to re-introduce optimistic/known value, we should put it in the SCCP analysis, not the sparse analysis API.

### Terminology

Please let me know if "entry state" is a good terminology.

I chose "entry" from Wikipedia (https://en.wikipedia.org/wiki/Data-flow_analysis#Basic_principles).

Another term I can think of is "boundary" (https://suif.stanford.edu/~courses/cs243/lectures/L3-DFA2-revised.pdf) which might be better since it also makes sense for backward analysis.

Reviewed By: Mogball

Differential Revision: https://reviews.llvm.org/D132086