[mlir][analysis] Add an analysis for preserving symbol tables

[Mogball]

Summary

This patch adds a SymbolTableAnalysis that can be used with the
analysis manager. It contains a symbol table collection. This analysis
allows symbol tables to be preserved across passes so that they do not
need to be recomputed. The analysis assumes it remains valid because
most transformations automatically keep symbol tables up-to-date using
its insert and erase methods.

[jpienaar]

mlir/include/mlir/Analysis/SymbolTableAnalysis.h

43

OOC is there a way to still invalidate the analysis manually given this? Or would it remain valid analysis always?

[Mogball]

mlir/include/mlir/Analysis/SymbolTableAnalysis.h

43

I don't think there's an API for invalidating analyses, but you can always reinitialize it.

[rriddle]

LGTM, this is desperately needed.

mlir/include/mlir/Analysis/SymbolTableAnalysis.h

43

This is slightly concerning-ish, given that you won't be able to know when this actually gets invalidated by a pass. Is it a problem to just explicitly say that this gets preserved in the various passes that we need to?

[Mogball]

mlir/include/mlir/Analysis/SymbolTableAnalysis.h

43

On the surface, it's a little annoying to have to explicitly mark the analysis as preserved for every single pass, but it also creates a composability issue: if this analysis were downstream, for example, any upstream pass will always invalidate the analysis: canonicalization, cse, etc. That requires the analysis to be upstreamed, which is problematic.

[Mogball]

rebase

mlir/include/mlir/Analysis/SymbolTableAnalysis.h

+//===- SymbolTableAnalysis.h - Analysis for cached symbol tables --*- C++ -*-=//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef MLIR_ANALYSIS_SYMBOLTABLEANALYSIS_H
+#define MLIR_ANALYSIS_SYMBOLTABLEANALYSIS_H
+
+#include "mlir/IR/SymbolTable.h"
+#include "mlir/Pass/AnalysisManager.h"
+
+namespace mlir {
+/// This is a simple analysis that contains a symbol table collection and, for
+/// simplicity, a reference to the top-level symbol table. This allows symbol
+/// tables to be preserved across passes. Most often, symbol tables are
+/// automatically kept up-to-date via the `insert` and `erase` functions.
+class SymbolTableAnalysis {
+public:
+  /// Create the symbol table analysis at the provided top-level operation and
+  /// instantiate the symbol table of the top-level operation.
+  SymbolTableAnalysis(Operation *op)
+      : topLevelSymbolTable(symbolTables.getSymbolTable(op)) {}
+
+  /// Get the symbol table collection.
+  SymbolTableCollection &getSymbolTables() { return symbolTables; }
+
+  /// Get the top-level symbol table.
+  SymbolTable &getTopLevelSymbolTable() { return topLevelSymbolTable; }
+
+  /// Get the top-level operation.
+  template <typename OpT>
+  OpT getTopLevelOp() {
+    return cast<OpT>(topLevelSymbolTable.getOp());
+  }
+
+  /// Symbol tables are kept up-to-date by passes. Assume that the analysis
+  /// remains valid.
+  bool isInvalidated(const AnalysisManager::PreservedAnalyses &pa) {
+    return false;
+  }
+
+private:
+  /// The symbol table collection containing cached symbol tables for all nested
+  /// symbol table operations.
+  SymbolTableCollection symbolTables;
+  /// The symbol table of the top-level operation.
+  SymbolTable &topLevelSymbolTable;
+};
+} // namespace mlir
+
+#endif // MLIR_ANALYSIS_SYMBOLTABLEANALYSIS_H