[clang] [CIR] Upstream initial support for CIR flattening (PR #130648)

[Bruno Cardoso Lopes via cfe-commits]

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@@ -0,0 +1,114 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements pass that inlines CIR operations regions into the parent
+// function region.
+//
+//===----------------------------------------------------------------------===//
+
+#include "PassDetail.h"
+#include "mlir/Dialect/Func/IR/FuncOps.h"
+#include "mlir/IR/PatternMatch.h"
+#include "mlir/Support/LogicalResult.h"
+#include "mlir/Transforms/DialectConversion.h"
+#include "mlir/Transforms/GreedyPatternRewriteDriver.h"
+#include "clang/CIR/Dialect/IR/CIRDialect.h"
+#include "clang/CIR/Dialect/Passes.h"
+#include "clang/CIR/MissingFeatures.h"
+
+using namespace mlir;
+using namespace cir;
+
+namespace {
+
+struct CIRFlattenCFGPass : public CIRFlattenCFGBase<CIRFlattenCFGPass> {
+
+  CIRFlattenCFGPass() = default;
+  void runOnOperation() override;
+};
+
+class CIRScopeOpFlattening : public mlir::OpRewritePattern<cir::ScopeOp> {
+public:
+  using OpRewritePattern<cir::ScopeOp>::OpRewritePattern;
+
+  mlir::LogicalResult
+  matchAndRewrite(cir::ScopeOp scopeOp,
+                  mlir::PatternRewriter &rewriter) const override {
+    mlir::OpBuilder::InsertionGuard guard(rewriter);
+    mlir::Location loc = scopeOp.getLoc();
+
+    // Empty scope: just remove it.
+    // TODO: Remove this logic once CIR uses MLIR infrastructure to remove
+    // trivially dead operations
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bcardosolopes wrote:

Context for other reviewers: MLIR canonicalizer is too aggressive and we need to either (a) make sure all our ops are modeling side-effects and/or (b) have more options in the canonicalizer in MLIR to temper aggressiveness level.

https://github.com/llvm/llvm-project/pull/130648