[clang] [CIR] Upstream cir-canonicalize pass (PR #131891)

[Erich Keane via cfe-commits]

================
@@ -0,0 +1,147 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 canonicalizes CIR operations, eliminating
+// redundant branches, empty scopes, and other unnecessary operations.
+//
+//===----------------------------------------------------------------------===//
+
+#include "PassDetail.h"
+#include "mlir/Dialect/Func/IR/FuncOps.h"
+#include "mlir/IR/Block.h"
+#include "mlir/IR/Operation.h"
+#include "mlir/IR/PatternMatch.h"
+#include "mlir/IR/Region.h"
+#include "mlir/Support/LogicalResult.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 {
+
+/// Removes branches between two blocks if it is the only branch.
+///
+/// From:
+///   ^bb0:
+///     cir.br ^bb1
+///   ^bb1:  // pred: ^bb0
+///     cir.return
+///
+/// To:
+///   ^bb0:
+///     cir.return
+struct RemoveRedundantBranches : public OpRewritePattern<BrOp> {
+  using OpRewritePattern<BrOp>::OpRewritePattern;
+
+  LogicalResult matchAndRewrite(BrOp op,
+                                PatternRewriter &rewriter) const final {
+    Block *block = op.getOperation()->getBlock();
+    Block *dest = op.getDest();
+
+    assert(!cir::MissingFeatures::labelOp());
+
+    // Single edge between blocks: merge it.
+    if (block->getNumSuccessors() == 1 &&
+        dest->getSinglePredecessor() == block) {
+      rewriter.eraseOp(op);
+      rewriter.mergeBlocks(dest, block);
+      return success();
+    }
+
+    return failure();
+  }
+};
+
+struct RemoveEmptyScope
+    : public OpRewritePattern<ScopeOp>::SplitMatchAndRewrite {
+  using SplitMatchAndRewrite::SplitMatchAndRewrite;
+
+  LogicalResult match(ScopeOp op) const final {
+    // TODO: Remove this logic once CIR uses MLIR infrastructure to remove
+    // trivially dead operations
+    if (op.isEmpty()) {
+      return success();
+    }
+
+    Region *region = &(op.getScopeRegion()); // getRegions().front();
----------------
erichkeane wrote:

This is VERY suspicious.  Address-of a thing resulting in nullptr?  I think we'd better ensure that there IS a scope region, then make `region` here a reference instead.

https://github.com/llvm/llvm-project/pull/131891