[mlir][CF] Add ub.unreachable canonicalization (llvm#169873)

Basic blocks with only a `ub.unreachable` terminator are unreachable.
This commit adds a canonicalization pattern that folds to `cf.cond_br`
to `cf.br` if one of the destination branches is unreachable.

Author: matthias-springer

mlir/lib/Dialect/ControlFlow/IR/CMakeLists.txt

@@ -12,4 +12,5 @@ add_mlir_dialect_library(MLIRControlFlowDialect
   MLIRControlFlowInterfaces
   MLIRIR
   MLIRSideEffectInterfaces
+  MLIRUBDialect
   )

mlir/lib/Dialect/ControlFlow/IR/ControlFlowOps.cpp

@@ -12,6 +12,7 @@
 #include "mlir/Dialect/Arith/IR/Arith.h"
 #include "mlir/Dialect/Bufferization/IR/BufferDeallocationOpInterface.h"
 #include "mlir/Dialect/Bufferization/IR/BufferizableOpInterface.h"
+#include "mlir/Dialect/UB/IR/UBOps.h"
 #include "mlir/IR/AffineExpr.h"
 #include "mlir/IR/AffineMap.h"
 #include "mlir/IR/Builders.h"
@@ -445,14 +446,45 @@ struct CondBranchTruthPropagation : public OpRewritePattern<CondBranchOp> {
     return success(replaced);
   }
 };
+
+/// If the destination block of a conditional branch contains only
+/// ub.unreachable, unconditionally branch to the other destination.
+struct DropUnreachableCondBranch : public OpRewritePattern<CondBranchOp> {
+  using OpRewritePattern<CondBranchOp>::OpRewritePattern;
+
+  LogicalResult matchAndRewrite(CondBranchOp condbr,
+                                PatternRewriter &rewriter) const override {
+    // If the "true" destination is unreachable, branch to the "false"
+    // destination.
+    Block *trueDest = condbr.getTrueDest();
+    Block *falseDest = condbr.getFalseDest();
+    if (llvm::hasSingleElement(*trueDest) &&
+        isa<ub::UnreachableOp>(trueDest->getTerminator())) {
+      rewriter.replaceOpWithNewOp<BranchOp>(condbr, falseDest,
+                                            condbr.getFalseOperands());
+      return success();
+    }
+
+    // If the "false" destination is unreachable, branch to the "true"
+    // destination.
+    if (llvm::hasSingleElement(*falseDest) &&
+        isa<ub::UnreachableOp>(falseDest->getTerminator())) {
+      rewriter.replaceOpWithNewOp<BranchOp>(condbr, trueDest,
+                                            condbr.getTrueOperands());
+      return success();
+    }
+
+    return failure();
+  }
+};
 } // namespace
 
 void CondBranchOp::getCanonicalizationPatterns(RewritePatternSet &results,
                                                MLIRContext *context) {
   results.add<SimplifyConstCondBranchPred, SimplifyPassThroughCondBranch,
               SimplifyCondBranchIdenticalSuccessors,
               SimplifyCondBranchFromCondBranchOnSameCondition,
-              CondBranchTruthPropagation>(context);
+              CondBranchTruthPropagation, DropUnreachableCondBranch>(context);
 }
 
 SuccessorOperands CondBranchOp::getSuccessorOperands(unsigned index) {

mlir/test/Dialect/ControlFlow/canonicalize.mlir

@@ -634,3 +634,25 @@ func.func @unsimplified_cycle_2(%c : i1) {
 ^bb7:
   cf.br ^bb6
 }
+
+// CHECK-LABEL: @drop_unreachable_branch_1
+//  CHECK-NEXT:   "test.foo"() : () -> ()
+//  CHECK-NEXT:   return
+func.func @drop_unreachable_branch_1(%c: i1) {
+  cf.cond_br %c, ^bb1, ^bb2
+^bb1:
+  "test.foo"() : () -> ()
+  return
+^bb2:
+  ub.unreachable
+}
+
+// CHECK-LABEL: @drop_unreachable_branch_2
+//  CHECK-NEXT:   ub.unreachable
+func.func @drop_unreachable_branch_2(%c: i1) {
+  cf.cond_br %c, ^bb1, ^bb2
+^bb1:
+  ub.unreachable
+^bb2:
+  ub.unreachable
+}