[FIRRTL] Erase empty layerblocks

Add a canonicalizer that will erase empty `firrtl.layerblock` ops.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>

Author: seldridge

include/circt/Dialect/FIRRTL/FIRRTLStatements.td

@@ -403,6 +403,7 @@ def LayerBlockOp : FIRRTLOp<
     $layerName $region attr-dict
   }];
   let hasVerifier = 1;
+  let hasCanonicalizeMethod = 1;
 }
 
 #endif // CIRCT_DIALECT_FIRRTL_FIRRTLSTATEMENTS_TD

lib/Dialect/FIRRTL/FIRRTLFolds.cpp

@@ -3410,3 +3410,19 @@ LogicalResult FPGAProbeIntrinsicOp::canonicalize(FPGAProbeIntrinsicOp op,
   rewriter.eraseOp(op);
   return success();
 }
+
+//===----------------------------------------------------------------------===//
+// Layer Block Op
+//===----------------------------------------------------------------------===//
+
+LogicalResult LayerBlockOp::canonicalize(LayerBlockOp op,
+                                         PatternRewriter &rewriter) {
+
+  // If the layerblock is empty, erase it.
+  if (op.getBody()->empty()) {
+    rewriter.eraseOp(op);
+    return success();
+  }
+
+  return failure();
+}

test/Dialect/FIRRTL/canonicalization.mlir

@@ -2,6 +2,8 @@
 
 firrtl.circuit "Casts" {
 
+firrtl.layer @A bind {}
+
 // CHECK-LABEL: firrtl.module @Casts
 firrtl.module @Casts(in %ui1 : !firrtl.uint<1>, in %si1 : !firrtl.sint<1>,
     in %clock : !firrtl.clock, in %asyncreset : !firrtl.asyncreset,
@@ -3588,4 +3590,10 @@ firrtl.class @PropertyArithmetic(in %in: !firrtl.integer, out %out0: !firrtl.int
   firrtl.propassign %out0, %3 : !firrtl.integer
   firrtl.propassign %out1, %4 : !firrtl.integer
 }
+
+// CHECK-LABEL: firrtl.module private @LayerBlocks
+firrtl.module private @LayerBlocks() {
+  // CHECK-NOT: firrtl.layerblock @A
+  firrtl.layerblock @A {}
+}
 }