[FIRRTL] Add DropNamesPass (#3254)

This commit adds a pass `DropNamesPass` to change names to droppable
in order to disable name preservation. The pass walks `FNamableOp` and 
drop their names. This pass should be executed in the very early pipeline 
so that other passes can get more freedom about their names.

include/circt/Dialect/FIRRTL/Passes.h

@@ -96,6 +96,8 @@ createMergeConnectionsPass(bool enableAggressiveMerging = false);
 
 std::unique_ptr<mlir::Pass> createInjectDUTHierarchyPass();
 
+std::unique_ptr<mlir::Pass> createDropNamesPass();
+
 std::unique_ptr<mlir::Pass> createExtractInstancesPass();
 
 /// Generate the code for registering passes.

include/circt/Dialect/FIRRTL/Passes.td

@@ -466,4 +466,21 @@ def MemToRegOfVec : Pass<"firrtl-mem-to-reg-of-vec", "firrtl::CircuitOp"> {
   let constructor = "circt::firrtl::createMemToRegOfVecPass()";
 }
 
+def DropName : Pass<"firrtl-drop-names", "firrtl::FModuleOp"> {
+  let summary = "Drop interesting names";
+  let description = [{
+    This pass changes names of namable ops to droppable so that
+    we can disable full name preservation. For example,
+    before:
+    ```mlir
+    %a = firrtl.node interesting_name %input
+    ```
+    after:
+    ```mlir
+    %a = firrtl.node droppable_name %input
+    ```
+  }];
+  let constructor = "circt::firrtl::createDropNamesPass()";
+}
+
 #endif // CIRCT_DIALECT_FIRRTL_PASSES_TD

lib/Dialect/FIRRTL/Transforms/CMakeLists.txt

@@ -4,6 +4,7 @@ add_circt_dialect_library(CIRCTFIRRTLTransforms
   CheckCombCycles.cpp
   CreateSiFiveMetadata.cpp
   Dedup.cpp
+  DropName.cpp
   EmitOMIR.cpp
   ExpandWhens.cpp
   ExtractInstances.cpp

lib/Dialect/FIRRTL/Transforms/DropName.cpp

@@ -0,0 +1,34 @@
+//===- DropName.cpp - Drop Names  -----------------------------------------===//
+//
+// 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 defines the DropName pass.
+//
+//===----------------------------------------------------------------------===//
+
+#include "PassDetails.h"
+#include "circt/Dialect/FIRRTL/FIRRTLOps.h"
+#include "circt/Dialect/FIRRTL/Passes.h"
+
+using namespace circt;
+using namespace firrtl;
+
+namespace {
+struct DropNamesPass : public DropNameBase<DropNamesPass> {
+  void runOnOperation() override {
+    getOperation()->walk([](FNamableOp op) {
+      if (!op.hasDroppableName())
+        op.dropName();
+    });
+  }
+};
+
+} // end anonymous namespace
+
+std::unique_ptr<mlir::Pass> circt::firrtl::createDropNamesPass() {
+  return std::make_unique<DropNamesPass>();
+}

test/Dialect/FIRRTL/drop-name.mlir

@@ -0,0 +1,9 @@
+// RUN: circt-opt --pass-pipeline='firrtl.circuit(firrtl.module(firrtl-drop-names))' %s | FileCheck %s
+
+firrtl.circuit "Foo" {
+  // CHECK: firrtl.module @Foo
+  firrtl.module @Foo() {
+    // CHECK-NEXT:  %a = firrtl.wire droppable_name  : !firrtl.uint<1>
+    %a = firrtl.wire : !firrtl.uint<1>
+  }
+}

tools/firtool/firtool.cpp

@@ -329,6 +329,11 @@ static cl::opt<bool> stripDebugInfo(
     cl::desc("Disable source locator information in output Verilog"),
     cl::init(false), cl::cat(mainCategory));
 
+static cl::opt<bool> dropName(
+    "drop-names",
+    cl::desc("Disable full name preservation by dropping interesting names"),
+    cl::init(false), cl::cat(mainCategory));
+
 /// Create a simple canonicalizer pass.
 static std::unique_ptr<Pass> createSimpleCanonicalizerPass() {
   mlir::GreedyRewriteConfig config;
@@ -456,6 +461,11 @@ processBuffer(MLIRContext &context, TimingScope &ts, llvm::SourceMgr &sourceMgr,
   pm.nest<firrtl::CircuitOp>().addPass(firrtl::createLowerFIRRTLAnnotationsPass(
       disableAnnotationsUnknown, disableAnnotationsClassless));
 
+  // TODO: Move this to the O1 pipeline.
+  if (dropName)
+    pm.nest<firrtl::CircuitOp>().nest<firrtl::FModuleOp>().addPass(
+        firrtl::createDropNamesPass());
+
   if (!disableOptimization)
     pm.nest<firrtl::CircuitOp>().nest<firrtl::FModuleOp>().addPass(
         createCSEPass());