[FIRRTL] Add GrandCentral no-bind option

Add an option to the GrandCentral pass that will _not_ bind in the
companion module and will _not_ emit the SV interface instantiation.
This is done to enable GrandCentral collateral to be synthesizable.
This is motivated by making all the assertions that are frequently
crammed into a Grand Central companion as eligible for synthesis.

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

include/circt/Dialect/FIRRTL/Passes.h

@@ -102,7 +102,8 @@ std::unique_ptr<mlir::Pass> createPrintNLATablePass();
 std::unique_ptr<mlir::Pass>
 createBlackBoxReaderPass(std::optional<mlir::StringRef> inputPrefix = {});
 
-std::unique_ptr<mlir::Pass> createGrandCentralPass();
+std::unique_ptr<mlir::Pass>
+createGrandCentralPass(bool instantiateCompanionOnly = false);
 
 std::unique_ptr<mlir::Pass> createCheckCombCyclesPass();
 

include/circt/Dialect/FIRRTL/Passes.td

@@ -420,6 +420,10 @@ def GrandCentral : Pass<"firrtl-grand-central", "CircuitOp"> {
 
   let constructor = "circt::firrtl::createGrandCentralPass()";
   let dependentDialects = ["circt::sv::SVDialect", "circt::hw::HWDialect"];
+  let options = [
+    Option<"instantiateCompanionOnly", "instantiate-companion-only", "bool", "false",
+      "Instantiate the companion without a bind and drop the interface">
+  ];
   let statistics = [
     Statistic<"numViews", "num-views-created",
       "Number of top-level SystemVerilog interfaces that were created">,

lib/Dialect/FIRRTL/Transforms/GrandCentral.cpp

@@ -577,6 +577,10 @@ struct InterfaceElemsBuilder {
 ///    instantiate interfaces and to generate the "mappings" file that produces
 ///    cross-module references (XMRs) to drive the interface.
 struct GrandCentralPass : public GrandCentralBase<GrandCentralPass> {
+  GrandCentralPass(bool instantiateCompanionOnlyFlag) {
+    instantiateCompanionOnly = instantiateCompanionOnlyFlag;
+  }
+
   void runOnOperation() override;
 
 private:
@@ -1116,7 +1120,8 @@ parseAugmentedType(ApplyState &state, DictionaryAttr augmentedType,
     NamedAttrList attrs;
     attrs.append("class", classAttr);
     attrs.append("name", name);
-    auto value = tryGetAs<Attribute>(augmentedType, root, "value", loc, clazz, path);
+    auto value =
+        tryGetAs<Attribute>(augmentedType, root, "value", loc, clazz, path);
     if (!value)
       return std::nullopt;
     attrs.append("value", value);
@@ -1890,7 +1895,11 @@ void GrandCentralPass::runOnOperation() {
                 return true;
               }
 
-              (*instance)->setAttr("lowerToBind", builder.getUnitAttr());
+              // Lower the companion to a bind unless the user told us
+              // explicitly not to.
+              if (!instantiateCompanionOnly)
+                (*instance)->setAttr("lowerToBind", builder.getUnitAttr());
+
               (*instance)->setAttr(
                   "output_file",
                   hw::OutputFileAttr::getFromFilename(
@@ -2024,6 +2033,14 @@ void GrandCentralPass::runOnOperation() {
     }
   });
 
+  // If we are in "instantiateCompanionOnly" mode, then just exit here.  We
+  // don't need to create the interface.  This is a janky hack for situations
+  // where you want to synthesize assertion logic included in the companion, but
+  // don't want to have a dead interface hanging around (or have problems with
+  // tools understanding interfaces).
+  if (instantiateCompanionOnly)
+    return;
+
   // Now, iterate over the worklist of interface-encoding annotations to create
   // the interface and all its sub-interfaces (interfaces that it instantiates),
   // instantiate the top-level interface, and generate a "mappings file" that
@@ -2250,6 +2267,7 @@ void GrandCentralPass::runOnOperation() {
 // Pass Creation
 //===----------------------------------------------------------------------===//
 
-std::unique_ptr<mlir::Pass> circt::firrtl::createGrandCentralPass() {
-  return std::make_unique<GrandCentralPass>();
+std::unique_ptr<mlir::Pass>
+circt::firrtl::createGrandCentralPass(bool instantiateCompanionOnly) {
+  return std::make_unique<GrandCentralPass>(instantiateCompanionOnly);
 }