[VerilogEmitter] Fix a race with sv.binds inside of hw.modules.

It turns out that sv.bind can exist inside of a hw.module as well
as at the top level.  These cause non-local references so we need
to make sure not to emit such modules concurrently with the modules
being emitted.

There is no good test for this, I noticed it due to a transient
failure of the existing ExportVerilog/sv-dialect.mlir test.

include/circt/Dialect/SV/SVStatements.td

@@ -560,7 +560,7 @@ def BindOp : SVOp<"bind", []> {
   let summary = "indirect instantiation statement";
   let description = [{
     Indirectly instantiate a module in the context of another module. This
-    operation pairs with rtl.instance which tracks all information except the
+    operation pairs with hw.instance which tracks all information except the
     emission point for the bind.  This requires that the parent module of the
     bind exist in the IR. See 23.11 of SV 2017 spec.
   }];

lib/Translation/ExportVerilog/ExportVerilog.cpp

@@ -311,6 +311,7 @@ static StringRef getVerilogDeclWord(Operation *op) {
   bool isProcedural = op->getParentOp()->hasTrait<ProceduralRegion>();
   return isProcedural ? "automatic logic" : "wire";
 }
+
 /// Return the name of a value without using the name map.  This is needed when
 /// looking into an instance from a different module as happens with bind.  It
 /// may return "" when unable to determine a name.  This works in situations
@@ -321,9 +322,13 @@ static StringRef getNameRemotely(Value value,
     return modulePorts[barg.getArgNumber()].getName();
 
   if (auto readinout = dyn_cast<ReadInOutOp>(value.getDefiningOp())) {
-    if (auto wire = dyn_cast<WireOp>(readinout.input().getDefiningOp()))
+    auto *wireInput = readinout.input().getDefiningOp();
+    if (!wireInput)
+      return {};
+
+    if (auto wire = dyn_cast<WireOp>(wireInput))
       return wire.name();
-    if (auto reg = dyn_cast<RegOp>(readinout.input().getDefiningOp()))
+    if (auto reg = dyn_cast<RegOp>(wireInput))
       return reg.name();
   }
   if (auto localparam = dyn_cast<LocalParamOp>(value.getDefiningOp()))
@@ -2909,7 +2914,7 @@ void ModuleEmitter::emitBind(BindOp op) {
       if (shouldPrintComma)
         os << ',';
     }
-    os << "\n";
+    os << '\n';
 
     // Emit the port's name.
     indent();
@@ -2940,7 +2945,7 @@ void ModuleEmitter::emitBind(BindOp op) {
     }
   }
   if (!isFirst) {
-    os << "\n";
+    os << '\n';
     indent();
   }
   os << ");\n";
@@ -3207,6 +3212,10 @@ struct SharedEmitterState {
   // Emitter options extracted from the top-level module.
   const LoweringOptions options;
 
+  /// This is a set is populated at "gather" time, containing the hw.module
+  /// operations that have a sv.bind in them.
+  SmallPtrSet<Operation *, 8> modulesContainingBinds;
+
   explicit SharedEmitterState(ModuleOp rootOp)
       : rootOp(rootOp), options(rootOp) {}
   void gatherFiles(bool separateModules);
@@ -3226,11 +3235,17 @@ struct SharedEmitterState {
 void SharedEmitterState::gatherFiles(bool separateModules) {
 
   /// Collect all the instance symbols from the specified module and add them to
-  /// the IRCache.  Instances only exist at the top level of the module.
-  auto collectInstanceSymbols = [&](HWModuleOp moduleOp) {
-    for (auto op : moduleOp.getBodyBlock()->getOps<InstanceOp>()) {
-      if (auto sym = op.sym_nameAttr())
-        symbolCache.addDefinition(sym, op);
+  /// the IRCache.  Instances only exist at the top level of the module.  Also
+  /// keep track of any modules that contain bind operations.  These are
+  /// non-hierarchical references which we need to be careful about during
+  /// emission.
+  auto collectInstanceSymbolsAndBinds = [&](HWModuleOp moduleOp) {
+    for (Operation &op : *moduleOp.getBodyBlock()) {
+      if (auto instance = dyn_cast<InstanceOp>(op))
+        if (auto sym = instance.sym_nameAttr())
+          symbolCache.addDefinition(sym, instance);
+      if (isa<BindOp>(op))
+        modulesContainingBinds.insert(moduleOp);
     }
   };
 
@@ -3290,7 +3305,7 @@ void SharedEmitterState::gatherFiles(bool separateModules) {
         .Case<HWModuleOp>([&](auto mod) {
           // Build the IR cache.
           symbolCache.addDefinition(mod.getNameAttr(), mod);
-          collectInstanceSymbols(mod);
+          collectInstanceSymbolsAndBinds(mod);
 
           // Emit into a separate file named after the module.
           if (attr || separateModules)
@@ -3432,9 +3447,9 @@ void SharedEmitterState::emitOps(EmissionList &thingsToEmit, raw_ostream &os,
       return; // Ignore things that are already strings.
 
     // BindOp emission reaches into the hw.module of the instance, and that
-    // body may be being transformed by its own emission.  Defer this to the
-    // serial phase.  They are speedy anyway.
-    if (isa<BindOp>(op))
+    // body may be being transformed by its own emission.  Defer their emission
+    // to the serial phase.  They are speedy to emit anyway.
+    if (isa<BindOp>(op) || modulesContainingBinds.count(op))
       return;
 
     SmallString<256> buffer;