[hw] Add hw.donttouch operation

include/circt/Dialect/HW/HWStructure.td

@@ -14,6 +14,7 @@
 #define CIRCT_DIALECT_HW_HWSTRUCTURE_TD
 
 include "circt/Dialect/HW/HWAttributes.td"
+include "circt/Dialect/HW/HWAttributesNaming.td"
 include "circt/Dialect/HW/HWDialect.td"
 include "circt/Dialect/HW/HWOpInterfaces.td"
 include "circt/Dialect/HW/HWTypes.td"
@@ -653,6 +654,24 @@ def HierPathOp : HWOp<"hierpath",
     }];
 }
 
+def DontTouchOp : HWOp<"donttouch",
+                       [DeclareOpInterfaceMethods<InnerRefUserOpInterface>,
+                        HasParent<"ModuleOp">]> {
+  let summary = "A user of an inner ref that blocks removal";
+  let description = [{
+    An operation that marks an InnerRef as having external users.  The operation
+    with the InnerRef should not be optimized in any way and may be used
+    externally by name.
+
+    This operation roughly implements the notion of "don't touch" that many
+    Verilog tools understand.
+  }];
+  let arguments = (ins InnerRefAttr:$ref);
+  let assemblyFormat = [{
+    $ref attr-dict
+  }];
+}
+
 // Edge behavior for trigger blocks. Currently these map 1:1 to SV event
 // control kinds.
 

lib/Dialect/HW/HWOps.cpp

@@ -1762,6 +1762,24 @@ LogicalResult OutputOp::verify() {
   return success();
 }
 
+//===----------------------------------------------------------------------===//
+// DontTouchOp
+//===----------------------------------------------------------------------===//
+
+LogicalResult DontTouchOp::verifyInnerRefs(hw::InnerRefNamespace &ns) {
+  auto ref = getRef();
+  if (ns.lookup(getRef()))
+    return success();
+  auto diag = emitOpError()
+              << "references non-existent inner symbol '" << ref << "'";
+  auto moduleName = ref.getModule();
+  if (auto *moduleOp = ns.symTable.lookup(moduleName))
+    return diag.attachNote(moduleOp->getLoc())
+           << "the inner symbol should exist in the body of this operation";
+  return diag.attachNote() << "no operation with symbol '@"
+                           << moduleName.getValue() << "' exists";
+}
+
 //===----------------------------------------------------------------------===//
 // Other Operations
 //===----------------------------------------------------------------------===//

test/Dialect/HW/basic.mlir

@@ -270,3 +270,9 @@ hw.module @aggregate_const(out o : !hw.array<1x!seq.clock>) {
   %0 = hw.aggregate_constant [#seq<clock_constant high> : !seq.clock] : !hw.array<1x!seq.clock>
   hw.output %0 : !hw.array<1x!seq.clock>
 }
+
+// CHECK-LABEL: hw.donttouch <@DontTouch::@a>
+hw.donttouch <@DontTouch::@a>
+hw.module public @DontTouch() {
+  %0 = sv.wire sym @a : !hw.inout<i1>
+}

test/Dialect/HW/cse.mlir

@@ -0,0 +1,9 @@
+// RUN: circt-opt -cse %s | FileCheck %s
+
+// CSE should not result in the deletion of `hw.donttouch`.
+//
+// CHECK-LABEL: hw.donttouch <@Foo::@a>
+hw.donttouch <@Foo::@a>
+hw.module public @Foo() {
+  %a = sv.wire sym @a : !hw.inout<i1>
+}

test/Dialect/HW/verify-irn.mlir

@@ -41,3 +41,16 @@ hw.module @XMRRefD() {
 hw.module @XMRRefOp() {
   hw.instance_choice "foo" sym @foo option "bar" @XMRRefA or @XMRRefB if "B" or @XMRRefC if "C"() -> ()
 }
+
+// -----
+
+// expected-error @below {{references non-existent inner symbol '#hw.innerNameRef<@Foo::@a>'}}
+// expected-note @below {{no operation with symbol '@Foo' exists}}
+hw.donttouch <@Foo::@a>
+
+// -----
+
+// expected-error @below {{references non-existent inner symbol '#hw.innerNameRef<@Foo::@a>'}}
+hw.donttouch <@Foo::@a>
+// expected-note @below {{the inner symbol should exist in the body of this operation}}
+hw.module public @Foo() {}