Add SetDefaultReadUnderWrite transform

* Optionally defines read-under-write behavior for all 'undefined' memories
* Use DefaultReadFirstAnnotation to choose read-first default
* Use DefaultWriteFirstAnnotation to choose write-first default
* Seal DefaultReadUnderWriteAnnotation based on Jack's feedback

1 files changed, 57 insertions, 0 deletions

diff --git a/src/main/scala/firrtl/passes/memlib/SetDefaultReadUnderWrite.scala b/src/main/scala/firrtl/passes/memlib/SetDefaultReadUnderWrite.scala
new file mode 100644
index 00000000..d5646099
--- /dev/null
+++ b/src/main/scala/firrtl/passes/memlib/SetDefaultReadUnderWrite.scala
@@ -0,0 +1,57 @@
+// SPDX-License-Identifier: Apache-2.0
+
+package firrtl.passes
+package memlib
+
+import firrtl._
+import firrtl.ir._
+import firrtl.options.{Dependency, OptionsException}
+import firrtl.annotations.NoTargetAnnotation
+
+sealed trait DefaultReadUnderWriteAnnotation extends NoTargetAnnotation
+
+/** This annotation directs the [[SetDefaultReadUnderWrite]] transform to assign a default value of 'old' (read-first
+  * behavior) to all synchronous-read memories with 'undefined' read-under-write parameters.
+  */
+case object DefaultReadFirstAnnotation extends DefaultReadUnderWriteAnnotation
+
+/** This annotation directs the [[SetDefaultReadUnderWrite]] transform to assign a default value of 'new' (write-first
+  * behavior) to all synchronous-read memories with 'undefined' read-under-write parameters.
+  */
+case object DefaultWriteFirstAnnotation extends DefaultReadUnderWriteAnnotation
+
+/**
+  * Adding a [[DefaultReadUnderWriteAnnotation]] and running the [[SetDefaultReadUnderWrite]] transform will cause all
+  * synchronous-read memories with 'undefined' read-under-write parameters to be assigned a default parameter value,
+  * either 'old' (read-first behavior) or 'new' (write-first behavior). This can help generate Verilog that is amenable
+  * to RAM macro inference for various FPGA tools, or it can be used to satisfy other downstream design constraints.
+  */
+class SetDefaultReadUnderWrite extends Transform with DependencyAPIMigration {
+  override def prerequisites = firrtl.stage.Forms.HighForm
+  override def optionalPrerequisites = Seq(Dependency[InferReadWrite])
+  override def optionalPrerequisiteOf = Seq(Dependency(VerilogMemDelays))
+  override def invalidates(a: Transform): Boolean = false
+
+  private def onStmt(defaultRUW: ReadUnderWrite.Value)(stmt: Statement): Statement = stmt match {
+    case mem: DefMemory if (mem.readLatency > 0 && mem.readUnderWrite == ReadUnderWrite.Undefined) =>
+      mem.copy(readUnderWrite = defaultRUW)
+    case s => s.mapStmt(onStmt(defaultRUW))
+  }
+
+  override def execute(state: CircuitState): CircuitState = {
+    val c = state.circuit
+    val ruwDefaults = state.annotations
+      .collect({
+        case DefaultReadFirstAnnotation  => ReadUnderWrite.Old
+        case DefaultWriteFirstAnnotation => ReadUnderWrite.New
+      })
+      .toSet
+    if (ruwDefaults.size == 0) {
+      state
+    } else if (ruwDefaults.size == 1) {
+      state.copy(circuit = c.copy(modules = c.modules.map(m => m.mapStmt(onStmt(ruwDefaults.head)))))
+    } else {
+      throw new OptionsException("Conflicting default read-under-write settings.")
+    }
+  }
+}