Optimize .readOnly to only create views when necessary (#4271) (#4273)

(cherry picked from commit 478eaef)

Co-authored-by: Jack Koenig <koenig@sifive.com>

Author: mergify[bot]

core/src/main/scala/chisel3/Data.scala

@@ -1015,7 +1015,12 @@ object Data {
       * This Data this method is called on must be a hardware type.
       */
     def readOnly(implicit sourceInfo: SourceInfo): T = {
-      self.viewAsReadOnly(_ => "Cannot connect to read-only value")
+      val alreadyReadOnly = self.isLit || self.topBindingOpt.exists(_.isInstanceOf[ReadOnlyBinding])
+      if (alreadyReadOnly) {
+        self
+      } else {
+        self.viewAsReadOnly(_ => "Cannot connect to read-only value")
+      }
     }
   }
 }

src/test/scala/chiselTests/ReadOnlySpec.scala

@@ -9,6 +9,8 @@ import chisel3.probe._
 import chisel3.properties._
 import chisel3.experimental.SourceInfo
 import chisel3.experimental.dataview._
+import chisel3.experimental.BundleLiterals._
+import chisel3.experimental.VecLiterals._
 
 import org.scalactic.source.Position
 
@@ -364,4 +366,31 @@ class ReadOnlySpec extends ChiselFlatSpec with Utils {
       })
     }
   }
+
+  it should "NOT create a view for literals" in {
+    ChiselStage.emitCHIRRTL(new RawModule {
+      val a = 123.U
+      val b = -27.S
+      val c = (new BiggerBundle).Lit(_.fizz -> 123.U, _.buzz -> 456.U)
+      val d = Vec.Lit(0.U, 23.U)
+      // Use referential equality to check they are the same objects (no view introduced)
+      assert(a.readOnly eq a)
+      assert(b.readOnly eq b)
+      assert(c.readOnly eq c)
+      assert(d.readOnly eq d)
+    })
+  }
+
+  it should "NOT create a view for op results" in {
+    ChiselStage.emitCHIRRTL(new RawModule {
+      val a = IO(Input(UInt(8.W)))
+      val b = IO(Input(new BiggerBundle))
+
+      val x = a + 1.U
+      val y = b.asUInt
+      // Use referential equality to check they are the same objects (no view introduced)
+      assert(x.readOnly eq x)
+      assert(y.readOnly eq y)
+    })
+  }
 }