Add dontTouch for annotating Data to not be removed

2 files changed, 64 insertions, 0 deletions

diff --git a/src/main/scala/chisel3/package.scala b/src/main/scala/chisel3/package.scala
index ac4e5441..1899b2ec 100644
--- a/src/main/scala/chisel3/package.scala
+++ b/src/main/scala/chisel3/package.scala
@@ -321,6 +321,8 @@ package object chisel3 {    // scalastyle:ignore package.object.name
     val withClock = chisel3.core.withClock
     val withReset = chisel3.core.withReset
 
+    val dontTouch = chisel3.core.dontTouch
+
     type BaseModule = chisel3.core.BaseModule
     type MultiIOModule = chisel3.core.ImplicitModule
     type RawModule = chisel3.core.UserModule
diff --git a/src/test/scala/chiselTests/DontTouchSpec.scala b/src/test/scala/chiselTests/DontTouchSpec.scala
new file mode 100644
index 00000000..6cd2c54d
--- /dev/null
+++ b/src/test/scala/chiselTests/DontTouchSpec.scala
@@ -0,0 +1,62 @@
+// See LICENSE for license details.
+
+package chiselTests
+
+import chisel3._
+import chisel3.experimental.dontTouch
+import firrtl.{FirrtlExecutionSuccess, Transform}
+
+class HasDeadCodeChild(withDontTouch: Boolean) extends Module {
+  val io = IO(new Bundle {
+    val a = Input(UInt(32.W))
+    val b = Output(UInt(32.W))
+    val c = Output(Vec(2, UInt(32.W)))
+  })
+  io.b := io.a
+  if (withDontTouch) {
+    dontTouch(io.c)
+  }
+}
+
+class HasDeadCode(withDontTouch: Boolean) extends Module {
+  val io = IO(new Bundle {
+    val a = Input(UInt(32.W))
+    val b = Output(UInt(32.W))
+  })
+  val inst = Module(new HasDeadCodeChild(withDontTouch))
+  inst.io.a := io.a
+  io.b := inst.io.b
+  val dead = Wire(init = io.a + 1.U)
+  if (withDontTouch) {
+    dontTouch(dead)
+  }
+}
+
+class DontTouchSpec extends ChiselFlatSpec {
+  val deadSignals = List(
+    "io_c_0",
+    "io_c_1",
+    "dead"
+  )
+  "Dead code" should "be removed by default" in {
+    val verilog = compile(new HasDeadCode(false))
+    for (signal <- deadSignals) {
+      verilog should not include (signal)
+    }
+  }
+  it should "NOT be removed if marked dontTouch" in {
+    val verilog = compile(new HasDeadCode(true))
+    for (signal <- deadSignals) {
+      verilog should include (signal)
+    }
+  }
+  "Dont touch" should "only work on bound hardware" in {
+    a [chisel3.core.Binding.BindingException] should be thrownBy {
+      compile(new Module {
+        val io = IO(new Bundle { })
+        dontTouch(new Bundle { val a = UInt(32.W) } )
+      })
+    }
+  }
+}
+