Support literals cast to aggregates as async reset reg init values (#…

…1225)

Accomplished by changing the code gen for casting literals to
aggregates. Rather than connecting the literal to a wire that is then
bit selected from, just bit select from the literal which saves the
creation of an intermediate wire and matches FIRRTL's semantics for
legal async reset initial values.

(cherry picked from commit 92d88ff)

chiselFrontend/src/main/scala/chisel3/Aggregate.scala

@@ -65,7 +65,7 @@ sealed abstract class Aggregate extends Data {
   private[chisel3] override def connectFromBits(that: Bits)(implicit sourceInfo: SourceInfo,
       compileOptions: CompileOptions): Unit = {
     var i = 0
-    val bits = WireDefault(UInt(this.width), that)  // handles width padding
+    val bits = if (that.isLit) that else WireDefault(UInt(this.width), that) // handles width padding
     for (x <- flatten) {
       val fieldWidth = x.getWidth
       if (fieldWidth > 0) {

src/test/scala/chiselTests/AsyncResetSpec.scala

@@ -169,4 +169,36 @@ class AsyncResetSpec extends ChiselFlatSpec {
     assertTesterPasses(new AsyncResetQueueTester)
   }
 
+  it should "allow literals cast to Bundles as reset values" in {
+    class MyBundle extends Bundle {
+      val x = UInt(16.W)
+      val y = UInt(16.W)
+    }
+    assertTesterPasses(new BasicTester {
+      val reg = withReset(reset.asAsyncReset) {
+        RegNext(0xbad0cad0L.U.asTypeOf(new MyBundle), 0xdeadbeefL.U.asTypeOf(new MyBundle))
+      }
+      val (count, done) = Counter(true.B, 4)
+      when (count === 0.U) {
+        chisel3.assert(reg.asUInt === 0xdeadbeefL.U)
+      } .otherwise {
+        chisel3.assert(reg.asUInt === 0xbad0cad0L.U)
+      }
+      when (done) { stop() }
+    })
+  }
+  it should "allow literals cast to Vecs as reset values" in {
+    assertTesterPasses(new BasicTester {
+      val reg = withReset(reset.asAsyncReset) {
+        RegNext(0xbad0cad0L.U.asTypeOf(Vec(4, UInt(8.W))), 0xdeadbeefL.U.asTypeOf(Vec(4, UInt(8.W))))
+      }
+      val (count, done) = Counter(true.B, 4)
+      when (count === 0.U) {
+        chisel3.assert(reg.asUInt === 0xdeadbeefL.U)
+      } .otherwise {
+        chisel3.assert(reg.asUInt === 0xbad0cad0L.U)
+      }
+      when (done) { stop() }
+    })
+  }
 }