Preserve UInt and SInt literals across .pad (backport #4156) (#4157)

* Preserve UInt and SInt literals across .pad (#4156)

(cherry picked from commit 65a05b2)

* Waive false binary compatibility issue

---------

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

build.sbt

@@ -386,7 +386,9 @@ lazy val unipublish =
         // chisel3.HasTarget is sealed
         ProblemFilters.exclude[ReversedMissingMethodProblem]("chisel3.package#HasTarget.suggestName"),
         // chisel3.internal.firrtl.ir.LitArg is package private
-        ProblemFilters.exclude[ReversedMissingMethodProblem]("chisel3.internal.firrtl.ir#LitArg.cloneWithValue")
+        ProblemFilters.exclude[ReversedMissingMethodProblem]("chisel3.internal.firrtl.ir#LitArg.cloneWithValue"),
+        // chisel3.Bits is sealed
+        ProblemFilters.exclude[ReversedMissingMethodProblem]("chisel3.Bits._padLit")
       ),
       // Forward doc command to unidoc
       Compile / doc := (ScalaUnidoc / doc).value,

core/src/main/scala/chisel3/Bits.scala

@@ -293,9 +293,13 @@ sealed abstract class Bits(private[chisel3] val width: Width) extends Element wi
     */
   final def pad(that: Int): this.type = macro SourceInfoTransform.thatArg
 
+  // Pad literal to that width
+  protected def _padLit(that: Int): this.type
+
   /** @group SourceInfoTransformMacro */
   def do_pad(that: Int)(implicit sourceInfo: SourceInfo): this.type = this.width match {
     case KnownWidth(w) if w >= that => this
+    case _ if this.isLit            => this._padLit(that)
     case _                          => binop(sourceInfo, cloneTypeWidth(this.width.max(Width(that))), PadOp, that)
   }
 
@@ -453,6 +457,11 @@ sealed class UInt private[chisel3] (width: Width) extends Bits(width) with Num[U
   private[chisel3] override def cloneTypeWidth(w: Width): this.type =
     new UInt(w).asInstanceOf[this.type]
 
+  override protected def _padLit(that: Int): this.type = {
+    val value = this.litValue
+    UInt.Lit(value, this.width.max(Width(that))).asInstanceOf[this.type]
+  }
+
   // TODO: refactor to share documentation with Num or add independent scaladoc
   /** Unary negation (expanding width)
     *
@@ -812,6 +821,11 @@ sealed class SInt private[chisel3] (width: Width) extends Bits(width) with Num[S
   private[chisel3] override def cloneTypeWidth(w: Width): this.type =
     new SInt(w).asInstanceOf[this.type]
 
+  override protected def _padLit(that: Int): this.type = {
+    val value = this.litValue
+    SInt.Lit(value, this.width.max(Width(that))).asInstanceOf[this.type]
+  }
+
   /** Unary negation (constant width)
     *
     * @return a hardware $coll equal to zero minus this $coll

src/test/scala/chiselTests/ProbeSpec.scala

@@ -537,10 +537,9 @@ class ProbeSpec extends ChiselFlatSpec with Utils {
       Array("--full-stacktrace")
     )
     (processChirrtl(chirrtl) should contain).allOf(
-      "node _T = pad(UInt<7>(0h7b), 16)",
-      "force_initial(p, _T)",
-      "node _T_2 = pad(in, 16)",
-      "force(clock, _T_1, p, _T_2)"
+      "force_initial(p, UInt<16>(0h7b))",
+      "node _T_1 = pad(in, 16)",
+      "force(clock, _T, p, _T_1)"
     )
   }
 

src/test/scala/chiselTests/SIntOps.scala

@@ -230,4 +230,22 @@ class SIntOpsSpec extends ChiselPropSpec with Utils {
     3.S.asSInt.litValue should be(3)
     -5.S.asSInt.litValue should be(-5)
   }
+
+  property("Calling .pad on a SInt literal should maintain the literal value") {
+    -5.S.getWidth should be(4)
+    -5.S.pad(2).litValue should be(-5)
+    -5.S.pad(2).getWidth should be(4)
+    -5.S.pad(4).litValue should be(-5)
+    -5.S.pad(4).getWidth should be(4)
+    -5.S.pad(6).litValue should be(-5)
+    -5.S.pad(6).getWidth should be(6)
+
+    -5.S(8.W).getWidth should be(8)
+    -5.S(8.W).pad(2).litValue should be(-5)
+    -5.S(8.W).pad(2).getWidth should be(8)
+    -5.S(8.W).pad(8).litValue should be(-5)
+    -5.S(8.W).pad(8).getWidth should be(8)
+    -5.S(8.W).pad(16).litValue should be(-5)
+    -5.S(8.W).pad(16).getWidth should be(16)
+  }
 }

src/test/scala/chiselTests/UIntOps.scala

@@ -512,4 +512,22 @@ class UIntOpsSpec extends ChiselPropSpec with Matchers with Utils {
     0.U.zext.widthOption should be(Some(2))
     0.U(0.W).zext.widthOption should be(Some(1))
   }
+
+  property("Calling .pad on a UInt literl should maintain the literal value") {
+    5.U.getWidth should be(3)
+    5.U.pad(2).litValue should be(5)
+    5.U.pad(2).getWidth should be(3)
+    5.U.pad(3).litValue should be(5)
+    5.U.pad(3).getWidth should be(3)
+    5.U.pad(4).litValue should be(5)
+    5.U.pad(4).getWidth should be(4)
+
+    5.U(8.W).getWidth should be(8)
+    5.U(8.W).pad(2).litValue should be(5)
+    5.U(8.W).pad(2).getWidth should be(8)
+    5.U(8.W).pad(8).litValue should be(5)
+    5.U(8.W).pad(8).getWidth should be(8)
+    5.U(8.W).pad(16).litValue should be(5)
+    5.U(8.W).pad(16).getWidth should be(16)
+  }
 }