Optimize BitPat factory from UInt literals (#3988)

src/main/scala/chisel3/util/BitPat.scala

@@ -98,8 +98,9 @@ object BitPat {
     */
   def apply(x: UInt): BitPat = {
     require(x.isLit, s"$x is not a literal, BitPat.apply(x: UInt) only accepts literals")
-    val len = if (x.isWidthKnown) x.getWidth else 0
-    apply("b" + x.litValue.toString(2).reverse.padTo(len, "0").reverse.mkString)
+    val width = x.getWidth.max(1) // BitPat doesn't support zero-width
+    val mask = (BigInt(1) << width) - 1
+    new BitPat(x.litValue, mask, width)
   }
 
   implicit class fromUIntToBitPatComparable(x: UInt) extends SourceInfoDoc {

src/test/scala/chiselTests/util/BitPatSpec.scala

@@ -2,6 +2,7 @@
 
 package chiselTests.util
 
+import chisel3._
 import chisel3.util.BitPat
 import _root_.circt.stage.ChiselStage
 import org.scalatest.flatspec.AnyFlatSpec
@@ -49,4 +50,13 @@ class BitPatSpec extends AnyFlatSpec with Matchers {
     b(4, 3) should be(BitPat("b01"))
     b(6, 6) should be(BitPat("b1"))
   }
+
+  it should "parse UInt literals correctly" in {
+    BitPat(0.U) should be(new BitPat(0, 1, 1))
+    // Note that this parses as 1-bit width, there are other APIs that don't support zero-width UInts correctly
+    BitPat(0.U(0.W)) should be(new BitPat(0, 1, 1))
+    BitPat(1.U) should be(new BitPat(1, 1, 1))
+    BitPat(2.U) should be(new BitPat(2, 3, 2))
+    BitPat(0xdeadbeefL.U) should be(new BitPat(BigInt("deadbeef", 16), BigInt("ffffffff", 16), 32))
+  }
 }