toBigInt should round identically for Double and BigDecimal (#3921)

---------

Co-authored-by: Schuyler Eldridge <schuyler.eldridge@gmail.com>
Co-authored-by: Jack Koenig <koenig@sifive.com>
(cherry picked from commit b9f5703)

core/src/main/scala/chisel3/Num.scala

@@ -5,6 +5,7 @@ package chisel3
 import chisel3.internal.sourceinfo.SourceInfoTransform
 
 import scala.language.experimental.macros
+import scala.math.BigDecimal.RoundingMode.{HALF_UP, RoundingMode}
 import chisel3.experimental.SourceInfo
 
 // REVIEW TODO: Further discussion needed on what Num actually is.
@@ -199,15 +200,24 @@ trait NumObject {
     result
   }
 
-  /**
-    * How to create a bigint from a big decimal with a specific binaryPoint (int)
-    * @param x           a BigDecimal value
-    * @param binaryPoint a binaryPoint that you would like to use
+  /** Create a bigint from a big decimal with a specific binaryPoint (Int)
+    *
+    * @param x           the BigDecimal value
+    * @param binaryPoint the binaryPoint to use
+    * @return
+    */
+  def toBigInt(x: BigDecimal, binaryPoint: Int): BigInt = toBigInt(x, binaryPoint, HALF_UP)
+
+  /** Create a bigint from a big decimal with a specific binaryPoint (Int)
+    *
+    * @param x           the BigDecimal value
+    * @param binaryPoint the binaryPoint to use
+    * @param roundingMode the RoundingMode to use
     * @return
     */
-  def toBigInt(x: BigDecimal, binaryPoint: Int): BigInt = {
+  def toBigInt(x: BigDecimal, binaryPoint: Int, roundingMode: RoundingMode): BigInt = {
     val multiplier = math.pow(2, binaryPoint)
-    val result = (x * multiplier).rounded.toBigInt
+    val result = (x * multiplier).setScale(0, roundingMode).toBigInt
     result
   }
 

src/test/scala/chiselTests/LiteralExtractorSpec.scala

@@ -59,6 +59,20 @@ class LiteralExtractorSpec extends ChiselFlatSpec {
     bigIntFromDouble should be(bigInt53)
   }
 
+  "doubles and big decimals" should "be rounded identically" in {
+
+    for (double <- Seq(1.0, 1.1, 1.5, 1.6, 2.5)) {
+
+      val bigDecimal = BigDecimal(double)
+
+      val bigIntFromDouble = Num.toBigInt(double, 0)
+      val bigIntFromBigDecimal = Num.toBigInt(bigDecimal, 0)
+
+      bigIntFromDouble should be(bigIntFromBigDecimal)
+    }
+
+  }
+
   "literals declared outside a builder context" should "compare with those inside builder context" in {
     class InsideBundle extends Bundle {
       val x = SInt(8.W)