TruthTable improvements: structural equality and delete sort (backport #2935) (#2936)

* TruthTable improvements: structural equality and delete sort (#2935)

We had implemented equals, but not hashCode. This commit also changes
the implemental of equals to just use the underlying values instead of
wasting the compute calling .toString.

Delete TruthTable.sort, it is unused.

(cherry picked from commit b5d9c08)

* Restore and deprecate TruthTable.sort

Co-authored-by: Jack Koenig <koenig@sifive.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

Author: mergify[bot]

src/main/scala/chisel3/util/experimental/decode/TruthTable.scala

@@ -3,29 +3,35 @@
 package chisel3.util.experimental.decode
 
 import chisel3.util.BitPat
+import scala.util.hashing.MurmurHash3
 import scala.collection.mutable
 
-sealed class TruthTable private (val table: Seq[(BitPat, BitPat)], val default: BitPat, val sort: Boolean) {
+sealed class TruthTable private (val table: Seq[(BitPat, BitPat)], val default: BitPat, _sort: Boolean) {
   def inputWidth = table.head._1.getWidth
 
   def outputWidth = table.head._2.getWidth
 
+  @deprecated("This field is unused and will be removed.", "Chisel 3.5")
+  def sort: Boolean = _sort
+
   override def toString: String = {
     def writeRow(map: (BitPat, BitPat)): String =
       s"${map._1.rawString}->${map._2.rawString}"
 
     (table.map(writeRow) ++ Seq(s"${" " * (inputWidth + 2)}${default.rawString}")).mkString("\n")
   }
 
-  def copy(table: Seq[(BitPat, BitPat)] = this.table, default: BitPat = this.default, sort: Boolean = this.sort) =
-    TruthTable(table, default, sort)
+  def copy(table: Seq[(BitPat, BitPat)] = this.table, default: BitPat = this.default, sort: Boolean = _sort) =
+    TruthTable(table, default)
 
   override def equals(y: Any): Boolean = {
     y match {
-      case y: TruthTable => toString == y.toString
+      case that: TruthTable => this.table == that.table && this.default == that.default
       case _ => false
     }
   }
+
+  override lazy val hashCode: Int = MurmurHash3.productHash((table, default))
 }
 
 object TruthTable {

src/test/scala/chiselTests/util/experimental/TruthTableSpec.scala

@@ -35,7 +35,9 @@ class TruthTableSpec extends AnyFlatSpec {
     assert(table.toString contains "     0")
   }
   "TruthTable" should "deserialize" in {
-    assert(TruthTable.fromString(str) == table)
+    val table2 = TruthTable.fromString(str)
+    assert(table2 === table)
+    assert(table2.hashCode === table.hashCode)
   }
   "TruthTable" should "merge same key" in {
     assert(