Fix Decoder bug for constant 0 and DC (#2363) (#2371)

* Fix the QMC bug for constant and dontcare output.
* Fix the Espresso bug for constant and dontcare output.

(cherry picked from commit 46b53320e9ec09fc26d63259f1821f4234640839)

Co-authored-by: Jiuyang Liu <liu@jiuyang.me>

2 files changed, 41 insertions, 19 deletions

diff --git a/src/main/scala/chisel3/util/experimental/decode/EspressoMinimizer.scala b/src/main/scala/chisel3/util/experimental/decode/EspressoMinimizer.scala
index de2f207b..86973e5b 100644
--- a/src/main/scala/chisel3/util/experimental/decode/EspressoMinimizer.scala
+++ b/src/main/scala/chisel3/util/experimental/decode/EspressoMinimizer.scala
@@ -57,11 +57,20 @@ object EspressoMinimizer extends Minimizer with LazyLogging {
     def readTable(espressoTable: String) = {
       def bitPat(espresso: String): BitPat = BitPat("b" + espresso.replace('-', '?'))
 
-      espressoTable
+      val out = espressoTable
         .split("\n")
         .filterNot(_.startsWith("."))
         .map(_.split(' '))
         .map(row => bitPat(row(0)) -> bitPat(row(1)))
+      // special case for 0 and DontCare, if output is not couple to input
+      if (out.isEmpty)
+        Array(
+          (
+            BitPat(s"b${"?" * table.inputWidth}"),
+            BitPat(s"b${"0" * table.outputWidth}")
+          )
+        )
+      else out
     }
 
     val input = writeTable(table)
diff --git a/src/main/scala/chisel3/util/experimental/decode/QMCMinimizer.scala b/src/main/scala/chisel3/util/experimental/decode/QMCMinimizer.scala
index a3481869..26a072f1 100644
--- a/src/main/scala/chisel3/util/experimental/decode/QMCMinimizer.scala
+++ b/src/main/scala/chisel3/util/experimental/decode/QMCMinimizer.scala
@@ -304,24 +304,37 @@ object QMCMinimizer extends Minimizer {
       )
     })
 
-    minimized.tail.foldLeft(table.copy(table = Seq(minimized.head))) {
-      case (tb, t) =>
-        if (tb.table.exists(x => x._1 == t._1)) {
-          tb.copy(table = tb.table.map {
-            case (k, v) =>
-              if (k == t._1) {
-                def ones(bitPat: BitPat) = bitPat.rawString.zipWithIndex.collect { case ('1', x) => x }
-                (
-                  k,
-                  BitPat(
-                    "b" + (0 until v.getWidth).map(i => if ((ones(v) ++ ones(t._2)).contains(i)) "1" else "?").mkString
+    // special case for 0 and DontCare, if output is not couple to input
+    if (minimized.isEmpty)
+      table.copy(
+        Seq(
+          (
+            BitPat(s"b${"?" * table.inputWidth}"),
+            BitPat(s"b${"0" * table.outputWidth}")
+          )
+        )
+      )
+    else
+      minimized.tail.foldLeft(table.copy(table = Seq(minimized.head))) {
+        case (tb, t) =>
+          if (tb.table.exists(x => x._1 == t._1)) {
+            tb.copy(table = tb.table.map {
+              case (k, v) =>
+                if (k == t._1) {
+                  def ones(bitPat: BitPat) = bitPat.rawString.zipWithIndex.collect { case ('1', x) => x }
+                  (
+                    k,
+                    BitPat(
+                      "b" + (0 until v.getWidth)
+                        .map(i => if ((ones(v) ++ ones(t._2)).contains(i)) "1" else "?")
+                        .mkString
+                    )
                   )
-                )
-              } else (k, v)
-          })
-        } else {
-          tb.copy(table = tb.table :+ t)
-        }
-    }
+                } else (k, v)
+            })
+          } else {
+            tb.copy(table = tb.table :+ t)
+          }
+      }
   }
 }