Fixing lint error: x + -1 (#1374)

* Generates lint-clean Verilog for the case: x + -1

...where x is anything and 1 is any literal.

Master behavior:

  input x : SInt<8>
  output z : SInt<9>
  z <= add(x, SInt(-2))

generates

  assign z = $signed(x) + -8'sh2;

After this PR:

  assign z = $signed(x) - 8'sh2;

If the literal is the maximum possible literal, a special case is triggered to properly trim the resulting subtraction.

Input:

  input x : SInt<2>
  output z : SInt<3>
  z <= add(x, SInt(-2))

now generates (after this PR)

  assign z = $signed(x) - 3'sh2;

* Updated documentation

* Change ArrayBuffer to ListBuffer

* Change name to minNegValue

* Remove mutable public interfaces

Co-authored-by: Albert Magyar <albert.magyar@gmail.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

1 files changed, 16 insertions, 2 deletions

diff --git a/src/main/scala/firrtl/transforms/ReplaceTruncatingArithmetic.scala b/src/main/scala/firrtl/transforms/ReplaceTruncatingArithmetic.scala
index 1fe9a723..8aa1553a 100644
--- a/src/main/scala/firrtl/transforms/ReplaceTruncatingArithmetic.scala
+++ b/src/main/scala/firrtl/transforms/ReplaceTruncatingArithmetic.scala
@@ -1,3 +1,5 @@
+// See LICENSE for license details.
+
 package firrtl
 package transforms
 
@@ -24,10 +26,22 @@ object ReplaceTruncatingArithmetic {
     */
   def onExpr(netlist: Netlist)(expr: Expression): Expression =
     expr.map(onExpr(netlist)) match {
+      // If an unsigned wrapping add/sub
       case orig @ DoPrim(Tail, Seq(e), SeqBIOne, tailtpe) =>
         netlist.getOrElse(we(e), e) match {
-          case DoPrim(Add, args, cs, _) => DoPrim(Addw, args, cs, tailtpe)
-          case DoPrim(Sub, args, cs, _) => DoPrim(Subw, args, cs, tailtpe)
+          case DoPrim(Add, args, cs, u: UIntType) => DoPrim(Addw, args, cs, tailtpe)
+          case DoPrim(Sub, args, cs, u: UIntType) => DoPrim(Subw, args, cs, tailtpe)
+          case _ => orig // Not a candidate
+        }
+      // If a signed wrapping add/sub, there should be a cast
+      case orig @ DoPrim(AsSInt, Seq(x), _, casttpe) =>
+        netlist.getOrElse(we(x), x) match {
+          case DoPrim(Tail, Seq(e), SeqBIOne, tailtpe) =>
+            netlist.getOrElse(we(e), e) match {
+              case DoPrim(Add, args, cs, s: SIntType) => DoPrim(Addw, args, cs, casttpe)
+              case DoPrim(Sub, args, cs, s: SIntType) => DoPrim(Subw, args, cs, casttpe)
+              case _ => orig // Not a candidate
+            }
           case _ => orig // Not a candidate
         }
       case other => other // Not a candidate