无法访问的is语句(Unreachable is statement)

简介

SpinalHDL将确保 switch 中的所有 is 语句均可访问。

示例

下面的代码：

class TopLevel extends Component {
  val sel = UInt(2 bits)
  val result = UInt(4 bits)
  switch(sel) {
    is(0){ result := 4 }
    is(1){ result := 6 }
    is(2){ result := 8 }
    is(3){ result := 9 }
    is(0){ result := 2 } // Duplicated is statement!
  }
}

会报错：

UNREACHABLE IS STATEMENT in the switch statement at
  ***
  Source file location of the is statement definition via the stack trace
  ***

一个可能的修复方法是：

class TopLevel extends Component {
  val sel = UInt(2 bits)
  val result = UInt(4 bits)
  switch(sel) {
    is(0){ result := 4 }
    is(1){ result := 6 }
    is(2){ result := 8 }
    is(3){ result := 9 }
  }
}