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Unreachable is statement

Introduction

SpinalHDL will check to ensure that all is statements in a switch are reachable.

Example

The following code:

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!
  }
}

will throw:

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

A fix could be:

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 }
  }
}