## 组合环(Combinatorial loop)

### 一、简介

SpinalHDL会检查设计中没有组合逻辑环的存在。

### 二、例子

下述代码：
```Scala
class TopLevel extends Component {
  val a = UInt(8 bits) // PlayDev.scala line 831
  val b = UInt(8 bits) // PlayDev.scala line 832
  val c = UInt(8 bits)
  val d = UInt(8 bits)

  a := b
  b := c | d
  d := a
  c := 0
}
```

会报错：
```
COMBINATORIAL LOOP :
  Partial chain :
    >>> (toplevel/a :  UInt[8 bits]) at ***(PlayDev.scala:831) >>>
    >>> (toplevel/d :  UInt[8 bits]) at ***(PlayDev.scala:834) >>>
    >>> (toplevel/b :  UInt[8 bits]) at ***(PlayDev.scala:832) >>>
    >>> (toplevel/a :  UInt[8 bits]) at ***(PlayDev.scala:831) >>>

  Full chain :
    (toplevel/a :  UInt[8 bits])
    (toplevel/d :  UInt[8 bits])
    (UInt | UInt)[8 bits]
    (toplevel/b :  UInt[8 bits])
    (toplevel/a :  UInt[8 bits])
```

可能的修复方式为:
```Scala
class TopLevel extends Component {
  val a = UInt(8 bits) // PlayDev.scala line 831
  val b = UInt(8 bits) // PlayDev.scala line 832
  val c = UInt(8 bits)
  val d = UInt(8 bits)

  a := b
  b := c | d
  d := 42
  c := 0
}
```

### 三、假阳性(false-positives)

SpinalHDL检测组合逻辑环的策略是悲观的, 因此可能出现误报的情况。假如发生了误报, 用户可以无效化对某一信号的环检测, 如：
```Scala
class TopLevel extends Component {
  val a = UInt(8 bits)
  a := 0
  a(1) := a(0) // 因为这行而发生了误报
}
```
可以被修复为：
```Scala
class TopLevel extends Component {
  val a = UInt(8 bits).noCombLoopCheck
  a := 0
  a(1) := a(0)
}
```

还应该指出, 诸如(a(1):= a(0))这样的赋值会使Verilator等工具无法匹配。在这种情况下, 使用Vec(Bool, 8)可能会更好。