# Combinatorial loop¶

## Introduction¶

SpinalHDL will check that there are no combinatorial loops in the design.

## Example¶

The following code:

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
}

will throw :

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])

A possible fix could be:

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
}

## False-positives¶

It should be said that SpinalHDL’s algorithm to detect combinatorial loops can be pessimistic, and it may give false positives. If it is giving a false positive, you can manually disable loop checking on one signal of the loop like so:

class TopLevel extends Component {
val a = UInt(8 bits)
a := 0
a(1) := a(0) // False positive because of this line
}

could be fixed by :

class TopLevel extends Component {
val a = UInt(8 bits).noCombLoopCheck
a := 0
a(1) := a(0)
}

It should also be said that assignments such as (a(1) := a(0)) can make some tools like Verilator unhappy. It may be better to use a Vec(Bool, 8) in this case.