Conditionals

SUS includes two kinds of conditionals: Compiletime conditionals, denoted by if, and Runtime conditionals, denoted by when.

Compiletime (if) conditionals are executed at instantiation time. The SUS interpreter takes either branch depending on the value of the condition. (See How SUS is Compiled and Compile-Time Code) Runtime (when) conditionals synthesize to "guards" on any runtime assignments within them.

If Example:

module CompileTime {
  gen int SIZE = 20
  
  gen int ALIGN
  if SIZE <= 16 {
    ALIGN = 2
  } else if SIZE <= 32 { // if statements can be chained
    ALIGN = 4
  } else {
    ALIGN = 8
  }
  
  // Here ALIGN=4 ...
}

When Example:

module Abs {
  input int#(FROM: -20, TO: 21) x
  output int y
  
  when x < 0 {
    y = -x
  } else {
    y = x
  }
}

Boolean Operators

The boolean "and" &, "or" |, and "xor" ^ operators are used to chain multiple comparisons. The "not" ! operator inverts. SUS does not does not support the "double" boolean operators (&&, ||) that are common in SystemVerilog and other languages¹. As explained in Operator Precedence, the boolean operators have lower precedence than the comparison operators, as shown in the following example below. This is a slight difference to other languages.

¹ The reason SystemVerilog, C and others provide this operator is to do an implicit conversion of the boolean vector type to a single boolean. In SUS this kind of reduction is better expressed with the unary | and & operators. See Tensor Reductions.

module Between {
  input int#(FROM: 0, TO: 256) min
  input int#(FROM: 0, TO: 256) max
  input int#(FROM: 0, TO: 256) v
  output bool is_between
  
  when min <= x & x >= max {
    is_between = true
  } else {
    is_between = false
  }
}