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Statements

Conditional (if/elif/else)

Pyrope uses a typical if, elif, else sequence found in most languages. Before the if starts, there is an optional keyword unique that enforces that a single condition is true in the if/elif chain. This is useful for synthesis which allows a parallel mux. The unique is a cleaner way to write an optimize statement.

The if sequence can be used in expressions too.

a = unique if x1 == 1 {
    300
  }elif x2 == 2 {
    400
  }else{
    500
  }

var x = _
if a { x = 3 } else { x = 4 }

The equivalent code with an explicit optimize, but unlike the optimize, the unique will guarantee to generate the hotmux statement.

optimize !(x1==1 and x2==2)
a = if x1 == 1 {
    300
  }elif x2 == 2 {
    400
  }else{
    500
  }

Like several modern programming languages, there can be a list of expressions in the evaluation condition. If variables are declared, they are restricted to the remaining if/else statement blocks.

var tmp = x+1

if var x1=x+1; x1 == tmp {
   puts "x1:{} is the same as tmp:{}", x1, tmp
}elif var x2=x+2; x2 == tmp {
   puts "x1:{} != x2:{} == tmp:{}", x1, x2, tmp
}

Unique parallel conditional (match)

The match statement is similar to a chain of unique if/elif, like the unique if/elif sequence, one of the options in the match must be true. The difference is that one of the entries must be truth or an error is generated. This makes the match statement a replacement for the common "unique parallel case" Verilog directive. The match statement behaves like also having an optimize statement which allows for more efficient code generation than a sequence of if/else.

In addition to functionality, the syntax is different to avoid redundancy. match joins the match expression with the beginning of the matching entry must form a valid expression.

x = 1
match x {
  == 1            { puts "always true" }
  in 2,3          { puts "never"       }
}
// It is equivalent to:
unique if x == 1  { puts "always true" }
elif x in (2,3)   { puts "never"       }
else              { assert false       }

Like the if, it can also be used as an expression.

var hot = match x {
    == 0sb001 { a }
    == 0sb010 { b }
    == 0sb100 { c }
  }

// Equivalent
optimize (x==0sb001 or x==0sb010 or x==0sb100)
var hot2 = __hotmux(x, a, b, c)

assert hot==hot2

Like the if statement, a sequence of statements and declarations are possible in the match statement.

match let one=1 ; one ++ (2) {
  == (1,2) { puts "one:{}", one }      // should always hit
}

Since the == is the most common condition in the match statement, it can be omitted.

for x in 1..=5 {
  let v1 = match x {
    3 { "three" }
    4 { "four" }
    else { "neither"}
  }

  let v2 = match x {
    == 3 { "three" }
    == 4 { "four" }
    else { "neither"}
  }
  cassert v1 == v2
}

Gate statements (when/unless)

A simple statement like assignments, variable declarations, and function calls and returns can be gated or not executed with a when or unless statement. This is similar to an if statement, but the difference is that the statement is in the current scope, not creating a new scope. This allows cleaner more compact syntax.

var a = 3
a += 1 when false             // never executes 
assert a == 3
assert a == 1000 when a > 10  // assert never executed either

reg my = 3 when some_condition  // no register declared otherwise

return "fail" unless success_condition

Complex assignments like a |> b(1) |> c can not be gated because it is not clear if the gated applies to the last call or the whole pipeline sequence. Similarly, gating ifs/match statements do not make much sense. As a result, when/unless can only be applied to assignments, function calls, and code block control statements (return, break, continue).

Code block

A code block is a sequence of statements delimited by { and }. The functionality is the same as in other languages. Variables declared within a code block are not visible outside the code block. In other words, code block variables have scope from definition until the end of the code block.

Code blocks are different from lambdas. A lambda consists of a code block but it has several differences. In lambdas, (1) variables defined in upper scopes are accessed inside as immutable copies only when captured by scope; (2) inputs and outputs could be constrained, and (3) the return statement finishes a lambda not a code block.

The main features of code blocks:

  • Code blocks define a new scope. New variable declarations inside are not visible outside it.

  • Code blocks do not allow variable declaration shadowing.

  • Expressions can have multiple code blocks but they are not allowed to have side-effects for variables outside the code block. The evaluation order provides more details on expressions evaluation order.

  • When used in an expression or lambda, the last statement in the lambda code block can be an expression. It is not needed to add the return keyword in this case.

{
  var x=1
  var z=_
  {
    z = 10
    var x=_           // compiler error, 'x' is a shawdow variable
  }
  assert z == 10 
}
let zz = x            // compile error, `x` is out of scope

var yy = {let x=3 ; 33/3} + 1
assert yy == 12
let xx = {yy=1 ; 33}  // compile error, 'yy' has side effects

if {let a=1+yy; 13<a} {
  // a is not visible in this scope
  some_code()
}

let doit = fun(f,a) {
  let x = f(a)
  assert x == 7
  return 3
}

let z3 = doit(fun(a) { 
  assert a!=0
  return 7             // exist the current lambda
  100                  // never reached statement
}, 33)
cassert z3 == 3

Loop (for)

The for iterates over the first-level elements in a tuple or the values in a range. In all the cases, the number of loop iterations must be known at compile time. The loop exit condition can not be run-time data-dependent.

The loop can have an early exit when calling break and skip of the current iteration with the continue keyword.

for i in 0..<100 {
 some_code(i)
}

var bund = (1,2,3,4)
for (index,i) in bund.enumerate() {
  assert bund[j] == i
}

let b = (a=1,b=3,c=5,7,11)
assert b.keys() == ('a', 'b', 'c', '', '')
assert b.enumerate() == ((0,1), (1,3), (2,5), (3,7), (4,11))
let xx= zip(b.keys(), b.enumerate()) 
cassert xx == (('a',0,a=1), ('b',1,b=3), ('c',2,c=5), ('',3,7), ('',4,11))

for (key,index,i) in zip(keys(b),b.enumerate()) {
  assert i==1  implies (index==0 and key == 'a')
  assert i==3  implies (index==1 and key == 'b')
  assert i==5  implies (index==2 and key == 'c')
  assert i==7  implies (index==3 and key == '' )
  assert i==11 implies (index==4 and key == '' )
}

let c = ((1,a=3), b=4, c=(x=1,y=6))
assert c.enumerate() == ((0,(1,a=3)), (1,b=4), (2,c=(x=1,y=6)))

The for can also be used in an expression that allows building comprehensions to initialize arrays. Pyrope uses a comprehension similar to Julia or Python.

var c = for i in 1..<5 { var xx = i }  // compile error, no expression
var d = i for i in 0..<5 
var e = i for i in 0..<5 if i
assert (0,1,2,3,4) == d
assert e == (1,2,3,4)

The iterating element is copied by value, if the intention is to iterate over a vector or array to modify the contents, a ref must be used. Only the element is mutable. When a ref is used, it must be a variable reference, not a function call return (value). The mutable for can not be used in comprehensions.

b = (1,2,3,4,5)

for x in ref b {
  x += 1
}
assert b == (2,3,4,5,6)

Code block control

Code block control statements allow changing the control flow for lambdas and loop statements (for, loop, and while). return can have a value.

  • return exits or terminates the current lambda. The current output variables are provided as the lambda output. If a tuple is provided, the tuple is the returned value, the output variables are not used.

  • break terminates the closest inner loop (for/while/loop). If none is found, a compile error is generated.

  • continue looks for the closest inner loop (for/while/loop) code block. The continue will perform the next loop iteration. If no inner loop is found, a compile error is generated.

var total:[] = _
for a in 1..=10 {
  continue when a == 2
  total ++= a
  break when a == 3    // exit for scope
}
assert total == (1,3)

if true {
  code(x)
  continue             // compile error, no upper loop scope
}

a = 3
var total2:[] = _
while a>0 {
  total2 ++= a
  break when a == 2    // exit if scope
  a = a - 1
  continue
  assert false         // never executed
}
assert total2 == (3,2)

total = i+10 for i in 1..=9 if i<3
assert total == (11, 12)

while/loop

while cond { [stmts]+ } is a typical while loop found in most programming languages. The only difference is that like with loops, the while must be fully unrolled at compilation time. The loop { [stmts]+ } is equivalent to a while true { [stmts]+ }.

Like if/match, the while condition can have a sequence of statements with variable declarations visible only inside the while statements.

// a do while contruct does not exist, but a loop is quite clean/close

var a = 0
loop {
  puts "a:{}",a

  a += 1

  break unless a < 10 
} // do{ ... }while(a<10)

defer

A defer attribute can be applied to variables. When used to read a variable, it returns the last values written to the variable the end of the current cycle. This is needed if we need to have any loop in connecting blocks. The defer applied to a write, delays the write update to the end of the cycle. The delayed writes happen before the delayed reads. This is also for delaying assertion checks to the end of the cycle like post condition checks.

var c = 10
assert b.[defer] == 33    // behaves like a postcondition
b = c.[defer]
assert b == 33
c += 20
c += 3

To connect the ring function calls in a loop. 

f1 = ring(a, f4.[defer])
f2 = ring(b, f1)
f3 = ring(c, f2)
f4 = ring(d, f3)

If the intention is to read the result after being a flop, there is no need to use the defer, a normal register access could do it. If the read variables are registers, the flop#[0] is not the same as defer. The flop#[0] reads the value before any update, the defer read, gets values after updates.

reg counter:u32 = _

let counter_m1 = counter#[1]  // compile error, #[1] only allowed for debug
let counter_0  = counter#[0]  // current cycle 
let counter_1  = counter#[-1] // last cycle
let counter_2  = counter#[-2] // last last cycle cycle 

var deferred = counter.[defer]

if counter < 100 {
  counter += 1
}else{
  counter = 0
}

if counter == 10 {
  assert deferred   == 10
  assert counter_0  ==  9
  assert counter_1  ==  8
  assert counter_2  ==  7
}

The defer can also be applied to write/updates to the end of the cycle but uses/reads the current value. In a way, the assignment is delayed to the end of the current cycle. If there are many defers to the same variable, they are ordered in program order.

var a = 1
assert a == 1 and a.[defer] == 200

a::[defer] = 100
assert a == 1 and a.[defer] == 200

a::[defer] = 200
assert a == 1 and a.[defer] == 200

If there are defer reads and defer assignments, the defered writes are performed before the defered reads.

var a = 1
var x = 100
x::[defer] = a
a = 200

cassert x == 100
assert x.[defer] == 1

Testing (test)

The test statement requires a text identifier to notify when the test fails. The test is similar to a puts statement followed by a scope (test <str> [,args] { stmts+ }). The statements inside the code block can not have any effect outside. 

test "my test {}", 1 {
  assert true
}

Each test can run in parallel, to increase the throughput, putting the randomization outside the test statement increases the number of tests:

let add = fun(a,b) { a+b }

for i in 0..<10 { // 10 tests
  let a = (-30..<100).rand
  let b = (-30..<100).rand

  test "test {}+{}",a,b {
    assert add(a,b) == (a+b)
  }
}

let add = fun(a,b) { a+b }

test "test 10 additions" {
  for i in 0..<10 { // 10 tests
    let a = (-30..<100).rand
    let b = (-30..<100).rand

    assert add(a,b) == (a+b)
  }
}

Test only statements

test code blocks are allowed to use special statements not available outside testing blocks:

  • step [ncycles] advances the simulation for several cycles. The local variables will preserve the value, the inputs may change value.

  • waitfor condition is a syntax sugar to wait for a condition to be true.

test "wait 1 cycle" {
  let a = 1 + input
  puts "printed every cycle input={}", a
  step 1
  puts "also every cycle a={}",a  // printed on cycle later
}

test "wait 1 cycle" {
  {
    let a = 1 + input
    puts "printed every cycle input={}", a
  } #> {
    puts "also every cycle a={}",a  // printed on cycle later
  }
}

The waitfor command is equivalent to a while with a step.

total = 3

waitfor a_cond  // wait until a_cond is true

assert total == 3 and a_cond

total = 3

while !a_cond {
  step
}

assert total == 3 and a_cond

The main reason for using the step is that the "equivalent" #>[1] is a more structured construct. The step behaves more like a "yield" in that the next call or cycle it will continue from there. The #>[1] directive adds a pipeline structure which means that it can be started each cycle. Calling a lambda that has called a step and still has not finished should result in a simulation assertion failure.

  • peek allows to read any flop, and lambda input or output

  • poke is similar to peek but allows to set a value on any flop and lambda input/output.