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DSLX Built-In Functions and Standard Library

This page documents the DSLX built-in functions (i.e. functions that don't require an import and are available in the top level namespace) and standard library modules (i.e. which are imported with an unqualified name like import std).

Generally built-in functions have some capabilities that cannot be easily done in a user-defined function, and thus do not live in the standard library.


This section describes the built-in functions provided for use in the DSL that do not need to be explicitly imported.

NOTE: A brief note on "Parallel Primitives": the DSL is expected to grow additional support for use of high-level parallel primitives over time, adding operators for order-insensitive reductions, scans, groupings, and similar. By making these operations known to the compiler in their high level form, we potentially enable optimizations and analyses on their higher level ("lifted") form. As of now, map is the sole parallel-primitive-oriented built-in.


Operation that produces the result of the add, as well as the carry bit as an output. The binary add operators works similar to software programming languages, preserving the length of the input operands, so this built-in can assist when easy access to the carry out value is desired. Has the following signature:

fn add_with_carry<N>(x: uN[N], y: uN[N]) -> (u1, uN[N])

widening_cast and checked_cast

widening_cast and checked_cast cast bits-type values to bits-type values with additional checks compared to casting with as.

widening_cast will report a static error if the type casted to is unable to respresent all values of the type casted from (ex. widening_cast<u5>(s3:0) will fail because s3:-1 cannot be respresented as an unsigned number).

checked_cast will cause a runtime error during dslx interpretation if the value being casted is unable to fit within the type casted to (ex. checked_cast<u5>(s3:0) will succeed while checked_cast<u5>(s3:-1) will cause the dslx interpreter to fail.

Currently both widening_cast and checked_cast will lower into a normal IR cast and will not generate additional assertions at the IR or Verilog level.

fn widening_cast<sN[N]>(value: uN[M]) -> sN[N]; where N > M
fn widening_cast<sN[N]>(value: sN[M]) -> sN[N]; where N >= M
fn widening_cast<uN[N]>(value: uN[M]) -> uN[N]; where N >= M

fn checked_cast<xN[M]>(value: uN[N]) -> xN[M]
fn checked_cast<xN[M]>(value: sN[N]) -> xN[M]

smulp and umulp

smulp and umulp perform signed and unsigned partial multiplications. These operations return a two-element tuple with the property that the sum of the two elements is equal to the product of the original inputs. Performing a partial multiplication allows for a pipeline stage in the middle of a multiply. These operations have the following signatures:

fn smulp<N>(lhs: sN[N], rhs: sN[N]) -> (sN[N], sN[N])
fn umulp<N>(lhs: uN[N], rhs: uN[N]) -> (uN[N], uN[N])


map, similarly to other languages, executes a transformation function on all the elements of an original array to produce the resulting "mapped' array. For example: taking the absolute value of each element in an input array:

import std;

fn main(x: s3[3]) -> s3[3] {
  let y: s3[3] = map(x, std::abs);

fn main_test() {
  let got: s3[3] = main(s3[3]:[-1, 1, 0]);
  assert_eq(s3[3]:[1, 1, 0], got)

Note that map is special, in that we can pass it a callee as if it were a value. As a function that "takes" a function as an argument, map is a special built-in -- in language implementor parlance it is a higher order function.

Implementation note: Functions are not first class values in the DSL, so the name of the function must be referred to directly.

Note: Novel higher order functions (e.g. if a user wanted to write their own map) cannot currently be written in user-level DSL code.


array_rev reverses the elements of an array.

fn test_array_rev() {
  assert_eq(array_rev(u8[1]:[42]), u8[1]:[42]);
  assert_eq(array_rev(u3[2]:[1, 2]), u3[2]:[2, 1]);
  assert_eq(array_rev(u3[3]:[2, 3, 4]), u3[3]:[4, 3, 2]);
  assert_eq(array_rev(u4[3]:[0xf, 0, 0]), u4[3]:[0, 0, 0xf]);
  assert_eq(array_rev(u3[0]:[]), u3[0]:[]);


Performs a check on a constant expression that only fails at compile-time (not at runtime).

Note that this can only be applied to compile-time-constant expressions.

fn test_const_assert() {
  const N = u32:42;
  const_assert!(N >= u32:1<<5);

Keep in mind that all const_assert!s in a function evaluate, similar to static_assert in C++ -- they are effectively part of the type system, so you cannot suppress them by putting them in a conditional:

fn f() {
  if false {
    const_assert!(false);  // <-- still fails even inside the "if false"

clz, ctz

DSLX provides the common "count leading zeroes" and "count trailing zeroes" functions:

  let x0 = u32:0x0FFFFFF8;
  let x1 = clz(x0);
  let x2 = ctz(x0);
  assert_eq(u32:4, x1);
  assert_eq(u32:3, x2)


Converts a binary-encoded value into a one-hot value. For an operand value of n``interpreted as an unsigned number, then-th result bit and only then`-th result bit is set. Has the following signature:

fn decode<uN[W]>(x: uN[N]) -> uN[W]

The width of the decode operation may be less than the maximum value expressible by the input (2**N - 1). If the encoded operand value is larger than the number of bits of the result, the result is zero.

Example usage: dslx/tests/decode.x.

See also the IR semantics for the decode op.


Converts a one-hot value to a binary-encoded value of the "hot" bit of the input. If the n-th bit and only the n-th bit of the operand is set, the result is equal to the value n as an unsigned number. Has the following signature:

fn encode(x: uN[N]) -> uN[ceil(log2(N))]

If multiple bits of the input are set, the result is equal to the logical or of the results produced by the input bits individually. For example, if bit 3 and bit 5 of an encode input are set the result is equal to 3 | 5 = 7.

Example usage: dslx/tests/encode.x.

See also the IR semantics for the encode op.


Converts a value to one-hot form. Has the following signature:

fn one_hot<N, NP1=N+1>(x: uN[N], lsb_is_prio: bool) -> uN[NP1]

When lsb_is_prio is true, the least significant bit that is set becomes the one-hot bit in the result. When it is false, the most significant bit that is set becomes the one-hot bit in the result.

When all bits in the input are unset, the additional bit present in the output value (MSb) becomes set.

Example usage: dslx/tests/one_hot.x.

See also the IR semantics for the one_hot op.


Produces the result of 'or'-ing all case values for which the corresponding bit of the selector is enabled. In cases where the selector has exactly one bit set (it is in one-hot form) this is equivalent to a match.

fn one_hot_sel<N, M>(selector: uN[N], cases: uN[N][M]) -> uN[N]

Evaluates each case value and ors each case together if the corresponding bit in the selector is set. The first element of cases is included if the LSB is set, the second if the next least significant bit and so on. If no selector bits are set this evaluates to zero. This function is not generally used directly though the compiler will when possible synthesize the equivalent code from a match expression. This is included for testing purposes and for bespoke 'intrinsic-style programming' use cases.


Casting has well-defined extension rules, but in some cases it is necessary to be explicit about sign-extensions, if just for code readability. For this, there is the signex built-in.

To invoke the signex built-in, provide it with the operand to sign extend (lhs), as well as the target type to extend to: these operands may be either signed or unsigned. Note that the value of the right hand side is ignored, only its type is used to determine the result type of the sign extension.

fn test_signex() {
  let x = u8:0xff;
  let s: s32 = signex(x, s32:0);
  let u: u32 = signex(x, u32:0);
  assert_eq(s as u32, u)

Note that both s and u contain the same bits in the above example.


Array-slice built-in operation. Note that the "want" argument is not used as a value, but is just used to reflect the desired slice type. (Prior to constexprs being passed to built-in functions, this was the canonical way to reflect a constexpr in the type system.) Has the following signature:

fn slice<T: type, N, M, S>(xs: T[N], start: uN[M], want: T[S]) -> T[S]


rev is used to reverse the bits in an unsigned bits value. The LSb in the input becomes the MSb in the result, the 2nd LSb becomes the 2nd MSb in the result, and so on.

// (Dummy) wrapper around reverse.
fn wrapper<N: u32>(x: bits[N]) -> bits[N] {

// Target for IR conversion that works on u3s.
fn main(x: u3) -> u3 {

// Reverse examples.
fn test_reverse() {
  assert_eq(u3:0b100, main(u3:0b001));
  assert_eq(u3:0b001, main(u3:0b100));
  assert_eq(bits[0]:0, rev(bits[0]:0));
  assert_eq(u1:1, rev(u1:1));
  assert_eq(u2:0b10, rev(u2:0b01));
  assert_eq(u2:0b00, rev(u2:0b00));


bit_slice_update(subject, start, value) returns a copy of the bits-typed value subject where the contiguous bits starting at index start (where 0 is the least-significant bit) are replaced with value. The bit-width of the returned value is the same as the bit-width of subject. Any updated bit indices which are out of bounds (if start + bit-width(value) >= bit-width(subject)) are ignored. Example usage: dslx/tests/bit_slice_update.x.

bit-wise reductions: and_reduce, or_reduce, xor_reduce

These are unary reduction operations applied to a bits-typed value:

  • and_reduce: evaluates to bool:1 if all bits of the input are set, and 0 otherwise.
  • or_reduce: evaluates to bool:1 if any bit of the input is set, and 0 otherwise.
  • xor_reduce: evaluates to bool:1 if there is an odd number of bits set in the input, and 0 otherwise.

These functions return the identity element of the respective operation for trivial (0 bit wide) inputs:

fn test_trivial_reduce() {
  assert_eq(and_reduce(bits[0]:0), true);
  assert_eq(or_reduce(bits[0]:0), false);
  assert_eq(xor_reduce(bits[0]:0), false);


update(array, index, new_value) returns a copy of array where array[index] has been replaced with new_value, and all other elements are unchanged. Note that this is not an in-place update of the array, it is an "evolution" of array. It is the compiler's responsibility to optimize by using mutation instead of copying, when it's safe to do. The compiler makes a best effort to do this, but can't guarantee the optimization is always made.

assert_eq, assert_lt

In a unit test pseudo function all valid DSLX code is allowed. To evaluate test results DSLX provides the assert_eq primitive (we'll add more of those in the future). Here is an example of a divceil implementation with its corresponding tests:

fn divceil(x: u32, y: u32) -> u32 {
  (x-u32:1) / y + u32:1

fn test_divceil() {
  assert_eq(u32:3, divceil(u32:5, u32:2));
  assert_eq(u32:2, divceil(u32:4, u32:2));
  assert_eq(u32:2, divceil(u32:3, u32:2));
  assert_eq(u32:1, divceil(u32:2, u32:2));

assert_eq cannot currently be synthesized into equivalent Verilog. Because of that it is recommended to use it within test constructs (interpretation) only.


DSLX has a macro for easy creation of zero values, even from aggregate types. Invoke the macro with the type parameter as follows:

struct MyPoint {
  x: u32,
  y: u32,

enum MyEnum : u2 {
  ZERO = u2:0,
  ONE = u2:1,

fn test_zero_macro() {
  assert_eq(zero!<u32>(), u32:0);
  assert_eq(zero!<MyPoint>(), MyPoint{x: u32:0, y: u32:0});
  assert_eq(zero!<MyEnum>(), MyEnum::ZERO);

The zero!<T> macro can also be used with the struct update syntax to initialize a subset of fields to zero. In the example below all fields except foo are initialized to zero in the struct returned by f.

struct MyStruct {
  foo: u1,
  bar: u2,
  baz: u3,
  bat: u4,

fn f() -> MyStruct {
  MyStruct{foo: u1:1,!<MyStruct>()}


DSLX supports printf-style debugging via the trace_fmt! builtin, which allows dumping of current values to stdout. For example:

// Note: to see `trace_fmt!` output you need to be seeing `INFO` level logging,
// enabled by adding the '--alsologtostderr' flag to the command line (among
// other means). For example:
// bazel run -c opt //xls/dslx:interpreter_main  /path/to/dslx/file.x -- --alsologtostderr

fn shifty(x: u8, y: u3) -> u8 {
  trace_fmt!("x: {:x} y: {}", x, y);
  // Note: y looks different as a negative number when the high bit is set.
  trace_fmt!("y as s8: {}", y as s2);
  x << y

fn test_shifty() {
  assert_eq(shifty(u8:0x42, u3:4), u8:0x20);
  assert_eq(shifty(u8:0x42, u3:7), u8:0);

would produce the following output, with each trace being annotated with its corresponding source position:

[ RUN UNITTEST  ] test_shifty
I0510 14:31:17.516227 1247677] x: 42 y: 4
I0510 14:31:17.516227 1247677] y as s8: 4
I0510 14:31:17.516227 1247677] x: 42 y: 7
I0510 14:31:17.516227 1247677] y as s8: -1
[            OK ]

Note: trace! currently exists as a builtin but is in the process of being removed, as it provided the user with only a "global flag" way of specifying the desired format for output values -- trace_fmt! is more powerful.

fail!: assertion failure

The fail! builtin indicates dataflow that should not be occurring in practice. Its general signature is:

fail!(label, fallback_value)

The fail! builtin can be thought of as a "fatal assertion macro". It is used to annotate dataflow that should not occur in practice and, if triggered, should raise a fatal error in simulation (e.g. via a JIT-execution failure status or a Verilog assertion when running in RTL simulation).

Note, however, that XLS will permit users to avoid inserting fatal-error-signaling hardware that correspond to this fail! -- assuming it will not be triggered in practice minimizes its cost in synthesized form. In this situation, when it is "erased", it acts as the identity function, propagating the fallback_value. This allows XLS to keep well defined semantics even when fatal assertion hardware is not present.

Example: if only these two enum values shown should be possible (say, as a documented precondition for main):

enum EnumType: u2 {
  FIRST = 0,
  SECOND = 1,

fn main(x: EnumType) -> u32 {
  match x {
    EnumType::FIRST => u32:0,
    EnumType::SECOND => u32:1,
    _ => fail!("unknown_EnumType", u32:0),

The fail!("unknown_EnumType", u32:0) above indicates that a) that match arm should not be reached (and if it is in the JIT or RTL simulation it will cause an error status or assertion failure respectively), but b) provides a fallback value to use (of the appropriate type) in case it were to happen in synthesized gates which did not insert fatal-error-indicating hardware.

The associated label (the first argument) must be a valid Verilog identifier and is used for identifying the failure when lowered to SystemVerilog. At higher levels in the stack, it's unused.


NOTE: Currently, cover! has no effect in RTL simulators supported in XLS open source (i.e. iverilog). See google/xls#436.

The cover! builtin tracks how often some condition is satisfied. It desugars into SystemVerilog cover points. Its signature is:

cover!(<name>, <condition>);

Where name is a function-unique literal string identifying the coverpoint and condition is a boolean element. When condition is true, a counter with the given name is incremented that can be inspected upon program termination. Coverpoints can be used to give an indication of code "coverage", i.e. to see what paths of a design are exercised in practice. The name of the coverpoint must begin with either a letter or underscore, and its remainder must consist of letters, digits, underscores, or dollar signs.


The gate! built-in is used for operand gating, of the form:

let gated_value = gate!(<pass_value>, <value>);

This will generally use a special Verilog macro to avoid the underlying synthesis tool doing boolean optimization, and will turn gated_value to 0 when the predicate pass_value is false. This can be used in attempts to manually avoid toggles based on the gating predicate.

It is expected that XLS will grow facilities to inserting gating ops automatically, but manual user insertion is a practical step in this direction. Additionally, it is expected that if, in the resulting Verilog, gating occurs on a value that originates from a flip flop, the operand gating may be promoted to register-based load-enable gating.

import std

Bits Type Properties


pub fn unsigned_min_value<N: u32>() -> uN[N]
pub fn signed_min_value<N: u32>() -> sN[N]

Returns the minimum signed or unsigned value contained in N bits.


pub fn unsigned_max_value<N: u32>() -> uN[N];
pub fn signed_max_value<N: u32>() -> sN[N];

Returns the maximum signed or unsigned value contained in N bits.


pub fn sizeof_unsigned<N: u32>(x : uN[N]) -> u32
pub fn sizeof_signed<N: u32>(x : sN[N]) -> u32

Returns the number of bits (sizeof) of unsigned or signed bit value.

Bit Manipulation Functions

std::to_signed & std::to_unsigned

pub fn to_signed<N: u32>(x: uN[N]) -> sN[N]
pub fn to_unsigned<N: u32>(x: sN[N]) -> uN[N] {

Convenience helper that converts an unsigned bits argument to its same-sized signed type, or visa-versa. This is morally equivalent to (but slightly more convenient than) a pattern like:

    x as sN[std::sizeof_unsigned(x)]


pub fn lsb<N: u32>(x: uN[N]) -> u1

Extracts the LSB (Least Significant Bit) from the value x and returns it.


pub fn convert_to_bits_msb0<N: u32>(x: bool[N]) -> uN[N]

Converts an array of N bools to a bits[N] value.

Note well: the boolean value at index 0 of the array becomes the most significant bit in the resulting bit value. Similarly, the last index of the array becomes the least significant bit in the resulting bit value.

import std;

fn convert_to_bits_test() {
  let _ = assert_eq(u3:0b001, std::convert_to_bits(bool[3]:[false, false, true]));
  let _ = assert_eq(u3:0b100, std::convert_to_bits(bool[3]:[true, false, false]));

There's always a source of confusion in these orderings:

  • Mathematically we often indicate the least significant digit as "digit 0"
  • But, in a number as we write the digits from left-to-right on a piece of paper, if you made an array from the written characters, the digit at "array index 0" would be the most significant bit.

So, it's somewhat ambiguous whether "index 0" in the array would become the least significant bit or the most significant bit. This routine uses the "as it looks on paper" conversion; e.g. [true, false, false] becomes 0b100.


pub fn fn convert_to_bools_lsb0<N:u32>(x: uN[N]) -> bool[N]

Convert a "word" of bits to a corresponding array of booleans.

Note well: The least significant bit of the word becomes index 0 in the array.


pub fn mask_bits<X: u32>() -> bits[X]

Returns a value with X bits set (of type bits[X]).


pub fn concat3<X: u32, Y: u32, Z: u32, R: u32 = X + Y + Z>(x: bits[X], y: bits[Y], z: bits[Z]) -> bits[R]

Concatenates 3 values of arbitrary bitwidths to a single value.


pub fn rrot<N: u32>(x: bits[N], y: bits[N]) -> bits[N]

Rotate x right by y bits.


pub fn popcount<N: u32>(x: bits[N]) -> bits[N]

Counts the number of bits in x that are '1'.


pub fn extract_bits<from_inclusive: u32, to_exclusive: u32, fixed_shift: u32,
                    N: u32>(x : bits[N]) -> bits[std::max(0, to_exclusive - from_inclusive)] {
    let x_extended = x as uN[max(unsigned_sizeof(x) + fixed_shift, to_exclusive)];
    (x_extended << fixed_shift)[from_inclusive:to_exclusive]

Extracts a bit-slice from x shifted left by fixed_shift. This function behaves as-if x as resonably infinite precision so that the shift does not drop any bits and that the bit slice will be in-range.

If to_exclusive <= from_excsuive, the result will be a zero-bit bits[0].


pub fn vslice<MSB: u32, LSB: u32>(x: bits[IN]) -> bits[OUT]

Similar to extract_bits above, but corresponds directly to the "part-select" Verilog syntax. That is:

y <= x[3:0];

Corresponds to:

let y: u4 = vslice<u32:3, u32:0>(x);

This is useful when porting code literally as a way to avoid transcription errors. For new code the DSLX first-class slicing syntax (either range-slicing or width-slicing) is preferred.

Mathematical Functions


pub fn bounded_minus_1<N: u32>(x: uN[N]) -> uN[N]

Returns the value of x - 1 with saturation at 0.


pub fn abs<BITS: u32>(x: sN[BITS]) -> sN[BITS]

Returns the absolute value of x as a signed number.


pub fn is_pow2<N: u32>(x: uN[N]) -> bool

Returns true when x is a non-zero power-of-two.


pub fn uadd<N: u32, M: u32, R: u32 = umax(N,M) + 1>(x: uN[N], y: uN[M]) -> uN[R]
pub fn sadd<N: u32, M: u32, R: u32 = umax(N,M) + 1>(x: sN[N], y: sN[M]) -> sN[R]

Returns sum of x (N bits) and y (M bits) as a umax(N,M)+1 bit value.


pub fn umul<N: u32, M: u32, R: u32 = N + M>(x: uN[N], y: uN[M]) -> uN[R]
pub fn smul<N: u32, M: u32, R: u32 = N + M>(x: sN[N], y: sN[M]) -> sN[R]

Returns product of x (N bits) and y (M bits) as an N+M bit value.


pub fn iterative_div<N: u32, DN: u32 = N * u32:2>(x: uN[N], y: uN[N]) -> uN[N]

Calculate x / y one bit at a time. This is an alternative to using the division operator '/' which may not synthesize nicely.


pub fn div_pow2<N: u32>(x: bits[N], y: bits[N]) -> bits[N]

Returns x / y where y must be a non-zero power-of-two.


pub fn mod_pow2<N: u32>(x: bits[N], y: bits[N]) -> bits[N]

Returns x % y where y must be a non-zero power-of-two.


pub fn ceil_div<N: u32>(x: uN[N], y: uN[N]) -> uN[N]

Returns the ceiling of (x divided by y).


pub fn round_up_to_nearest(x: u32, y: u32) -> u32

Returns x rounded up to the nearest multiple of y.


pub fn upow<N: u32>(x: uN[N], n: uN[N]) -> uN[N]
pub fn spow<N: u32>(x: sN[N], n: uN[N]) -> sN[N]

Performs integer exponentiation as in Hacker's Delight, Section 11-3. Only non-negative exponents are allowed, hence the uN parameter for spow.


pub fn clog2<N: u32>(x: bits[N]) -> bits[N]

Returns ceiling(log2(x)), with one exception: When x = 0, this function differs from the true mathematical function: clog2(0) = 0 where as ceil(log2(0)) = -infinity

This function is frequently used to calculate the number of bits required to represent x possibilities. With this interpretation, it is sensible to define clog2(0) = 0.

Example: clog2(7) = 3.


pub fn flog2<N: u32>(x: bits[N]) -> bits[N]

Returns floor(log2(x)), with one exception:

When x=0, this function differs from the true mathematical function: flog2(0) = 0 where as floor(log2(0)) = -infinity

This function is frequently used to calculate the number of bits required to represent an unsigned integer n to define flog2(0) = 0, so that flog(n)+1 represents the number of bits needed to represent the n.

Example: flog2(7) = 2, flog2(8) = 3.


pub fn smax<N: u32>(x: sN[N], y: sN[N]) -> sN[N]
pub fn umax<N: u32>(x: uN[N], y: uN[N]) -> uN[N]

Returns the maximum of two integers.


pub fn smin<N: u32>(x: sN[N], y: sN[N]) -> sN[N]
pub fn umin<N: u32>(x: uN[N], y: uN[N]) -> uN[N]

Returns the minimum of two unsigned integers.


pub fn uadd_with_overflow<V: u32>(x: uN[N], y: uN[M]) -> (bool, uN[V])

Returns a 2-tuple indicating overflow (boolean) and a sum (x + y) as uN[V]. An overflow occurs if the result does not fit within a uN[V].


pub fn umul_with_overflow<V: u32>(x: uN[N], y: uN[M]) -> (bool, uN[V])

Returns a 2-tuple indicating overflow (boolean) and a product (x * y) as uN[V]. An overflow occurs if the result does not fit within a uN[V].

Misc Functions

Signed comparison - std::{sge, sgt, sle, slt}

pub fn sge<N: u32>(x: uN[N], y: uN[N]) -> bool
pub fn sgt<N: u32>(x: uN[N], y: uN[N]) -> bool
pub fn sle<N: u32>(x: uN[N], y: uN[N]) -> bool
pub fn slt<N: u32>(x: uN[N], y: uN[N]) -> bool

Explicit signed comparison helpers for working with unsigned values, can be a bit more convenient and a bit more explicit intent than doing casting of left hand side and right hand side.


pub fn find_index<BITS: u32, ELEMS: u32>( array: uN[BITS][ELEMS], x: uN[BITS]) -> (bool, u32)

Returns (found, index) given an array and the element to find within the array.

Note that when found is false, the index is 0 -- 0 is provided instead of a value like -1 to prevent out-of-bounds accesses from occurring if the index is used in a match expression (which will eagerly evaluate all of its arms), to prevent it from creating an error at simulation time if the value is ultimately discarded from the unselected match arm.

import acm_random

Port of ACM random number generator to DSLX.

DO NOT use acm_random.x for any application where security -- unpredictability of subsequent output and previous output -- is needed. ACMRandom is in NO WAY a cryptographically secure pseudorandom number generator, and using it where recipients of its output may wish to guess earlier/later output values would be very bad.


pub fn rng_deterministic_seed() -> u32

Returns a fixed seed for use in the random number generator.


pub fn rng_new(seed: u32) -> State

Create the state for a new random number generator using the given seed.


pub fn rng_next(s: State) -> (State, u32)

Returns a pseudo-random number in the range [1, 2^31-2].

Note that this is one number short on both ends of the full range of non-negative 32-bit integers, which range from 0 to 2^31-1.


pub fn rng_next(s: State) -> (State, u64)

Returns a pseudo random number in the range [1, (2^31-2)^2].

Note that this does not cover all non-negative values of int64, which range from 0 to 2^63-1. The top two bits are ALWAYS ZERO.