Struct fidget::vm::VmData

pub struct VmData<const N: usize = { u8::MAX as usize }> { /* private fields */ }

A flattened math expression, ready for evaluation or further compilation.

Under the hood, VmData stores two different representations:

• A tape in single static assignment form (SsaTape), which is suitable for use during tape simplification
• A tape in register-allocated form (RegTape), which can be efficiently evaluated or lowered into machine assembly

Example

Consider the expression x + y. The SSA tape will look something like this:

$0 = INPUT 0   // X
$1 = INPUT 1   // Y
$2 = ADD $0 $1 // (X + Y)

This will be lowered into a tape using real (or VM) registers:

r0 = INPUT 0 // X
r1 = INPUT 1 // Y
r0 = ADD r0 r1 // (X + Y)

Note that in this form, registers are reused (e.g. r0 stores both X and X + Y).

We can peek at the internals and see this register-allocated tape:

use fidget::{compiler::RegOp, eval::MathShape, vm::{VmShape, VmData}};

let (sum, ctx) = fidget::rhai::eval("x + y")?;
let data = VmData::<255>::new(&ctx, sum)?;
assert_eq!(data.len(), 3); // X, Y, and (X + Y)

let mut iter = data.iter_asm();
assert_eq!(iter.next().unwrap(), RegOp::Input(0, 0));
assert_eq!(iter.next().unwrap(), RegOp::Input(1, 1));
assert_eq!(iter.next().unwrap(), RegOp::AddRegReg(0, 0, 1));

Implementations

impl<const N: usize> VmData<N>

pub fn new(context: &Context, node: Node) -> Result<Self, Error>

Builds a new tape for the given node

pub fn vars(&self) -> &HashMap<String, u32>

Returns this tape’s mapping of variable names to indexes

pub fn len(&self) -> usize

Returns the length of the internal VM tape

pub fn is_empty(&self) -> bool

Returns true if the internal VM tape is empty

pub fn choice_count(&self) -> usize

Returns the number of choice (min/max) nodes in the tape.

This is required because some evaluators pre-allocate spaces for the choice array.

pub fn slot_count(&self) -> usize

Returns the number of slots used by the inner VM tape

pub fn var_count(&self) -> usize

Returns the number of variables used in this tape

pub fn simplify( &self, choices: &[Choice], workspace: &mut VmWorkspace<N>, tape: VmData<N> ) -> Result<Self, Error>

Simplifies both inner tapes, using the provided choice array

To minimize allocations, this function takes a VmWorkspace and spare VmData; it will reuse those allocations.

pub fn iter_asm(&self) -> impl Iterator<Item = RegOp> + '_

Produces an iterator that visits RegOp values in evaluation order

pub fn pretty_print(&self)

Pretty-prints the inner SSA tape

Trait Implementations

impl<const N: usize> Default for VmData<N>

fn default() -> VmData<N>

Returns the “default value” for a type.