Enum Quality 

pub enum Quality {
    Unknown,
    Guessed,
    Adjusted,
    Precise,
}

How much a probability or a frequency is worth believing.

The order is the point, and it is GCC’s order out of enum profile_quality in gcc/profile-count.h: worse first, so the quality of a computed value is the smaller of what went into it and Ord says so without a table.

Variants

Unknown

Nobody has said anything about this one.

What a block a pass created gets until the pass says otherwise. It is not zero and it is not one, it is the absence of a claim, and a consumer that treats it as either is the third failure mode in section 11.6.

Guessed

A static predictor said so, from the shape of the code and nothing else.

Everything in M4 is this, because there is no profile data yet. A guess is still much better than nothing: the hit rates in section 11.2 are measurements of how people write programs, and those have held up for thirty years.

Adjusted

It came from a measurement, and then a transformation scaled it.

Splitting a block, unrolling a loop or threading a jump all divide a measured count across paths that were not measured separately. The result is worth more than a guess and less than what was measured, which is exactly what this says.

Precise

Measured, and nothing has touched it since.

Also what a branch on a constant gets, because that one is not a measurement or a guess. It is arithmetic.

Implementations

impl Quality

pub const fn as_str(self) -> &'static str

How the quality reads in a dump.

pub const fn is_measured(self) -> bool

Whether this came from running the program rather than from looking at it.

The question a consumer asks when it is about to do something it could not undo. Section 40.5’s rule that a statically predicted branch never counts as predictable is this predicate, and it is here rather than at each call site so that the rule is one thing.

Trait Implementations

impl Clone for Quality

fn clone(&self) -> Quality

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Copy for Quality

impl Debug for Quality

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Display for Quality

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Eq for Quality

impl Hash for Quality

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl Ord for Quality

fn cmp(&self, other: &Quality) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

where Self: Sized,

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

where Self: Sized,

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

where Self: Sized,

Restrict a value to a certain interval.

fn clamp_to<R>(self, range: R) -> Self

where Self: Sized, R: ClampBounds<Self>,

🔬This is a nightly-only experimental API. (clamp_to)

Restrict a value to a certain range.

impl PartialEq for Quality

fn eq(&self, other: &Quality) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl PartialOrd for Quality

fn partial_cmp(&self, other: &Quality) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &Rhs) -> bool

Tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Rhs) -> bool

Tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &Rhs) -> bool

Tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Rhs) -> bool

Tests greater than or equal to (for self and other) and is used by the >= operator.

impl StructuralPartialEq for Quality