Enum OptLevel 

pub enum OptLevel {
    O0,
    O1,
    O2,
    O3,
    Os,
    Oz,
}

An optimisation level.

spec/16-performance.md section 16.4 gives each level a throughput budget and a code quality budget, and the levels exist to make that tradeoff explicit rather than to be a dial. There is no -O4, because a level nobody can state the contract for is a level nobody can test.

Variants

O0

-O0. Compile as fast as possible and keep every variable inspectable.

O1

-O1. The cheap wins, at roughly the cost of -O0.

O2

-O2. The full pipeline. This is the level the code quality claim is about.

O3

-O3. -O2 plus the transformations that trade size for speed.

Os

-Os. Optimise for size, at roughly -O2 compile time.

Oz

-Oz. Optimise for size, aggressively.

Implementations

impl OptLevel

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

The flag that selects this level.

pub const fn is_size(self) -> bool

Whether this level optimises for size rather than speed.

pub const fn runs_optimizer(self) -> bool

Whether the middle end runs at all.

pub const fn schedules(self) -> bool

Whether the instructions of a block are put in the order the machine finishes soonest.

spec/optimizer/38-scheduling-and-layout.md section 38.6 says -O2 and above, and gcc turns -fschedule-insns2 on at the two size levels as well, which costs nothing: a schedule is a permutation of instructions that were all going to be written anyway, so it is the one optimization here that cannot make a function larger.

pub const fn sibling_calls(self) -> bool

Whether a call in tail position becomes a jump.

-O2 and above and the two size levels, which is where gcc turns -foptimize-sibling-calls on. Not at -O1, where gcc leaves it off so that a backtrace still shows every function the program went through.

pub const fn frame_pointer(self) -> bool

Whether every function keeps a frame pointer when the command line did not say.

Only at -O0, which is where gcc keeps one. Code built without optimization is code somebody is going to step through, and an asm statement written for that build may walk the frame through %rbp itself, the way chibicc’s own test of asm returns early.

Trait Implementations

impl Clone for OptLevel

fn clone(&self) -> Self

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl Copy for OptLevel

impl Debug for OptLevel

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

Formats the value using the given formatter.

impl Default for OptLevel

fn default() -> Self

Returns the “default value” for a type.

impl Display for OptLevel

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

Formats the value using the given formatter.

impl Eq for OptLevel

impl FromStr for OptLevel

fn from_str(s: &str) -> Result<Self, ()>

Parses the part after -O, so "" is -O which GCC treats as -O1.

type Err = ()

The associated error which can be returned from parsing.

impl Hash for OptLevel

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 OptLevel

fn cmp(&self, other: &Self) -> 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 OptLevel

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

Equality operator ==.

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

Inequality operator !=.

impl PartialOrd for OptLevel

fn partial_cmp(&self, other: &Self) -> 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 OptLevel