Can compiler do a default implementation's inlining of the trait's method?

I understand that the trait's method doesn't have a body, so there is nothing to inline. But is there any sense to mark its default implementation like this?

trait Magnitude
fn square_magnitude( &self ) -> f64;

#[inline]
fn magnitude( &self ) -> f64
self.square_magnitude().sqrt()




Do I need to rewrite whole method body and mark this impl's method with #[inline] when implementing the trait for type rather than marking just trait's method as above?

#[inline]

1 Answer
1

If I understand the question correctly, you are asking two things:

magnitude

square_magnitude

magnitude

square_magnitude

inline

square_magnitude

As to the first, there is no reason why it couldn't. As to the second the answer is yes, the compiler will be able to inline both functions because by the time it generates the code, the source for both functions is available. This can be seen in the disassembly:

trait Magnitude
fn square_magnitude( &self ) -> f64;

#[inline]
fn magnitude( &self ) -> f64
self.square_magnitude().sqrt()



struct Vector x: f64, y: f64

impl Magnitude for Vector
#[inline]
fn square_magnitude (&self) -> f64
self.x*self.x + self.y*self.y



pub fn test (x: f64, y: f64) -> f64
let v = Vector x: x, y: y ;
v.magnitude()



Compiled with rustc v1.28.0 and option -O:

-O

example::test:
mulsd xmm0, xmm0
mulsd xmm1, xmm1
addsd xmm1, xmm0
xorps xmm0, xmm0
sqrtsd xmm0, xmm1
ret


Note however that the compiler will not inline square_magnitude inside magnitude if square_magnitude is not declared inline itself:

square_magnitude

magnitude

square_magnitude

inline

impl Magnitude for Vector
fn square_magnitude (&self) -> f64
self.x*self.x + self.y*self.y




Generates:

<example::Vector as example::Magnitude>::square_magnitude:
movsd xmm1, qword ptr [rdi]
movsd xmm0, qword ptr [rdi + 8]
mulsd xmm1, xmm1
mulsd xmm0, xmm0
addsd xmm0, xmm1
ret

example::test:
mulsd xmm0, xmm0
mulsd xmm1, xmm1
addsd xmm1, xmm0
xorps xmm0, xmm0
sqrtsd xmm0, xmm1
ret


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