Possible performance loss with f32 arithmetic #91447
Description
I've tried, out of curiosity, a floating point arithmetic test and found quite a big difference between C++ and Rust.
The code used in rust
pub struct Stats
{
x: f32,
y: f32,
z: f32
}
pub fn sum(a: &Stats, b: &Stats) -> Stats
{
Stats {
x: a.x + b.x,
y: a.y + b.y,
z: a.z + b.z
}
}The code used in C++
struct Stats
{
float x;
float y;
float z;
};
Stats sum(const Stats &a, const Stats &b)
{
return Stats {
a.x + b.x,
a.y + b.y,
a.z + b.z
};
}Here is a link to a godbolt for side-by-side comparision of assembly output: https://godbolt.org/z/dqc4b74rv
Rust seem to absolutely want the floats back into e* registers instead of keeping them in xmm registers, C++ leaves them into the xmm registers. In some cases it might more advantageous to leave the floats in xmm registers for future operations on them rather then passing them back into the e* registers.