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Hi,
When playing with some of my code on compiler explorer, I found a case where rust would not compute the result of some functions at compile time. I managed to reduce the code to the following:
pub fn test() -> i32 {
let mut s = 0;
for i in 0..102 {
if i == 0 {
s = i;
}
s += 1;
}
s
}
pub fn test2() -> i32 {
let mut s = 0;
for i in 0..101 {
if i == 0 {
s = i;
}
s += 1;
}
s
}
pub fn test3() -> i32 {
let mut s = 0;
for _ in 0..102 {
s += 1;
}
s
}Both test2 and test3 are completely optimized and are compiled down to a singled "mov constant + ret" combo. But the test function generates something quite convoluted: the compiler loads the result (102) and removes 6 from it until it reaches 0, at which point it returns the value of another register to which 6s have been added at each step.
I tried a few versions of rustc but couldn't find any which completely optimized away the first function. Using mir-opt-level didn't help.