Description
| Bugzilla Link | 48965 |
| Version | 11.0 |
| OS | Linux |
| CC | @dwblaikie,@fhahn,@jrmuizel,@LebedevRI,@nikic |
Extended Description
This was originally reported here: rust-lang/rust#75935
In the end it boils down to the following minimal testcases in C/C++/Rust.
Godbolt links: https://godbolt.org/z/s6reoz and https://rust.godbolt.org/z/a7f6rj
All of them have in common that the iteration happens not by having a counter but instead iterating until the end pointer. When additionally counting the iterations, the maximum (or here: exact) number of iterations does not seem to be available during optimizations. As such the two assertions are not detected as dead code and not optimized away. In this specific minimal case, the whole function should've been possible to optimize away.
The testcase is extremely contrived but it also happens in real code (the Rust issue contains something closer to my original code) and causes missed optimization opportunities, and is especially problematic with Rust because of automatic bounds checks for array indexing that can be impossible to optimize away because of this and then preventing other optimizations (like auto-vectorization or loop unrolling) to kick in.
C
void foo(const uint32_t *y, size_t y_len) {
const uint32_t *y_end = y + y_len;
size_t c = 0;
for (const uint32_t *y_iter = y; y_iter != y_end; y_iter++, c++) {
assert(c < y_len);
}
assert(c == y_len);
}
C++
void foo(const std::vector<uint32_t>& y) {
size_t c = 0;
for (auto y_iter = y.cbegin(); y_iter != y.cend(); y_iter++, c++) {
assert(c < y.size());
}
assert(c == y.size());
}
Rust
pub fn foo(y: &[u32]) {
let mut x = 0;
for (c, _y) in y.iter().enumerate() {
assert!(c < y.len());
x = c;
}
assert!(x == y.len());
}