Failed to vectorize the for_each iteration with min function
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A-codegen
A-LLVM
C-optimization
I-slow
- Dominant language
- Rust
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Description
I tried this code:
#[inline]
fn min_update(dst: &mut u32, lhs: u32){
if *dst > lhs{
*dst = lhs;
}
}
#[inline]
fn min(mut dst: u32, lhs: u32) -> u32 {
min_update(&mut dst, lhs);
dst
}
pub fn vectorize_fail(lhs: &[u32], dst: &mut u32) {
lhs.iter().for_each(|&x|{
min_update(dst, x);
});
}
pub fn vectorize(lhs: &[u32], dst: &mut u32){
lhs.iter().for_each(|&x|{
*dst = min(*dst, x);
});
}
I expected to see this happen: The vectorize_fail function will get vectorized
Instead, this happened: The auto-vectorization is not performed, see godbolt for details
Meta
rustc --version --verbose:
1.77.0
Backtrace
<backtrace>
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First steps
- Read the whole issue, then the project's contributing guide.
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Research direction
Reproduce the difference between vectorize_fail and vectorize using the Rust snippet and the linked Godbolt example. Compare the generated output for both functions and investigate why the min_update form is not auto-vectorized. Done means the failing form is vectorized consistently with the equivalent min form.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- rust
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100