rust-lang / rust-lang/rust-clippy
`div_ceil` does not yield optimal code for unsigned const divisor
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- Rust
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Description
Let's say we have a function:
fn foo(begin: usize) -> usize {
(begin + 128 - 1) / 128
}
This compiles to:
foo:
leaq 127(%rdi), %rax
shrq $7, %rax
retq
But clippy suggests that I should use div_ceil:
fn foo(begin: usize) -> usize {
begin.div_ceil(128)
}
... which compiles to:
foo:
movq %rdi, %rax
shrq $7, %rax
andl $127, %edi
cmpq $1, %rdi
sbbq $-1, %rax
retq
It seems that the original version produces better code on x86_64 with only two instructions (without the retq), but 5 instructions (without the retq) for the div_ceil version.
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
The issue names no repository files or tests. Start with the Rust example and linked Godbolt comparison, then inspect how Clippy produces the div_ceil suggestion for unsigned constant divisors. Done means deciding the expected behavior and verifying it with an appropriate regression test and codegen comparison.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- rust
- Domain
- performance, tooling
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100