rust-lang / rust-lang/rustc_codegen_gcc
Support #![feature(unchecked_math)]
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libgccjit
medium
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Description
The following code:
#![feature(unchecked_math)]
pub fn nop(num: i32) -> i32 {
unsafe { (num.unchecked_mul(2)) / 2 }
}
gives:
example::nop:
lea eax, [rdi+rdi]
sar eax
ret
while cg_llvm gives:
example::nop:
mov eax, edi
ret
because it seems cg_gcc always enable checked maths.
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
Start with the Rust reproducer in the issue and inspect how rustc_codegen_gcc handles unchecked arithmetic compared with cg_llvm. Confirm the generated assembly for unchecked multiplication and division, then verify that support for #![feature(unchecked_math)] produces the expected unchecked behavior.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- rust
- Domain
- compilers
- Issue type
- Feature
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100