Exploring the difference in compiler output on Rust code

Tools used

Assembly generated with cargo-asm https://github.com/gnzlbg/cargo-asm

Machine code analysis done with llvm-mca https://llvm.org/docs/CommandGuide/llvm-mca.html

diffs generated with delta https://github.com/dandavison/delta

diff output converted to html with aha https://github.com/theZiz/aha

Workflow

Comparing two functions:

1. Generate assembly with cargo asm <target function A>
2. Copy assembly output of target function
3. Repeat 1-2 for <target function B>
4. Generate diff with delta asmA asmB -w <column_width> --side-by-side | aha --black > <my_diff>.html
5. Feed the assembly into llvm-mca and examine output

Unfortunately it is not feasible to pipe the output of cargo asm into delta directly as the output includes debug strings and the assembly of any other function in the crate (with mangled names)

Comparisons

Function signature argument types: &Vec<u32> vs. &[u32]

Branching vs Branchless with data dependency

Ideas for investigation

• for-range vs. iter-loops
• for-range vs. raw-loops
• zero-initialized storage vs. maybe-uninit storage
• Function signature argument types: Box vs. String
• Enum variant layout: Box vs. String / Box<[u8]> vs. Vec