rust-performance

name	rust-performance
description	Rust performance optimization. Activates for benchmarking, profiling, allocation reduction, cache optimization, SIMD, zero-copy patterns, and performance-related questions. Invoke with /rust-performance.

Rust Performance Skill

Profiling Workflow

1. Measure first — never optimize blindly
2. Profile → identify hotspots
3. Benchmark → establish baseline
4. Optimize → apply targeted fix
5. Benchmark again → verify improvement

Profiling Tools

Tool	Use For	Command
cargo flamegraph	CPU profiling	cargo flamegraph --bin my-app
perf	Linux CPU profiling	perf record --call-graph dwarf ./target/release/my-app
DHAT (valgrind)	Heap allocation profiling	valgrind --tool=dhat ./target/release/my-app
cargo-show-asm	Inspect generated assembly	cargo asm my_crate::my_func
criterion	Microbenchmarks	In benches/ directory
hyperfine	CLI benchmark	hyperfine './target/release/my-app'

Common Optimizations

Allocation Reduction

// BAD: allocates on every iteration
for item in items {
    let s = format!("prefix_{}", item);
    process(&s);
}

// GOOD: reuse buffer
let mut buf = String::new();
for item in items {
    buf.clear();
    write!(&mut buf, "prefix_{}", item).unwrap();
    process(&buf);
}

Pre-allocation

// BAD
let mut v = Vec::new();
for i in 0..1000 { v.push(i); }

// GOOD
let mut v = Vec::with_capacity(1000);
for i in 0..1000 { v.push(i); }

String Optimization

• &str over String where possible
• SmallString / compact_str for short strings
• String::with_capacity when size is known
• Cow<'_, str> for conditionally owned strings

Collection Selection

Need	Collection	Why
Sequential access	Vec<T>	Cache-friendly, contiguous memory
Key-value lookup	HashMap / BTreeMap	O(1) / O(log n) lookup
Unique set	HashSet / BTreeSet	Deduplication
FIFO queue	VecDeque<T>	Efficient push/pop at both ends
Sorted data	BTreeMap/BTreeSet	Ordered iteration
Small fixed set	[T; N] or ArrayVec	Stack-allocated, no heap

Zero-Copy Patterns

• bytes::Bytes for shared byte buffers
• memmap2 for memory-mapped files
• zerocopy for safe transmutation
• nom/winnow for zero-copy parsing

Compile-Time Optimization

# Cargo.toml
[profile.release]
lto = "thin"        # Link-time optimization
codegen-units = 1   # Better optimization, slower build
strip = true        # Strip debug symbols
panic = "abort"     # Smaller binary

[profile.release.build-override]
opt-level = 3

Red Flags

• clone() in hot loops
• String allocation where &str suffices
• Box<dyn Trait> in performance-critical paths (vtable indirection)
• HashMap with small N (linear scan on Vec is faster for N < ~20)
• Collecting into Vec just to iterate again
• format!() for string building in loops