Inline assembly for critical performance hotspots

Rust's inline assembly (asm! macro) lets you write assembly for performance-critical code or to access hardware features. It's an unstable feature (nightly only) and requires unsafe. I use it sparingly for SIMD intrinsics not exposed by std::arch, cryptography, or tight loops where the compiler can't optimize well. The syntax is similar to GCC's inline assembly. You specify inputs, outputs, and clobbers. For most cases, Rust's abstractions are fast enough, but inline assembly is the escape hatch when you need it. Always benchmark before resorting to assembly, and document why it's necessary. It sacrifices portability and readability for performance.