optlevel

Optlevel, or opt-level, is a compiler setting that controls how aggressively code is optimized during compilation. It is commonly used in LLVM-based toolchains and in language ecosystems such as Rust, where it can be configured to balance runtime performance, binary size, and compilation speed.

In Rust, the opt-level setting can be specified in compiler flags or in profile configurations. Typical values are 0, 1, 2, 3, or size-oriented options "s" and "z". A level of 0 disables most optimizations, prioritizing fast compile times and easier debugging. Higher levels enable more aggressive optimizations, which can improve runtime performance but may increase compile time and code size. The "s" option aims to reduce code size with some performance trade-offs, while "z" focuses even more on shrinking the binary.

Impact and use cases. Increasing the opt-level generally yields faster executables at the cost of longer compile times and sometimes reduced debuggability. For debugging and development, opt-level 0 or low levels are common to keep builds quick and straightforward. For production or performance-critical applications, opt-level 2 or 3 is typical, often combined with other optimizations like inlining, inlining thresholds, or link-time optimization (LTO) to maximize speed. Size-focused builds may prefer "s" or "z" when memory constraints or download sizes are critical.

Setting and examples. In Rust, you can set opt-level in Cargo.toml under a profile, for example [profile.release] opt-level = 3, or pass -C opt-level=3 to rustc. Cargo can also apply different levels for different profiles (dev, release, bench). The optlevel choice is a primary lever for tuning performance versus compile-time costs and binary size.