Matt Godbolt's blog

16 hours ago

Giving the compiler compile-time known loop count (e.g., via `std::span<int, 8>`) enables loop unrolling optimization.
A dynamically-sized span (`std::span<int>`) results in a straightforward loop with counter and branch per iteration.
Unrolling eliminates loop counter and conditional branch, saving instructions and allowing further pattern detection like using `ldmib` for multiple loads.
Compilers may partially unroll, speculatively unroll, or fall back to regular looping depending on heuristics and iteration count (e.g., at 50 iterations it gives up).
Checking hot loops and providing compile-time loop counts sets the compiler up for success; profile-guided optimization (PGO) can also help improve guesses.

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