Loop Optimization: Hand vs Compiler

A decision rule for the five common loop transformations. Its value is knowing when manual application is redundant (the compiler already does it) or harmful (it defeats the vectorizer or fools your benchmark).

When To Use

• Reviewing a hot loop and a hand-rolled transform appears (unrolled body, shift-instead-of-multiply, bespoke SIMD).
• Authoring a loop that profiling proved hot, deciding whether to optimize it by hand.
• Pushing back on a "this is faster" claim about a loop micro-opt.

When NOT To Use