C++ and Microarchitecture Nuances

C++ source code is written in order. That does not mean the processor executes it in order. This is the first correction. It is also the one many performance discussions manage to avoid. Modern high-performance cores use out-of-order execution . They accept a sequential instruction stream, break it into internal operations, rename registers, place work into scheduling structures, execute ready operations early, and then retire the results in program order. The machine preserves the visible behavior of sequential execution. Internally, it is not taking attendance line by line. For ordinary software, this is mostly invisible. For C++ intended to run in tens of nanoseconds, it is not invisible. At that scale, performance is not just about the number of instructions. It is about whether those instructions can be scheduled in parallel or whether the program quietly built a dependency chain and then acted surprised. The processor is a dependency scheduler Out-of-order execution exists because in-order pipelines waste time. If an older instruction stalls, an in-order processor must often wait even if later instructions are independent and ready. That is a poor use of hardware. The chip has execution units available. The instruction stream has more work. Dynamic scheduling fixes part of this problem. The processor tracks which operations have their inputs ready. When an operation is ready and an execution unit is available, it can issue. Older operations may still be waiting. Later operations may run first. The final architectural state is still committed in order, so the program behaves correctly. Tomasulo’s algorithm is the classic model for this idea. It used reservation stations and register renaming to allow instructions to execute when their operands became available rather than strictly when they appeared in the original program (Tomasulo, 1967). Later superscalar processors extended the same general approach with speculation and reorder buffers, but the central idea