[sakras]

To be honest I kind of understand this “passing the buck”. In computing in general you never trust the guy up the stack to give you good input. Query engines do filter reordering because they don’t trust the optimizer to get it right. Compilers do optimizations because they don’t trust the programmer to get the order of operations right (rightfully). CPUs do OOO because they don’t trust compilers to get the order of instructions right. The way I see it is there are 2 variants: 1) make a specific instruction (clutters the instruction set, makes processors who don’t care implement it), 2) rely on processors who care to implement instruction fusion, and those who don’t will do it the slow way. Either way, it gets implemented in hardware, and processors who care need to make a change in the front end.

[atq2119]

> CPUs do OOO because they don’t trust compilers to get the order of instructions right.

Not really. CPUs do out-of-order because cache hits are unpredictable and it is crucial for single-threaded performance to make progress on dependent operations as soon as a loaded value is available.

There may be other, lower order, factors, but variable memory latency is the real reason.