alt Hacker NewsOut-of-order execution is an internal optimization, from the outside results are still guaranteed to be available in order - e.g. the instruction stream appears to be executed in order when observing the CPU from the outside.
For instance you don't need to be afraid that an instruction uses garbage inputs just because a previous instruction hadn't finished computing an input value to the instruction. At worst you'll get a pipeline stall if the CPU can't fill the gap with out-of-order executed instructions.
On some CPUs it does get tricky once memory is involved though (on ARM, but not on x86).
> from the outside results are still guaranteed to be available in order - e.g. the instruction stream appears to be executed in order when observing the CPU from the outside.
> […]
> On some CPUs it does get tricky once memory is involved though (on ARM, but not on x86).
https://en.wikipedia.org/wiki/Memory_ordering:
“Among the commonly used architectures, x86-64 processors have the strongest memory order, but may still defer memory store instructions until after memory load instructions.”