I agree it’s hard to believe it could be faster. But we expect there must be _some_ cut off where more registers is worse, it’s just a question of where. But mo

I agree it’s hard to believe it could be faster. But we expect there must be _some_ cut off where more registers is worse, it’s just a question of where. But more-than-one being usually worse seems surprising to me too. I expect it’s unlikely someone actually benchmarked when making that decision, but just went with the “obvious must” reaction too though. On reflection now, I feel like there could be some reasons the stack pattern may sometimes be better or equivalent for multi-return. To your point though, curiously the Win64 ABI does only use one return register (unlike the SysV ABI which allocates up to two integer registers as you say). I’m guessing their microbenchmark may have been that if the compiler had to spill the value earlier, it’s better to be able to spill directly to the sret pointer, than to need extra code to reload it at the end. The bigger reason I usually see that this matters none is that if you’ve missed the inlining opportunity of something that small, you’re already so far away from optimal {code size, performance, memory usage} it is really too premature to optimize the shape of this code (and the ABI).