It's that structs of two simple fields need to be passed in registers. And more specifically that this rule is relevant for mixed integer and floating point fields.
It's a very specific rule that requires a ton of code to implement compared to the integer calling convention. And again like the weird AMD64 convention likely invented to squeeze out a theoretical few cycles that never occur outside microbenchmarks
It's that structs of two simple fields need to be passed in registers. And more specifically that this rule is relevant for mixed integer and floating point fields.
It's a very specific rule that requires a ton of code to implement compared to the integer calling convention. And again like the weird AMD64 convention likely invented to squeeze out a theoretical few cycles that never occur outside microbenchmarks