18.104.22.168. Stack Frame for a Function with Structures Passed By Value
Functions that take struct value arguments still have their first 16 bytes of arguments arriving in registers r4 through r7, just like other functions.
If part of a structure is passed using registers, the function might need to copy the register contents back to the stack. This operation is similar to that required in the variable arguments case as shown in the figure above, Stack Frame Using Variable Arguments.
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