Question

How are stack based structures passed to subroutines in ARM assembly?

For example:

typedef struct SomeStruct { 
    uint32_t one;
    uint32_t two;
    uint32_t three;
    uint32_t four;
} SomeStruct;

void SomeFunction(uint32_t someValue, SomeStruct someStruct, uint32_t otherValue);

In this case, how would the structure be passed? My guess is:

• r0 – someValue
• r1 – someStruct.one
• r2 – someStruct.two
• r3 – someStruct.three
• *sp – someStruct.four
• *sp,#4 – otherValue

Is that correct or does something else happen? Or does it just pass the structure’s sp based address?

Answer 1

There’s a “procedure call standard” for an ARM ABI here: http://infocenter.arm.com/help/topic/com.arm.doc.ihi0042d/IHI0042D_aapcs.pdf

There’s more than one ABI for ARM, and more again for 64 bit ARM, but just looking at this one, the relevant section is “5.5 Parameter Passing”. At a glance, I think C.5 says you’re right, this struct will be split between r1-r3 and the stack.

B.1, which can replace an argument of composite type with a pointer to a copy in memory, never applies in C, since both caller and callee must have SomeStruct as a complete type in order to pass by value.