ARM Return Values

The following list shows the standard registers that a called function uses to return values:

R0 for single-word integer and integer-equivalent values.
R0 for single-precision floating-point return values.
R0 and R1 for 64-bit integer return values.
R0 and R1 for double-precision floating-point values.
For structure return values, the calling function allocates re-entrant memory on the stack in the local argument area. It passes a pointer to that memory as the first argument in R0. This stored value may be associated with a variable allocated in the calling function stack frame. Before returning, the called function copies the return value to the memory locations indicated by the first argument.

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