The ARM-THUMB Procedure Call Standard

The ARM-THUMB Procedure Call StandardCost of shared library relative to RWPICode built RWPI is not compatible with shared library code. However, code built for shared library use is RWPIand so is compatible with RWPI code if the run-time environment maintains the invariant 0[0[SB]] = SB (byconvention, shared library index 0 is reserved for client code).The additional cost of building for shared library 0 is 2 instructions (8 bytes) per static-data-using function in ARMstateand 3 instructions (6 bytes) in Thumb state.C subset of the ARM C Library ARM-state Thumb-state#(static-data-using functions) 120 120Cost per function 8 bytes 6 bytesApproximate % size increase 2.3% 2.3%While 2.3% is an unacceptable overhead to impose on all user code, it may well be an acceptable overhead on arun-time library that will form a small proportion of the final application.Shared library supportIn comparison with the base standard, shared library support costs approximately:oIn ARM-state- One dedicated register (SB).- Two instructions per static-data-using function.- One instruction per data address constant.oIn Thumb-state- Three instructions per static-data-using function.- One instruction per data address constant.C subset of the ARM C Library ARM-state Thumb-stateCost of SB 1% 0%#(static-data-using functions) * cost 120 * 8 bytes 120 * 6 bytes#(data address constants) * cost 249 * 4 bytes 283 * 2 bytesApproximate % size increase 5.6% 4.1%Stack limit checkingA stack limit check costs approximately:oOne dedicated register (SL) and 2 instructions (8 bytes) per non-leaf function, in ARM-state.o Four instructions (8 bytes) per non-leaf function, in Thumb-state.Checking a large frame is slightly more expensive than this, especially in Thumb-state, but large frames are rare(there are none in the ARM C library, for example).SWS ESPC 0002 A-05 Page 36 of 37