“fp” is an alias for r11; despite its name, it was never used as a frame pointer in iOS (the iOS tools set up a frame pointer in r7 by default, as many of the o

“fp” is an alias for r11; despite its name, it was never used as a frame pointer in iOS (the iOS tools set up a frame pointer in r7 by default, as many of the original thumb instructions cannot easily access r8-r12). More generally, a frame pointer of some sort is necessary to deal with variable-sized stack allocations, and often useful for performance analysis and debugging tools, so many compilers set them up by default even when they aren’t strictly necessary.