According to ARM Architecture Procedure Call Standard (AAPCS) on the ARMv6-M, and ARMv7-M architecture in it says:
"Although the processor hardware allows SP to be at any word aligned address at function boundaries, standard programming practice requires C program code to ensure that the SP is at a 64-bit (doubleword) aligned address."
What does it mean that the Stack pointer has to be at a 64 bit aligned address?