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?
Yes. For processors with 64-bit interface (e.g. Cortex-M7), having such data alignment would be more efficient. It could also made debugging easier (e.g. when setting up data watchpoint).
It also avoids corner cases where a 64-bit access go across boundaries of different memory types, which can result in unpredictable behavior according the the architecture specification.
regards,
Joseph