Stack alignment constrains the stack pointer to a multiple of a specified byte boundary. AAPCS32 requires word alignment generally and eight-byte alignment at public interfaces. Saving four 32-bit words preserves eight-byte alignment; saving three would change it by four bytes and may require padding before another call. Frame size and the alignment of local objects must be considered together.[ref]
Procedure Call Standard for the Arm Architecture by ARM Limited. Base standard, universal and public-interface stack constraints
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