From the sample code below which I got here, I don't understand why the value of registers are move to specific part in stack when byte-spilling is performed. pushq %rbp movq %rsp, %rbp subq $96, %rsp leaq L__unnamed_cfstring_23(%rip), %rax leaq L__unnamed_cfstring_26(%rip), %rcx movl $42, %edx leaq l_objc_msgSend_fixup_alloc(%rip), %r8 movl $0, -4(%rbp) movl %edi, -8(%rbp) movq %rsi, -16(%rbp) movq %rax, -48(%rbp) ## 8-byte Spill movq %rcx, -56(%rbp) ## 8-byte Spill movq %r8, -64(%rbp) ## 8-byte Spill movl %edx, -68(%rbp) ## 4-byte Spill callq _objc_autoreleasePoolPush movq L_OBJC_CLASSLIST_REFERENCES_$_(%rip), %rcx movq %rcx, %rdi movq -64(%rbp), %rsi ## 8-byte Reload movq %rax, -80(%rbp) ## 8-byte Spill callq *l_objc_msgSend_fixup_alloc(%rip) movq L_OBJC_SELECTOR_REFERENCES_27(%rip), %rsi movq %rax, %rdi movq -56(%rbp), %rdx ## 8-byte Reload movl -68(%rbp), %ecx ## 4-byte Reload And also, I don't know what is the purpose of byte-spilling since the program logic can still be achieved if the function is the one saving the value of the registers it will be used inside it. I really have no idea why is this happening. Please help me understand this.