OS X-x64:堆栈不是16字节对齐错误

Question

libdyld.dylib`STACK_NOT_16_BYTE_ALIGNED_ERROR:->0x7FFFC12DA2FA<+0>:movdqa%xMM0,(%RSP)0x7FFFC12DA2FF<+5>:int3

libdyld.dylib`_dyld_func_lookup:0x7fffc12da300<+0>:pushq%rbp 0x7fffc12da301<+1>:movq%rsp,%rbp

代码如下：

Object_copy:
    pushq %rbp
    movq %rbp, %rsp

    subq $8, %rsp
    movq %rdi, 8(%rsp)          # save self address
    movq obj_size(%rdi), %rax   # get object size
    imul $8, %rax          
    movq %rax, %rdi 
    callq _malloc             <------------------- error in this call

    # rsi old object address
    # rax new object address
    # rdi object size, mutiple of 8

    # rcx temp reg

    # copy object tag
    movq 0(%rsi), %rcx
    movq %rcx, 0(%rax)

    # set rdx to counter, starting from 8
    movq $8, %rdx

    # add 8 to object size, since we are starting from 8
    addq $8, %rdi

    start_loop:
        cmpq %rdx, %rdi
        jle end_loop

        movq (%rdx, %rsi, 1), %rcx
        movq %rcx, (%rdx, %rax, 1)

        addq $8, %rdx
        jmp start_loop

    end_loop:
        leave 
        ret



Main_protoObj:
    .quad    5                          ; object tag
    .quad    3                          ; object size
    .quad    Main_dispatch_table        ; dispatch table

_main:
    leaq Main_protoObj(%rip), %rdi
    callq Object_copy                # copy main proto object
    subq $8, %rsp                    # save the main object on the stack
    movq %rax, 8(%rsp)
    movq %rax, %rdi                 # set rdi point to SELF
    callq Main_init
    callq Main_main

    addq $8, %rsp                    # restore stack

    leaq _term_msg(%rip), %rax
    callq _print_string

Answer 1

正如您所说，MacOS X有一个16字节的堆栈对齐方式，这意味着机器希望堆栈上的每个变量从当前堆栈指针的16倍数字节开始。

当堆栈不对齐时，这意味着我们开始尝试从16字节窗口的中间读取变量，通常以分段错误告终。

在调用代码中的例程之前，需要确保堆栈对齐正确；在这种情况下，意味着基指针寄存器可以被16整除。

subq $8, %rsp               # stack is misaligned by 8 bytes
movq %rdi, 8(%rsp)          #
movq obj_size(%rdi), %rax   #
imul $8, %rax               #
movq %rax, %rdi             #
callq _malloc               # stack is still misaligned when this is called

subq $16, %rsp               # stack is still aligned
movq %rdi, 16(%rsp)          #
...                          #
callq _malloc                # stack is still aligned when this is called, good