.cfi指令解读（一）

转自：http://51hired.com/questions/14933/%E5%A6%82%E4%BD%95%E7%90%86%E8%A7%A3%E8%BF%99%E4%BA%9B.cfi%E6%8C%87%E4%BB%A4%EF%BC%9F

CFI(calling frame info)的作用是出现异常时stack的回滚(unwind)

而回滚的过程是一级级CFA往上回退，直到异常被catch。

DWARF4标准的section 6.4:

The call frame is identified by an address on the stack. We refer to this address as the Canonical Frame Address orCFA. Typically, the CFA is defined to be the value of the stack pointer at the call site in the previous frame (which may be different from its value on entry to the current frame).

即CFA定义为执行call xxx时SP(stack pointer)所指向的地址。

pushl %ebp .cfi_def_cfa_offset 8 .cfi_offset 5, -8 

表示执行完pushl %ebp后SP与CFA偏了8字节（4字节return address,4字节ebp）

movl %esp, %ebp .cfi_def_cfa_register 5 

表示执行完movl %esp, %ebp后cfa_register不再是esp,而是ebp

leave .cfi_restore 5 .cfi_def_cfa 4, 4 

表示执行完leave后ebp的值已经恢复到初始状态，并且CFA的计算方式应该是esp+4

其中寄存器对应的数字是架构相关的，详细参考xxx_map_dwarf_register：

1. i386
2. x86_64
3. sparc
4. arm

参考:

Dwarf2 Exception Handler HOWTO

CFI directives