switch与ifelse.doc

在程序中我们经常用到switch case，它的用法就不再用多说了。计算switch中的值然后比较，跳转到相应的分支。很多人说在编译是时通过转化成if。。else来实现的。但实际使用时尤其在调试代码时，每次的case跳转都是直接跳转到匹配值的。这样就与用if。。。else有些矛盾。那么switch。。case究竟如何实现的那？实际试验一下： //switch_test1.c #include stdio.h #include stdlib.h int main() { int i; i=1; switch(i) { case 1: printf(%d\n,i); break; case 2: printf(%d\n,i); break; case 3: printf(%d\n,i); break; case 4: printf(%d\n,i); break; deflaut: break; } return 1; } 对test1.c用Gcc进行到汇编程序的编译 Gcc -S test1.c –o test.s 得到文件内容如下： .file switch-test1.c .def ___main; .scl 2; .type 32; .endef .section .rdata,dr LC0: .ascii %d\12\0 .text .globl _main .def _main; .scl 2; .type 32; .endef _main: pushl %ebp movl %esp, %ebp subl $24, %esp andl $-16, %esp movl $0, %eax addl $15, %eax addl $15, %eax shrl $4, %eax sall $4, %eax movl %eax, -12(%ebp) movl -12(%ebp), %eax call __alloca call ___main movl $1, -4(%ebp) movl -4(%ebp), %eax movl %eax, -8(%ebp) cmpl $2, -8(%ebp) je L4 cmpl $2, -8(%ebp) jg L8 cmpl $1, -8(%ebp) je L3 jmp L2 L8: cmpl $3, -8(%ebp) je L5 cmpl $4, -8(%ebp) je L6 jmp L2 L3: movl -4(%ebp), %eax movl %eax, 4(%esp) movl $LC0, (%esp) call _printf jmp L2 L4: movl -4(%ebp), %eax movl %eax, 4(%esp) movl $LC0, (%esp) call _printf jmp L2 L5: movl -4(%ebp), %eax movl %eax, 4(%esp) movl $LC0, (%esp) call _printf jmp L2 L6: movl -4(%ebp), %eax movl %eax, 4(%esp) movl $LC0, (%esp) call _printf L7: L2: movl $1, %eax leave