我做了一个简单的测试:
int b;
for (int i = 0; i < 10; i++) {
b = i;
}
对比
for (int i = 0; i < 10; i++) {
int b = i;
}
我使用gcc-5.2.0编译了这些代码。然后我反汇编了这两个代码的主函数,得到以下结果:
1º:
0x00000000004004b6 <+0>: push rbp
0x00000000004004b7 <+1>: mov rbp,rsp
0x00000000004004ba <+4>: mov DWORD PTR [rbp-0x4],0x0
0x00000000004004c1 <+11>: jmp 0x4004cd <main+23>
0x00000000004004c3 <+13>: mov eax,DWORD PTR [rbp-0x4]
0x00000000004004c6 <+16>: mov DWORD PTR [rbp-0x8],eax
0x00000000004004c9 <+19>: add DWORD PTR [rbp-0x4],0x1
0x00000000004004cd <+23>: cmp DWORD PTR [rbp-0x4],0x9
0x00000000004004d1 <+27>: jle 0x4004c3 <main+13>
0x00000000004004d3 <+29>: mov eax,0x0
0x00000000004004d8 <+34>: pop rbp
0x00000000004004d9 <+35>: ret
对比
第二个
0x00000000004004b6 <+0>: push rbp
0x00000000004004b7 <+1>: mov rbp,rsp
0x00000000004004ba <+4>: mov DWORD PTR [rbp-0x4],0x0
0x00000000004004c1 <+11>: jmp 0x4004cd <main+23>
0x00000000004004c3 <+13>: mov eax,DWORD PTR [rbp-0x4]
0x00000000004004c6 <+16>: mov DWORD PTR [rbp-0x8],eax
0x00000000004004c9 <+19>: add DWORD PTR [rbp-0x4],0x1
0x00000000004004cd <+23>: cmp DWORD PTR [rbp-0x4],0x9
0x00000000004004d1 <+27>: jle 0x4004c3 <main+13>
0x00000000004004d3 <+29>: mov eax,0x0
0x00000000004004d8 <+34>: pop rbp
0x00000000004004d9 <+35>: ret
两个代码生成完全相同的汇编结果,这难道不证明这两个代码产生了相同的结果吗?