Why does gcc generates strange code without flag -fno-pie?

问题

I am trying to compile dummy function in gcc with flag -fno-pie and without.

void dummy_test_entrypoint() { }

When i compile without the flag.

gcc -m32 -ffreestanding -c test.c -o test.o

I get the following disassembled code.

00000000 <dummy_test_entrypoint>:
0:  55                      push   ebp
1:  89 e5                   mov    ebp,esp
3:  e8 fc ff ff ff          call   4 <dummy_test_entrypoint+0x4>
8:  05 01 00 00 00          add    eax,0x1
d:  90                      nop
e:  5d                      pop    ebp
f:  c3                      ret    

When i compile with the flag.

00000000 <dummy_test_entrypoint>:
0:  55                      push   ebp
1:  89 e5                   mov    ebp,esp
3:  90                      nop
4:  5d                      pop    ebp
5:  c3                      ret 

My question.

What is it???

3:  e8 fc ff ff ff          call   4 <dummy_test_entrypoint+0x4>
8:  05 01 00 00 00          add    eax,0x1

回答1:

You disassembled the object file without the --reloc flag, so the output is misleading. With the --reloc flag, you'll see this:

   3:   e8 fc ff ff ff          call   4 <dummy_test_entrypoint+0x4>
            4: R_386_PC32   __x86.get_pc_thunk.ax
   8:   05 01 00 00 00          add    $0x1,%eax
            9: R_386_GOTPC  _GLOBAL_OFFSET_TABLE_

And the subroutine looks like this:

00000000 <__x86.get_pc_thunk.ax>:
   0:   8b 04 24                mov    (%esp),%eax
   3:   c3                      ret    

This construct loads the GOT pointer into %eax, in case the function needs to reference global data. The function does not contain such a reference, but because you compiled the code without optimization, GCC did not remove the dead code.

来源：https://stackoverflow.com/questions/57831288/why-does-gcc-generates-strange-code-without-flag-fno-pie