Prevent gcc from optimizing printf into puts-CodePudding

This is my test code:

#include <stdio.h>

int main(void)
{
    printf("Hello, world!\n");

    return 0;
}

Pretty simple, and I compiled it with gcc -o helloworld helloworld.c (and I also tried -g too).

However when I objdump -tT helloworld, the output is:


helloworld:     file format elf64-x86-64

SYMBOL TABLE:
0000000000000000 l    df *ABS*  0000000000000000              abi-note.c
000000000000039c l     O .note.ABI-tag  0000000000000020              __abi_tag
0000000000000000 l    df *ABS*  0000000000000000              init.c
0000000000000000 l    df *ABS*  0000000000000000              crtstuff.c
0000000000001070 l     F .text  0000000000000000              deregister_tm_clones
00000000000010a0 l     F .text  0000000000000000              register_tm_clones
00000000000010e0 l     F .text  0000000000000000              __do_global_dtors_aux
0000000000004030 l     O .bss   0000000000000001              completed.0
0000000000003df0 l     O .fini_array    0000000000000000              __do_global_dtors_aux_fini_array_entry
0000000000001130 l     F .text  0000000000000000              frame_dummy
0000000000003de8 l     O .init_array    0000000000000000              __frame_dummy_init_array_entry
0000000000000000 l    df *ABS*  0000000000000000              helloworld.c
0000000000000000 l    df *ABS*  0000000000000000              crtstuff.c
000000000000211c l     O .eh_frame  0000000000000000              __FRAME_END__
0000000000000000 l    df *ABS*  0000000000000000              
0000000000003df0 l       .init_array    0000000000000000              __init_array_end
0000000000003df8 l     O .dynamic   0000000000000000              _DYNAMIC
0000000000003de8 l       .init_array    0000000000000000              __init_array_start
0000000000002014 l       .eh_frame_hdr  0000000000000000              __GNU_EH_FRAME_HDR
0000000000004000 l     O .got.plt   0000000000000000              _GLOBAL_OFFSET_TABLE_
0000000000001000 l     F .init  0000000000000000              _init
00000000000011d0 g     F .text  0000000000000005              __libc_csu_fini
0000000000000000  w      *UND*  0000000000000000              _ITM_deregisterTMCloneTable
0000000000004020  w      .data  0000000000000000              data_start
0000000000000000       F *UND*  0000000000000000              puts@GLIBC_2.2.5
0000000000004030 g       .data  0000000000000000              _edata
00000000000011d8 g     F .fini  0000000000000000              .hidden _fini
0000000000000000       F *UND*  0000000000000000              __libc_start_main@GLIBC_2.2.5
0000000000004020 g       .data  0000000000000000              __data_start
0000000000000000  w      *UND*  0000000000000000              __gmon_start__
0000000000004028 g     O .data  0000000000000000              .hidden __dso_handle
0000000000002000 g     O .rodata    0000000000000004              _IO_stdin_used
0000000000001160 g     F .text  0000000000000065              __libc_csu_init
0000000000004038 g       .bss   0000000000000000              _end
0000000000001040 g     F .text  000000000000002f              _start
0000000000004030 g       .bss   0000000000000000              __bss_start
0000000000001139 g     F .text  000000000000001a              main
0000000000004030 g     O .data  0000000000000000              .hidden __TMC_END__
0000000000000000  w      *UND*  0000000000000000              _ITM_registerTMCloneTable
0000000000000000  w    F *UND*  0000000000000000              __cxa_finalize@GLIBC_2.2.5


DYNAMIC SYMBOL TABLE:
0000000000000000  w   D  *UND*  0000000000000000              _ITM_deregisterTMCloneTable
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 puts
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 __libc_start_main
0000000000000000  w   D  *UND*  0000000000000000              __gmon_start__
0000000000000000  w   D  *UND*  0000000000000000              _ITM_registerTMCloneTable
0000000000000000  w   DF *UND*  0000000000000000  GLIBC_2.2.5 __cxa_finalize

as you can see, there's no printf symbol... what went go wrong..? and interestingly, I didn't use puts though, we can see puts there.

and then I found this

ok if I used printf with literal string, I got puts instead of printf because there's no need to use it. so I compiled this again with -O0 but gcc still optimizes it.

what should I do for keeping gcc from optimizing it?

CodePudding user response：

With gcc, you can use the command-line option

-fno-builtin-printf

so that the compiler doesn't recognize the function printf as a built-in function (which allows further optimizations, such as redirecting it to puts).

Or you can use

-fno-builtin

so that the compiler doesn't recognize any functions as built-in, except for functions starting with the __builtin_ prefix.

However, I generally don't recommend doing this, because this will probably have a negative performance impact. You should only do this when you have a special reason to do so.

CodePudding user response：

Add a trivial specifier:

printf("Hello, world%c", '\n');