g++ doesn't inline functions

问题

There is such code:

int fun1(){
   return 2 + 3;
}

inline int fun2(){  
   return 4 + 5;
}

int main(){
    int a = fun1();
    int b = fun2();
    return 0;
}

and corresponding assembly code:

    .file   "prog47.cpp"
    .text
.globl _Z4fun1v
    .type   _Z4fun1v, @function
_Z4fun1v:
.LFB0:
    .cfi_startproc
    .cfi_personality 0x0,__gxx_personality_v0
    pushl   %ebp
    .cfi_def_cfa_offset 8
    movl    %esp, %ebp
    .cfi_offset 5, -8
    .cfi_def_cfa_register 5
    movl    $5, %eax
    popl    %ebp
    ret
    .cfi_endproc
.LFE0:
    .size   _Z4fun1v, .-_Z4fun1v
    .section    .text._Z4fun2v,"axG",@progbits,_Z4fun2v,comdat
    .weak   _Z4fun2v
    .type   _Z4fun2v, @function
_Z4fun2v:
.LFB1:
    .cfi_startproc
    .cfi_personality 0x0,__gxx_personality_v0
    pushl   %ebp
    .cfi_def_cfa_offset 8
    movl    %esp, %ebp
    .cfi_offset 5, -8
    .cfi_def_cfa_register 5
    movl    $9, %eax
    popl    %ebp
    ret
    .cfi_endproc
.LFE1:
    .size   _Z4fun2v, .-_Z4fun2v
    .text
.globl main
    .type   main, @function
main:
.LFB2:
    .cfi_startproc
    .cfi_personality 0x0,__gxx_personality_v0
    pushl   %ebp
    .cfi_def_cfa_offset 8
    movl    %esp, %ebp
    .cfi_offset 5, -8
    .cfi_def_cfa_register 5
    andl    $-16, %esp
    subl    $16, %esp
    call    _Z4fun1v
    movl    %eax, 12(%esp)
    call    _Z4fun2v        # why fun2 is called?
    movl    %eax, 8(%esp)
    movl    $0, %eax
    leave
    ret
    .cfi_endproc
.LFE2:
    .size   main, .-main
    .section    .note.GNU-stack,"",@progbits

Why function fun2 is not inlined and called like normal function? I have read that inline keyword is only hint for compiler and it doesn't have to inline function, however definition of fun2 is so simple, so it could be inlined. How to force g++ to inline functions?

回答1:

GCC has an attribute which forces inlining: always_inline

inline int fun2()  __attribute__((always_inline));
inline int fun2() {  
   return 4 + 5;
}

Will cause it to work on any optimization settings.

回答2:

Turn on optimizations. This is what you get for the main function with -O2 (x86_64):

0000000000400560 <main>:
  400560:   31 c0                   xor    %eax,%eax
  400562:   c3                      retq

It's not only inline, it's removed.

Without optimizations, the compiler is much less likely to inline. (Inlining makes the code harder to debug, so having only very moderate levels of inlining with default non-optimizing options is a good idea.)

回答3:

inline is a compiler hint. The compiler may or may not actually inline your code.

来源：https://stackoverflow.com/questions/7866861/g-doesnt-inline-functions

标签

c++

inline