How to enable optimization in G++ with #pragma

Question

I want to enable optimization in g++ without command line parameter. I know GCC can do it by writing #pragma GCC optimize (2) in my code. But it seems won\'t work i

Answer 1

This appears to be a bug in g++ (Bug 48026, references another related issue.)

As a workaround, you can mark each function with __attribute__((optimize("whatever"))). Not great.

int main() __attribute__((optimize("-O2")));
int main()
{
    long x;
    x=11;
    x+=12;
    x*=13;
    x/=14;
    return 0;
}

$ g++ -Wall -c t.c
$ objdump -d t.o

t.o:     file format elf64-x86-64


Disassembly of section .text.startup:

0000000000000000 <main>:
   0:   55                      push   %rbp
   1:   31 c0                   xor    %eax,%eax
   3:   48 89 e5                mov    %rsp,%rbp
   6:   5d                      pop    %rbp
   7:   c3                      retq