How to enable optimization in g ++ C # pragma

Question

I want to enable optimization in g ++ without a command line parameter. I know that GCC can do this by writing #pragma GCC optimize (2) in my code. But it doesn't seem to work in g ++.

This page can help: http://gcc.gnu.org/onlinedocs/gcc/Function-Specific-Option-Pragmas.html

My compiler version:

 $ g++ --version g++ (Ubuntu/Linaro 4.6.1-9ubuntu3) 4.6.1 <suppressed copyright message> $ gcc --version gcc (Ubuntu/Linaro 4.6.1-9ubuntu3) 4.6.1 <suppressed copyright message> 

I worte the code like this:

 #pragma GCC optimize (2) int main(){ long x; x=11; x+=12; x*=13; x/=14; return 0; } 

And compiled it using GCC Not g ++ . Then I used objdump, which outputs

 08048300 <main>: 8048300: 55 push %ebp 8048301: 31 c0 xor %eax,%eax 8048303: 89 e5 mov %esp,%ebp 8048305: 5d pop %ebp 8048306: c3 ret 8048307: 90 nop 

When I removed #param GCC optimize(2) . Objdump output:

 080483b4 <main>: 80483b4: 55 push %ebp 80483b5: 89 e5 mov %esp,%ebp 80483b7: 83 ec 10 sub $0x10,%esp 80483ba: c7 45 fc 0b 00 00 00 movl $0xb,-0x4(%ebp) 80483c1: 83 45 fc 0c addl $0xc,-0x4(%ebp) 80483c5: 8b 55 fc mov -0x4(%ebp),%edx 80483c8: 89 d0 mov %edx,%eax 80483ca: 01 c0 add %eax,%eax 80483cc: 01 d0 add %edx,%eax 80483ce: c1 e0 02 shl $0x2,%eax 80483d1: 01 d0 add %edx,%eax 80483d3: 89 45 fc mov %eax,-0x4(%ebp) 80483d6: 8b 4d fc mov -0x4(%ebp),%ecx 80483d9: ba 93 24 49 92 mov $0x92492493,%edx 80483de: 89 c8 mov %ecx,%eax 80483e0: f7 ea imul %edx 80483e2: 8d 04 0a lea (%edx,%ecx,1),%eax 80483e5: 89 c2 mov %eax,%edx 80483e7: c1 fa 03 sar $0x3,%edx 80483ea: 89 c8 mov %ecx,%eax 80483ec: c1 f8 1f sar $0x1f,%eax 80483ef: 89 d1 mov %edx,%ecx 80483f1: 29 c1 sub %eax,%ecx 80483f3: 89 c8 mov %ecx,%eax 80483f5: 89 45 fc mov %eax,-0x4(%ebp) 80483f8: b8 00 00 00 00 mov $0x0,%eax 80483fd: c9 leave 80483fe: c3 ret 80483ff: 90 nop 

However, it will not work with g ++!

Answer 1

This is apparently a bug in g++ ( error 48026 , referencing another related issue.)

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

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

 $ g++ -Wall -c tc $ objdump -d to to: 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