Question

Are there any flags needed to compile code with inlined assembly instructions?

I am trying to get g++ compile the following code (cloned from an answer here on SO):

#include <iostream>

using namespace std;

inline unsigned int get_cpu_feature_flags()
{
    unsigned int features;

    __asm
    {                             // <- Line 10
        // Save registers
        push    eax
        push    ebx
        push    ecx
        push    edx

        // Get the feature flags (eax=1) from edx
        mov     eax, 1
        cpuid
        mov     features, edx

        // Restore registers
        pop     edx
        pop     ecx
        pop     ebx
        pop     eax
    }

    return features;
}

int main() {
    // Bit 26 for SSE2 support
    static const bool cpu_supports_sse2 = (get_cpu_feature_flags() & 0x04000000)!=0;
    cout << (cpu_supports_sse2? "Supports SSE" : "Does NOT support SSE");
}

but I get the following error:

$ g++ t2.cpp 
t2.cpp: In function ‘unsigned int get_cpu_feature_flags()’:
t2.cpp:10:5: error: expected ‘(’ before ‘{’ token
t2.cpp:12:9: error: ‘push’ was not declared in this scope
t2.cpp:12:17: error: expected ‘;’ before ‘eax’
$

Answer 1

As others have hinted but not said explicitly, this is incorrect syntax for gcc (which uses a string-based asm(“…”) language instead of true inline assembly code) and for gas (which uses AT&T syntax instead of Intel syntax).

A google for “gcc inline assembly” pulls up this tutorial, which looks good:

http://www.ibiblio.org/gferg/ldp/GCC-Inline-Assembly-HOWTO.html

And you can find the relevant section of the gcc documentation here:

http://gcc.gnu.org/onlinedocs/gcc-4.7.1/gcc/Extended-Asm.html