Question

i’m writting handler for OpenGL texture and i’m thinking about safety and performance. Which level of optimization should remove marked if statements?

struct Texture2D {
    GLuint ID;

    inline Texture2D(): ID(0) {};
    inline explicit Texture2D(GLuint id): ID(id) {};
    ~Texture2D();

    void GenTexture(bool regen = false);
    void DeleteTexture();

    void BindTexture();

    void Parameterf( GLenum pname, GLfloat param );
    void Parameteri( GLenum pname, GLint param );
    void glTexParameterfv( GLenum target, GLenum pname, const GLfloat *params );
    void glTexParameteriv( GLenum target, GLenum pname, const GLint *params );

    static Texture2D binded;
};
inline void Texture2D::GenTexture(bool regen) {
    if(ID){
        if(regen)
            DeleteTexture();
        else
            return;
    }

    glGenTextures(1,&ID);
}

inline void Texture2D::DeleteTexture() {
    glDeleteTextures(1,&ID);
    ID = 0;
}

inline void Texture2D::BindTexture() {
    glBindTexture(GL_TEXTURE_2D, ID);
    binded.ID = ID;
}

inline void Texture2D::Parameterf( GLenum pname, GLfloat param ){
    if(binded.ID == ID)                          // THIS
        BindTexture();                           // THIS

    glTexParameterf(GL_TEXTURE_2D,pname,param);
}

inline void Texture2D::Parameteri( GLenum pname, GLint param ){
    if(binded.ID == ID)                          // THIS
        BindTexture();                           // THIS

    glTexParameterf(GL_TEXTURE_2D,pname,param);
}

inline Texture2D::~Texture2D() {
    DeleteTexture();
}

// in this function
void loadTexture(...) {
    Texture2D t;
    t.GenTexture();
    t.BindTexture();
    // if statements in next functions
    t.Parameterf(...);
    t.Parameterf(...);
    t.Parameterf(...);
    t.Parameterf(...);
    t.Parameterf(...);
}

Answer 1

None.

Sad story, but C++ assumes that if you call a function, then this function might produce all kinds of side effects, including changing the value of binded.ID (which the function somehow knows to do)

Except

If you make sure that the functions you invoke have absolutely no legal way to know about your bindend.ID, either directly (by referencing it) or indirectly (because somebody else take a pointer of it and passed it around). Here’s a simple example (assuming that side_effect() is in a different translation unit)

int side_effect();
int k=1; 

int main()
{
    side_effect(); 
    if (k!=0) return 0;
    side_effect(); 
    if (k!=0) return 0;
    side_effect(); 
    if (k!=0) return 0;
}

side_effect() can use and change k legally by declaring it as an external. No call of side_effect can be optimized away.

int side_effect();
static int k=1; 

int main()
{
    side_effect(); 
    if (k!=0) return 0;
    side_effect(); 
    if (k!=0) return 0;
    side_effect(); 
    if (k!=0) return 0;
}

It is not possible for side_effect to access k in an allowed manner, because you can’t access statics in another translation unit. Therefore the code can be optimized to side_effect(); return 0 because k will not change, as long as side_effect() does not poke around in the memory. Which would be undefined behavior of course.

int side_effect();
void snitch(int*);

static int k=1; 

int main()
{
    snitch(&k); // !!!
    side_effect(); 
    if (k!=0) return 0;
    side_effect(); 
    if (k!=0) return 0;
    side_effect(); 
    if (k!=0) return 0;
}

The compiler has no way to know, if snitch() saves its argument in a place where side_effect() can change it, therefore no call to side_effect() can be eliminated.

You get the same situation, if you have k as a local variable: If there is a possibility that some unknown routine can access k in a legal way, then the compiler can not make optimizations based on the value of k.

PS: Making k const does not help, because it is legal to cast a const away. const-ness can not be used as a optimization hint.