Question

According to the GCC documentation, __attribute__((pure)) tells the compiler that a function has no side-effects, and so it can be subject to common subexpression elimination.

This attribute appears to work for non-virtual functions, but not for virtual functions. For example, consider the following code:

extern void f( int );

class C {
 public:
   int a1();
   int a2() __attribute__((pure));
   virtual int b1();
   virtual int b2() __attribute__((pure));
};

void test_a1( C *c ) {
   if( c->a1() ) {
      f( c->a1() );
   }
}

void test_a2( C *c ) {
   if( c->a2() ) {
      f( c->a2() );
   }
}

void test_b1( C *c ) {
   if( c->b1() ) {
      f( c->b1() );
   }
}

void test_b2( C *c ) {
   if( c->b2() ) {
      f( c->b2() );
   }
}

When compiled with optimization enabled (either -O2 or -Os), test_a2() only calls C::a2() once, but test_b2() calls b2() twice.

Is there a reason for this? Is it because, even though the implementation in class C is pure, g++ can’t assume that the implementation in every subclass will also be pure? If so, is there a way to tell g++ that this virtual function and every subclass’s implementation will be pure?

Answer 1

Without looking into g++’s internals, I suspect it’s because g++ can’t assume that every subclass’s implementation will be pure (like you said).

Can you turn b2 into a non-virtual pure wrapper around a virtual non-pure method?