Strict aliasing and overlay inheritance

Question

Consider this code example:

#include 

typedef struct A A;

struct A {
   int x;
   int y;
};

typedef struct B B;

struct B {
   int x;
   in         


        

Answer 1

The line with *ap = is a strict aliasing violation: an object of type B is written using an lvalue expression of type A.

Supposing that line was not present, and we moved onto ap->x = 10; ap->y = 20;. In this case an lvalue of type int is used to write objects of type int.

There is disagreement about whether this is a strict aliasing violation or not. I think that the letter of the Standard says that it is not, but others (including gcc and clang developers) consider ap->x as implying that *ap was accessed. Most agree that the standard's definition of strict aliasing is too vague and needs improvement.

Sample code using your struct definitions:

void f(A* ap, B* bp)
{
  ap->x = 213;
  ++bp->x;
  ap->x = 213;
  ++bp->x;
}

int main()
{
   B b = { 0 };
   f( (A *)&b, &b );
   printf("%d\n", b.x);
}

For me this outputs 214 at -O2, and 2 at -O3 , with gcc. The generated assembly on godbolt for gcc 6.3 was:

f:
    movl    (%rsi), %eax
    movl    $213, (%rdi)
    addl    $2, %eax
    movl    %eax, (%rsi)
    ret

which shows that the compiler has rearranged the function to:

int temp = bp->x + 2;
ap->x = 213;
bp->x = temp;

and therefore the compiler must be considering that ap->x may not alias bp->x.