How is the Union Type derived in LLVM

Question

union u{
 char ch[41];
 int b[10];  
}un;

The LLVM compiles to this

%union.u = type { [10 x i32], [4 x i8] }

and this

union un{
  struct s{
    int a;
    float f;
    double d;
  }st;

  int intArr[10];
}uno;

compiles to this

%union.un = type { %struct.s, [24 x i8] }
%struct.s = type { i32, float, double }

Can any one explain how the union type is derived ?

Answer 1

The first member is just the most aligned member of the union (it arbitrarily picks one if there are multiple such members), and an array of i8 is appended to make it the right size.