Re: [std-discussion] Is this union aliasing code well-defined?

[Yubin Ruan <ablacktshirt at gmail dot com>]

+Cc gcc-list.

Does any gcc developer have any comments?

On Mon, Sep 25, 2017 at 01:41:55PM -0700, Myriachan wrote:
> This question that "supercat" posted on Stack Overflow ran into an 
> interesting problem:
> 
> https://stackoverflow.com/questions/46205744/is-this-use-of-unions-strictly-conforming/
> 
> A copy of the code involved is as follows:
> 
> struct s1 {unsigned short x;};
> struct s2 {unsigned short x;};
> union s1s2 { struct s1 v1; struct s2 v2; };
> 
> static int read_s1x(struct s1 *p) { return p->x; }
> static void write_s2x(struct s2 *p, int v) { p->x=v;}
> 
> int test(union s1s2 *p1, union s1s2 *p2, union s1s2 *p3)
> {
>   if (read_s1x(&p1->v1))
>   {
>     unsigned short temp;
>     temp = p3->v1.x;
>     p3->v2.x = temp;
>     write_s2x(&p2->v2,1234);
>     temp = p3->v2.x;
>     p3->v1.x = temp;
>   }
>   return read_s1x(&p1->v1);
> }
> int test2(int x)
> {
>   union s1s2 q[2];
>   q->v1.x = 4321;
>   return test(q,q+x,q+x);
> }
> #include <stdio.h>
> int main(void)
> {
>   printf("%d\n",test2(0));
> }
> 
> 
> Both GCC and Clang in -fstrict-aliasing mode with optimizations are acting 
> as if they ran into undefined behavior, and return 4321 instead of the 
> expected 1234.  This happens in both C and C++ mode.  Intel C++ and Visual 
> C++ return the expected 1234.  All four compilers hardwire the result as a 
> constant parameter to printf rather than call test2 or modify memory at 
> runtime.
> 
> From my reading of the C++ Standard, particularly [class.union]/5, 
> assignment expressions through a union member access changes the active 
> member of the union (if the union member has a trivial default constructor, 
> which it does here, being C code).  Taking the address of p2->v2 and p1->v1 
> ought to be legal because those are the active members of the union at the 
> time their pointers are taken.
> 
> Is this a well-defined program, or is there subtle undefined behavior 
> happening here?
> 
> Melissa
> 
> -- 
> 
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