phc-discussions - Re: [PHC] On type aliasing and similar issues

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Date: Wed, 08 Apr 2015 04:03:01 +0100
From: Samuel Neves <sneves@....uc.pt>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] On type aliasing and similar issues

On 04/08/2015 03:37 AM, Alexander Cherepanov wrote:
> AFACT this is implementation-defined in C89 (3.3.2.3) and fully defined in C99 and C11 (6.5.2.3p3). 

Yes, type punning with unions is now OK (though implementation-defined; accessing the wrong member may still trap) in
C99 and above. What is being dereferenced in the example is the pointer to the union, not the members, so I'm not sure
strict aliasing's undefined behavior applies. The example could be further improved to demonstrate this:

  #include <stdint.h>
  #include <stdio.h>

  union U {
    uint32_t x[2];
    uint64_t y;
  };

  extern union U * v;

  void f() {
    uint32_t * p = &v->x[0];
    uint64_t * q = &v->y;
    *p = 17;
    *q = 42;
    printf("%u\n", v->x[0]);
  }

While Clang and recent GCC do what one would hope (print 42), GCC 3.4 and Intel compiler print 17.