Aliasing bug

[Andrew Stubbs <ams at codesourcery dot com>]

Hi all,

I'm fairly sure I have found an aliasing bug in GCC, although I could be 
wrong. I've reproduced it in both 4.4 and mainline.

Consider this testcase, aliasing.c:

    extern void *foo;

    extern inline short **
    f1 (void)
    {
      union
        {
          void **v;
          short **s;
        } u;

      u.v = (&foo);
      if (*u.s == 0) *u.s = (short *)42;
      return u.s;
    }

    const short *a, *b;

    int
    f ()
    {
      a = *f1();
      b = *f1();
    }

The (not very useful) testcase initialises foo to 42, if necessary, and 
then sets both 'a' and 'b' to equal foo. There should be no way that 'a' 
and 'b' can ever be set to zero.

Compile the code as follows:

  sh-linux-gnu-gcc -c aliasing.c -O2 -fdump-tree-all

The dump file aliasing.c.133t.optimized (the last tree dump) then contains:

    f ()
    {
      void * foo.3;
      short int * D.1982;
      short int * * D.1973;

    <bb 2>:
      foo.3_10 = foo;
      D.1982_26 = (short int *) foo.3_10;
      if (D.1982_26 == 0B)
        goto <bb 3>;
      else
        goto <bb 5>;

    <bb 3>:
      D.1973_13 = (short int * *) &foo;
      *D.1973_13 = 42B;
      a = 0B;

    <bb 4>:
      b = D.1982_26;
      return;

    <bb 5>:
      a = D.1982_26;
      goto <bb 4>;

    }

This is the state of the code after the tree optimisations. Both 'a' and 
'b' are set to the initial value of foo, before it was initialised. Not 
only that, but 'a' is explicitly set to zero.

This problem goes away if -fno-strict-aliasing is used.

Is this a compiler bug? Or have I got something wrong in my code?

Thanks

Andrew