From mboxrd@z Thu Jan  1 00:00:00 1970
From: Bernd Schmidt <bernds@cygnus.co.uk>
To: Alexandre Oliva <oliva@lsd.ic.unicamp.br>
Cc: Joe Buck <jbuck@synopsys.COM>, gcc@gcc.gnu.org, rms@gnu.org
Subject: Re: type based aliasing again
Date: Thu, 30 Sep 1999 18:02:00 -0000
Message-ID: <Pine.LNX.4.10.9909141328450.20868-100000@biriani.cygnus.co.uk>
References: <oryae97qt7.fsf@cupuacu.lsd.ic.unicamp.br>
X-SW-Source: 1999-09n/msg00525.html
Message-ID: <19990930180200.UNJu93QyCu_1R9IVGMSCMqTDRR1r65WmEQWm7F89H6k@z>

> BTW, it seems to me that this `cast to union' is something particular
> to gcc, not a general solution, right?  I mean, other compilers might
> not pay as much attention to the unions as gcc does, since accessing
> any type other than the one that was actually stored in the union is
> undefined behavior anyway.

If, by "cast to union", you mean code of the form

   union foo
   {
      double a;
      int b[2];
   };
   double x;
   x = 2.0;
   ((union foo *)&x)->b[1] = 0;

then this is not even going to work with gcc, regardless of whether
-fstrict-aliasing is on or not.  We found code like this in some math
library code here at Cygnus a few weeks ago.  The problem is that gcc's
MEM_IN_STRUCT/MEM_SCALAR_P alias analysis will catch cases like this and
determine that the integer memory access can't alias the variable x.
The code should be written

   union foo
   {
      double a;
      int b[2];
   };
   union foo x;
   x.a = 2.0;
   x.b[1] = 0;

The guarantee that this will work is a gcc extension.

Bernd