Re: optimization/4733: Incorrect code when aliasing double and struct on ARM -O2

[Richard Earnshaw <rearnsha at arm dot com>]

The following reply was made to PR optimization/4733; it has been noted by GNATS.

From: Richard Earnshaw <rearnsha@arm.com>
To: bjh21@netbsd.org
Cc: gcc-gnats@gcc.gnu.org, Richard.Earnshaw@arm.com
Subject: Re: optimization/4733: Incorrect code when aliasing double and 
 struct on ARM -O2
Date: Mon, 29 Oct 2001 17:43:44 +0000

 > >How-To-Repeat:
 > use "./cc1 -O2"
 > 
 > typedef	unsigned int	u_int;
 > struct ieee_double {
 > 	u_int	dbl_fracl;
 > 	u_int	dbl_frach:20;
 > 	u_int	dbl_exp:11;
 > 	u_int	dbl_sign:1;
 > };
 > int
 > isinf (d)
 > 	double d;
 > {
 > 	register struct ieee_double *p = (struct ieee_double *)(void *)&d;
 > 
 > 	return (p->dbl_exp == 2047  &&
 > 	    (p->dbl_frach == 0 && p->dbl_fracl == 0));
 > }
 
 Nope, this code violates the ANSI memory aliasing rules.  According to 
 ANSI, the only way to do this portably is to memcpy between the types; gcc 
 will do the right thing if you use a union, but not otherwise.
 
 R.