From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Martin v. Loewis" <martin@mira.isdn.cs.tu-berlin.de>
To: mark@codesourcery.com
Cc: nik@tiuk.ti.com, tim@franck.Princeton.EDU, egcs@egcs.cygnus.com, george@moberg.com
Subject: Re: Clarification anyone?  --  was Re: Linux and aliasing?
Date: Tue, 08 Jun 1999 15:47:00 -0000
Message-id: <199906082213.AAA00692@mira.isdn.cs.tu-berlin.de>
References: <199906081731.KAA16675@franck.Princeton.EDU> <199906081732.SAA13273@tiuk.ti.com> <19990608104553V.mitchell@codesourcery.com> <19990608104553V.mitchell@codesourcery.com>
X-SW-Source: 1999-06/msg00308.html

>     Nick> Even in the same function?
> 
> Right.

In some cases, flow analysis will detect that the value is clobbered.
Consider

unsigned long foo()
{
   unsigned long a=15;
    *((short *) &a) = 123;
   return a;
}

Compiling this with gcc-2.95 19990606 -O2 -fomit-frame-pointer gives
me

foo:
	movl $123,%eax
	ret

so the compiler clearly detects that a has been changed. The case that
doesn't work as before is

short *bar(unsigned long *a)
{
  return (short*)a;
}

unsigned long foo()
{
   unsigned long a;
   short *addr =  *bar(&a);
   a = 15;
   *addr = 123;
   return a;
}

Regards,
Martin

From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Martin v. Loewis" <martin@mira.isdn.cs.tu-berlin.de>
To: mark@codesourcery.com
Cc: nik@tiuk.ti.com, tim@franck.Princeton.EDU, egcs@egcs.cygnus.com, george@moberg.com
Subject: Re: Clarification anyone?  --  was Re: Linux and aliasing?
Date: Wed, 30 Jun 1999 15:43:00 -0000
Message-ID: <199906082213.AAA00692@mira.isdn.cs.tu-berlin.de>
References: <199906081731.KAA16675@franck.Princeton.EDU> <199906081732.SAA13273@tiuk.ti.com> <19990608104553V.mitchell@codesourcery.com>
X-SW-Source: 1999-06n/msg00308.html
Message-ID: <19990630154300.FIj_Oq_qneg4iv3OgdZlCBoNQpORWgBgcxuo0SWZECA@z>

>     Nick> Even in the same function?
> 
> Right.

In some cases, flow analysis will detect that the value is clobbered.
Consider

unsigned long foo()
{
   unsigned long a=15;
    *((short *) &a) = 123;
   return a;
}

Compiling this with gcc-2.95 19990606 -O2 -fomit-frame-pointer gives
me

foo:
	movl $123,%eax
	ret

so the compiler clearly detects that a has been changed. The case that
doesn't work as before is

short *bar(unsigned long *a)
{
  return (short*)a;
}

unsigned long foo()
{
   unsigned long a;
   short *addr =  *bar(&a);
   a = 15;
   *addr = 123;
   return a;
}

Regards,
Martin