c/10360: __alignof__(double) answer 8

c/10360: __alignof__(double) answer 8

[thoran]

>Number:         10360
>Category:       c
>Synopsis:       __alignof__(double) answer 8
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Apr 09 09:06:00 UTC 2003
>Closed-Date:
>Last-Modified:
>Originator:     Frederic De Jaeger
>Release:        gcc 3.2.3
>Organization:
>Environment:
Reading specs from /usr/lib/gcc-lib/i386-linux/3.2.3/specs
Configured with: ../src/configure -v --enable-languages=c,c++,java,f77,proto,pascal,objc,ada --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --with-gxx-include-dir=/usr/include/c++/3.2 --enable-shared --with-system-zlib --enable-nls --without-included-gettext --enable-__cxa_atexit --enable-clocale=gnu --enable-java-gc=boehm --enable-objc-gc i386-linux
Thread model: posix
gcc version 3.2.3 20030309 (Debian prerelease)

Linux thc 2.4.20-k7 #1 Tue Jan 14 00:29:06 EST 2003 i686 unknown unknown GNU/Linux
>Description:
__alignof__(double) answer 8.  But actually, it is 4 (that's what gcc generates for record).

Apparently the problem happened in gcc 3.0
>How-To-Repeat:
#include <stdio.h>

struct bla
{
  char c;
  double v;
};

void main (void)
{
  struct bla b;
  printf("align double %d\neffective alignement %d\n", __alignof__(double),
	 ((char *)(&b.v)) - ((char *)(&b.c)));
}
>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted:

Re: c/10360: __alignof__(double) answer 8

[Richard Henderson]

The following reply was made to PR c/10360; it has been noted by GNATS.

From: Richard Henderson <rth@redhat.com>
To: Richard Frith-Macdonald <richard@brainstorm.co.uk>
Cc: gcc-gnats@gcc.gnu.org, thoran@free.fr, nobody@gcc.gnu.org,
   gcc-prs@gcc.gnu.org, gcc-bugs@gcc.gnu.org, discuss-gnustep@gnu.org,
   rth@gcc.gnu.org, Frederic De Jaeger <dejaeger@free.fr>
Subject: Re: c/10360: __alignof__(double) answer 8
Date: Thu, 10 Apr 2003 01:00:00 -0700

 On Thu, Apr 10, 2003 at 06:06:44AM +0100, Richard Frith-Macdonald wrote:
 > It would be interesting to know in what sense the alignment is 
 > 'preferred'.
 
 In that it takes one less cycle to load that way?  In that that's
 how data gets aligned which isn't constrained by the ABI?
 
 > Shouldn't this either be fixed as Frederic suggests, or the documentation
 > be changed so that instead of saying 'preferred' it says 'misleading and
 > largely useless'
 
 And indeed, that's what we ALREADY say, except you didn't read it.
 To wit:
 
 	Some machines never actually require alignment; they allow
 	reference to any data type even at an odd addresses. For
 	these machines, __alignof__ reports the recommended alignment
 	of a type.
 
 
 r~

Re: c/10360: __alignof__(double) answer 8

[Richard Frith-Macdonald]

The following reply was made to PR c/10360; it has been noted by GNATS.

From: Richard Frith-Macdonald <richard@brainstorm.co.uk>
To: Richard Henderson <rth@redhat.com>
Cc: gcc-gnats@gcc.gnu.org, thoran@free.fr, nobody@gcc.gnu.org,
   gcc-prs@gcc.gnu.org, gcc-bugs@gcc.gnu.org, discuss-gnustep@gnu.org,
   rth@gcc.gnu.org, Frederic De Jaeger <dejaeger@free.fr>
Subject: Re: c/10360: __alignof__(double) answer 8
Date: Thu, 10 Apr 2003 06:06:44 +0100

 On Wednesday, April 9, 2003, at 11:40  pm, Richard Henderson wrote:
 
 > On Wed, Apr 09, 2003 at 11:59:58PM +0200, Frederic De Jaeger wrote:
 >> Thus, why gcc does not align fields with respect to this *preferred*
 >> alignment?
 >
 > Because the ABI says not to.
 
 It would be interesting to know in what sense the alignment is 
 'preferred'.
 Surely if the 'ABI says not to' then the alignment is not preferred?
 
 >> ... and we expect it to return the alignment used by the compiler 
 >> (and not
 >> the *preferred* alignment).
 >
 > A meaningless number.  Because "the alignment used by the compiler"
 > is going to depend on the context in which it is used.
 
 I think that Frederic means the alignment the compiler uses to lay out
 structures in memory.
 
 It's hard to see how that can be context sensistive since the compiler
 needs to know it to access the structure elements via a pointer, so
 the code that stored the structure in memory and the code that
 retrieves it through the pointer have to use the same alignments.
 
 I imagine this is *by far* the most common context in which anyone
 would want to know a type alignment (actually it's the only one I can
 think of off hand).
 
 So if __alignof__ is not returning the alignment of a type for structure
 layout, how can it be returning the 'preferred' alignment.  Shouldn't 
 this
 either be fixed as Frederic suggests, or the documentation be changed
 so that instead of saying 'preferred' it says 'misleading and largely
 useless'
 
 Sorry if this sounds overly aggressive, but I do think that compiler 
 extensions
 should be useful and behave as expected, and this behavior seems to
 mean that __alignof__ fails in this.
 

Re: c/10360: __alignof__(double) answer 8

[Richard Henderson]

The following reply was made to PR c/10360; it has been noted by GNATS.

From: Richard Henderson <rth@redhat.com>
To: Frederic De Jaeger <dejaeger@free.fr>
Cc: rth@gcc.gnu.org, discuss-gnustep@gnu.org, gcc-bugs@gcc.gnu.org,
   gcc-prs@gcc.gnu.org, nobody@gcc.gnu.org, richard@brainstorm.co.uk,
   thoran@free.fr, gcc-gnats@gcc.gnu.org
Subject: Re: c/10360: __alignof__(double) answer 8
Date: Wed, 9 Apr 2003 15:40:39 -0700

 On Wed, Apr 09, 2003 at 11:59:58PM +0200, Frederic De Jaeger wrote:
 > Thus, why gcc does not align fields with respect to this *preferred*
 > alignment?
 
 Because the ABI says not to.
 
 > How can I compute the address of a field in a record?
 
 offsetof.
 
 > I need to do this uniformly on all the types.  That means I cannot use
 > the trick : 
 >  offset = (char *)&foo.bla - (char *)&foo.
 > or the "offsetof" macro.
 
 Tough luck then.
 
 > ... and we expect it to return the alignment used by the compiler (and not
 > the *preferred* alignment).
 
 A meaningless number.  Because "the alignment used by the compiler"
 is going to depend on the context in which it is used.
 
 
 r~

Re: c/10360: __alignof__(double) answer 8

[Frederic De Jaeger]

The following reply was made to PR c/10360; it has been noted by GNATS.

From: Frederic De Jaeger <dejaeger@free.fr>
To: rth@gcc.gnu.org, discuss-gnustep@gnu.org, gcc-bugs@gcc.gnu.org,
   gcc-prs@gcc.gnu.org, nobody@gcc.gnu.org, richard@brainstorm.co.uk,
   thoran@free.fr, gcc-gnats@gcc.gnu.org
Cc:  
Subject: Re: c/10360: __alignof__(double) answer 8
Date: 09 Apr 2003 23:59:58 +0200

 rth> Synopsis: __alignof__(double) answer 8
 rth> State-Changed-From-To: open->closed
 rth> State-Changed-By: rth
 rth> State-Changed-When: Wed Apr  9 17:29:53 2003
 rth> State-Changed-Why:
 rth>     Not a bug.  __alignof__ returns the *preferred* alignment.
 rth>     You're seeing other SVr4 ABI rules concerning structures.
 
 Thus, why gcc does not align fields with respect to this *preferred*
 alignment?
 How can I compute the address of a field in a record?
 
 I need to do this uniformly on all the types.  That means I cannot use
 the trick : 
  offset = (char *)&foo.bla - (char *)&foo.
 or the "offsetof" macro.
 because I don't know, statically, the type of the record from which I
 need to compute offsets.  
 
 The type information comes from the @encode directive of
 objective-C (ie:  it's a characters string that describe a type).
 Actually, we are using "objc_alignof_type" (from the objective-c API)
 and we expect it to return the alignment used by the compiler (and not
 the *preferred* alignment).
 
 Thanks for your help,
 
 
        Frederic De Jaeger

Re: c/10360: __alignof__(double) answer 8

[rth]

Synopsis: __alignof__(double) answer 8

State-Changed-From-To: open->closed
State-Changed-By: rth
State-Changed-When: Wed Apr  9 17:29:53 2003
State-Changed-Why:
    Not a bug.  __alignof__ returns the *preferred* alignment.
    You're seeing other SVr4 ABI rules concerning structures.

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=10360