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To: Nicola Pero <nicola@brainstorm.co.uk>
Cc: Andrew Haley <aph@redhat.com>, Robert Dewar <dewar@gnat.com>,
   <nathan@codesourcery.com>, <shebs@apple.com>, <gcc@gcc.gnu.org>
Subject: Re: On alignment
From: Jason Merrill <jason@redhat.com>
In-Reply-To: <Pine.LNX.4.44.0304221648080.31614-100000@nicola.brainstorm.co.uk> (Nicola
 Pero's message of "Tue, 22 Apr 2003 17:04:38 +0100 (BST)")
References: <Pine.LNX.4.44.0304221648080.31614-100000@nicola.brainstorm.co.uk>
Date: Tue, 22 Apr 2003 18:46:00 -0000
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On Tue, 22 Apr 2003 17:04:38 +0100 (BST), Nicola Pero <nicola@brainstorm.co.uk> wrote:

> At runtime, the ObjC runtime library has information about the struct -
> the type of each member.  At the moment, it loops on the struct members,
> and uses __alignof__ on each of them to get the alignment of that member
> inside the struct (then, can read/write the struct members).  As far as I
> understand from this discussion, this doesn't work any longer.

If you actually take __alignof the member, this should work fine.  If you
do __alignof the type, you will get the larger number.

On Tue, 22 Apr 2003 17:19:28 +0100, Andrew Haley <aph@redhat.com> wrote:

> It's rather scary.  It means, for example, that
>
> template<class T> void copy (T *a, T *b)
> {
>   if (__alignof__ (T) >= 8)
>     copy_by_dwords (a, b);
>   else
>     copy_by_bytes (a, b);
> }    
>
> won't do what is expected when applied to a member of a struct.  I
> suppose we get away with this because the x86 never generates
> alignment traps in such cases.

Exactly.  A STRICT_ALIGNMENT target wouldn't do this sort of thing.

Jason