RE: Unaligned access to packed structs on ppc405

["John Yates" <jyates@netezza.com>]

David,

Do I read this correctly?  Are you truly saying that two structs
with identical layout will trigger different code sequences just
because one was declared packed?

(This is just the sort of thing that drives me crazy when trying
to coax a compiler into producing good code.)

/john

-----Original Message-----
From: David Edelsohn [mailto:dje@watson.ibm.com]
Sent: Friday, February 03, 2006 11:26 PM
To: Yaro Pollak
Cc: gcc-help@gcc.gnu.org
Subject: Re: Unaligned access to packed structs on ppc405


>>>>> Yaro Pollak writes:

> If on the other hand I compile:
> 
>    Test pt;
>    return pt.b + pt.c + pt.d;
> 
>    lwz 3,8(1)

	That is an artifact of the example.  If I declare "pt" as a global
variable, GCC 4.1 prerelease produces three lbz.

	The lbz has to do with the size and the packed alignment.  With
the packed structure, GCC chooses the smallest memory access that covers
the bitfield.  Once GCC has chosen bytes, it cannot merge the loads
together.  If the structure were not declared packed, GCC would use wider
loads with masking, and then determine that the loads refer to the same
object. 

David