Subject: Re: powerpc gcc alignment problem
To: Dan Winship <danw@MIT.EDU>
From: Jason Thorpe <>
List: tech-toolchain
Date: 01/13/2000 17:10:29
On Tue, 28 Dec 1999 12:44:48 -0500 (EST) 
 Dan Winship <danw@MIT.EDU> wrote:

 > I'd like to fix this in time for 1.4.2.


 > #defining STRICT_ALIGNMENT in gcc makes it not generate the unaligned
 > accesses, but that's an overly large hammer for this, since (a) we
 > only care about 64-bit types, not smaller ones, and (b) they don't
 > actually have to be fully aligned, just half-aligned.

Technically speaking, the PowerPC *does* have strict alignment, because
unaligned accesses require fixups (right?  Or, did I totally misunderstand
that part of the book when I read it...)

I think STRICT_ALIGNMENT is the way to go.  Although, you should make
sure it doesn't have any side-effects on e.g. how structure members are
padded.  I sure hope 32-bit data types are always aligned to 32 bits, etc.

 > #defining SLOW_UNALIGNED_ACCESS also fixes the code, but it seems to
 > result in much more code than the STRICT_ALIGNMENT case in the sample
 > program in PR 8952.
 > As far as I've been able to tell, there's no way for the MD part of
 > the gcc sources to express "floats must be half-aligned and other
 > things don't need to be aligned" to the MI part. I have some ideas on
 > how it could be done, but it would require changes to lots of files,
 > so it seems like the sort of thing we'd want to get put into the real
 > gcc tree and then we could import it from them.
 > So... for now, does anyone object to my turning on STRICT_ALIGNMENT in
 > /usr/src/gnu/dist/gcc/config/rs6000/netbsd.h and then getting that
 > pulled up to the release branch?
 > -- Dan

        -- Jason R. Thorpe <>