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Re: powerpc & unaligned block moves with fp registers
- From: Dale Johannesen <dalej at apple dot com>
- To: dewar at gnat dot com (Robert Dewar)
- Cc: Dale Johannesen <dalej at apple dot com>, mrs at apple dot com, dj at redhat dot com, dje at watson dot ibm dot com, gcc at gcc dot gnu dot org
- Date: Sat, 1 Nov 2003 11:46:41 -0800
- Subject: Re: powerpc & unaligned block moves with fp registers
On Saturday, November 1, 2003, at 04:21 AM, Robert Dewar wrote:
We should be very careful to not remove `working' and reasonable
semantics that are widely implemented and depended on by existing
code,
just because some standard can be read in such a way that suggest that
something might not be mandated to work.
I agree with this, actually, but the group seems to be moving more in
the
direction of not supporting anything nonstandard (strict aliasing, which
broke lots of existing code including gcc itself, springs to mind).
I would think that many C programs assume that access to a volatile
variable
is in fact atomic if the machine can easily generate instructions to
make
it so, given that standard C lacks a mechanism for specifying atomic
access,
so I would agree with Mike that you want to be careful here on making
changes.
I would have to be shown that there is existing code that expects
volatile
accesses to misaligned doubles to be atomic. This doesn't even work in
many environments.