[PATCH, PR43814] Assume function arguments of pointer type are aligned.

[Jakub Jelinek]

On Wed, Sep 28, 2011 at 03:00:09PM -0400, David Miller wrote:
> From: Maxim Kuvyrkov <maxim@codesourcery.com>
> Date: Thu, 29 Sep 2011 07:58:01 +1300
> 
> > To summarize, your opinion seems to be to not enable the
> > optimization by default, correct?
> 
> Yes, and I don't think we ever could do so.
> 
> BTW, another common paradigm is using the "always clear" bits of a
> pointer to encode state bits.

Yeah, I think it is a bad optimization that is going to break lots of code
and the glibc patch is definitely unacceptable (and not doing what you think
it is doing, your change didn't do anything at all, as the aligned attribute
applies to the pointer type and not to the pointed type).

The alignment of the argument pointer can be IMHO only hard assumed
if there is a load or store using that type dominating the spot where you
are checking it, and the target is strict alignment target.
Then we can both assume that alignment, optimize away tests on the low bits
etc.
Otherwise, what you could do is just use the pointed type's alignment as
a hint, likely alignment, e.g. on non-strict alignment target you could
expand code optimistically assuming that it is very likely aligned, but
still shouldn't optimize away low bits tests.

	Jakub