possible gcse failure: not able to eliminate redundant loads

[Alex Rosenberg]

on 12/16/02 1:12 PM, Richard Henderson at rth@redhat.com wrote:

> On Mon, Dec 16, 2002 at 10:04:01PM +0100, Toon Moene wrote:
>> BTW, I wasn't aware it's legal to dereference NULL C pointers.
> 
> It isn't, of course.  But that's the whole point.  You've
> got code that says
> 
> x = (condition ? *p : 0);
> 
> i.e. the dereference is protected by a conditional.  Thus
> we can't hoist the dereference past the conditional (and
> thence out of the loop that started this thread).

This may be obvious, but... this transformation is legal on some platforms.

AIX maps page 0 as read-only and zero-filled. IBM's xlC compiler will hoist
loads past a conditional with this knowledge. (It doesn't map the high end
of the address space similarly -- one of IBM's countless Java
implementations takes advantage of this discrepancy.)

IA64, Transmeta and other architectures split the job of a load into two
parts to enable exactly this sort of optimization.

+------------------------------------------------------------+
| Alexander M. Rosenberg           <mailto:alexr@_spies.com> |
| Nobody cares what I say. Remove the underscore to mail me. |