Re: Indirect memory addresses vs. lra

[Jeff Law <law@redhat.com>]

On 8/9/19 2:14 AM, John Darrington wrote:
> On Thu, Aug 08, 2019 at 01:57:41PM -0600, Jeff Law wrote:
> 
>      Yea, it's certainly designed with the more mainstream architectures in
>      mind.  THe double-indirect case that's being talked about here is well
>      out of the mainstream and not a feature of anything LRA has targetted to
>      date.  So I'm not surprised it's not working.
>      
>      My suggestion would be to ignore the double-indirect aspect of the
>      architecture right now, get the port working, then come back and try to
>      make double-indirect addressing modes work.
>      
> This sounds like sensible advice.  However I wonder if this issue is
> related to the other major outstanding problem I have, viz: the large 
> number of test failures which report "Unable to find a register to
> spill" - So far, nobody has been able to explain how to solve that
> issue and even the people who appear to be more knowlegeable have
> expressed suprise that it is even happening at all.
You're going to have to debug what LRA is doing and why.  There's really
no short-cuts here.  We can't really do it for you.  Even if you weren't
using LRA you'd be doing the same process, just on even more difficult
to understand codebase.

> 
> Even if it should turn out not to be related, the message I've been
> receiving in this thread is lra should not be expected to work for
> non "mainstream" backends.  So perhaps there is another, yet to be
> discovered, restriction which prevents my backend from ever working?
It's possible.  But that's not really any different than reload.
There's certainly various aspects of architectures that reload can't
handle as well -- even on architectures that were mainstream processors
when reload was under active development and maintenance.  THere's even
a good chance reload won't handle double-indirect addressing modes well
-- they were far from mainstream and as a result the code which does
purport to handle double-indirect addressing modes hasn't been
used/tested all that much over the last 25+ years.

> 
> On the other hand, given my lack of experience with gcc,  it could be
> that lra is working perfectly, and I have simply done something
> incorrectly.    But the uncertainty voiced in this thread means that it
> is hard to be sure that I'm not trying to do something which is
> currently unsupported.
My recommendation is to continue with the LRA path.

jeff