Re: syncing the GCC vax port, atomic issue

[Jeff Law <law%redhat.com@localhost>]

On 9/20/19 7:18 PM, coypu%sdf.org@localhost wrote:
> On Fri, Sep 20, 2019 at 10:07:59PM +0000, coypu%sdf.org@localhost wrote:
>> Introducing the reversed jbb* patterns doesn't seem to help with the
>> original issue. It crashes building libatomic.
> 
> My loose understanding of what is going on:
> - GCC emits this atomic in expand.
> - When cleaning up, it looks for optimizations.
> - It decides this is a branch to another branch situation, so maybe
>   can be improved.
> - This fails to output an instruction for unrelated reasons.
> - Hit an assert.
> 
> I don't think that we should be trying to combine regular branch +
> atomic branch in very generic code.
I can see how you might think that, but the way the RTL optimizers work,
particularly the combiner is it tries to smash together two or more
insns that are related by dataflow into a single insn (I'm
over-simplifying, but it's good enough for this discussion).

That's an inherent design decision for the combiner, it's not likely to
change.   If you want to prevent the combiner from doing that, you have
to use UNSPECs

Jeff