Re: syncing the GCC vax port, atomic issue

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

On 9/21/19 12:27 PM, Paul Koning wrote:
> 
> 
>> On Sep 20, 2019, at 9:18 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.
>> My guess is that, if it didn't crash now, it might emit a different kind
>> of branch which loses the atomic qualities, and result in wrong code.
> 
> Or it might leave the atomic branch, in a place where it isn't really wanted.
> 
> I wonder if this could be avoided by representing the atomic branch by an UNSPEC rather than by a branch, since it isn't a "normal branch" that GCC knows about.
If the atomic branch is special from an architectural standpoint, then
that may be advisable.

Jeff