Re: syncing the GCC vax port, atomic issue

[coypu%sdf.org@localhost]

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.


I tried to single-step GCC, and it might be trying entirely different
instruction patterns.
I'm not sure whether I should put a lot of trust in the line numbers
shown from .md files, but it's trying nonlocal_goto in vax.md.
In any case, nothing from builtins.md.