[llvm] r205409 - Remove duplicated DMB instructions

[Renato Golin]

Hi Tim,

I'm not against removing it, if it's creating problems. AFAIK, Yi Kong
is working with it, so before we rush to remove it, we may start a
discussion on how to move it to a better place and extend the
implementation so that it works as intended without breaking other
stuff.

If Yi doesn't have the time/opportunity to work on this in the
foreseeable future, feel free to remove it and open a bug to reapply
once someone has time to do it properly.

cheers,
--renato


On 9 June 2014 16:42, Tim Northover <t.p.northover at gmail.com> wrote:
> Hi,
>
>> ARM specific optimiztion, finding places in ARM machine code where 2 dmbs
>> follow one another, and eliminating one of them.
>
> Saleem's pointed me at a few more oddities with this pass, and on the
> whole I'm really not sure it should have been committed:
>
> 1. It runs after unpacking machine bundles, so it would completely
> hose predicated Thumb code.
> 2. But that's OK! Because it doesn't actually touch t2DMBs at all.
>
> Basically, it seems like a very incomplete solution (ARM-only; at the
> wrong level; combining barriers too conservatively anyway) that would
> be difficult to extend to a pass that handles things properly and has
> been effectively abandoned by its writer.
>
> It's a shame, but I suspect more problems will be caused than solved
> by its presence. Unless someone actually wants to step forward and
> maintain it properly, of course.
>
> Cheers.
>
> Tim.