Question about how to fix PR69052

[Jeff Law]

On 01/25/2016 11:42 AM, Bin.Cheng wrote:
>
> Yuri Rumyantsev suggested we may add a hook to handle GOT related
> instruction propagation specially so it won't be hoisted out.  Is a
> hook at this stage sounds feasible?
I think that would be a mistake.  ISTM this really needs to be cost driven.

>
> Also it would be great if we can abstract an interface out of combine,
> and the interface can be used in other passes checking whether
> instructions can be merged or not.  I know this will be even harder
> than emulation of combine behavior.
>
> Another point is if we can do combine among different basic blocks?
No, the combiner works within a basic block only.  There was a group, I 
believe in Moscow, that worked on a cross-block combiner.  It was 
discussed at the Cauldron in California a few years back.  I don't know 
if they did any further work on those ideas.

Is the issue here not knowing when the loop invariant can be forward 
propagated back into the memory reference -- and for complex RTL like 
you cited, you ultimately determine that no it can't be propagated and 
thus increase its invariant cost?

jeff