[LLVMdev] hazard scheduling nodes

[Andrew Trick]

On Mar 12, 2013, at 3:43 AM, Jonas Paulsson <jonas.paulsson at ericsson.com> wrote:

> Hi Andy,
>  
> The thing is that I was trying to build a sched graph in other places than these two standard scheduling passes. For instance, in pre-emit. I would like to reschedule a basic block on my vliw target just before assembly emission.
>  
> I tried to add SUnits for hazards in an experiment, but this gave very weird errors… even while allocating extra space in SUnits vector. For some function, I could not even build the sched-graph, so that’s when I mailed you. My question is still whether it is possible to use the scheduler classes at an arbitrary point, and what the pitfalls might be…

Each SUnit requires a real instruction currently.

You have to be sure not to create cycles in your DAG.

If instructions are already bundled, the scheduler should see them as a single instruction. To reschedule within a bundle, you need to unbundle them first. That could be tricky if you have readers and writers of the same register in the bundle.

I can't think of any other issues. In theory, the scheduler can work at any point after register rewriting.

-Andy

> 
> From: Andrew Trick [mailto:atrick at apple.com] 
> Sent: Saturday, March 09, 2013 3:05 AM
> To: Jonas Paulsson
> Cc: llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] hazard scheduling nodes
>  
>  
> On Feb 21, 2013, at 9:11 AM, Jonas Paulsson <jonas.paulsson at ericsson.com> wrote:
> 
> 
> Hi,
>  
> I am trying to add Hazard scheduling nodes after buildSchedGraph(), with a scheduler derived from ScheduleDAGInstrs. I get weird errors, so I wonder what I am doing wrong?
>  
> What I am doing right now is:
>  
> I have a created MI with opcode HAZARD that does not have parent, and I greate a SUnit(HazardMI). I use this one HazardMI for all hazard nodes.
> I remove all edges using removePred.
> I insert edges between Node -> hazardNode -> pred.
>  
> What is there to think about in this approach? I must be missing some detail…
>  
> Any help is appreciated,
>  
> Jonas Paulsson
>  
> Jonas, I'm sorry I forgot to respond to this. Hopefully you figured out how to debug it. I'm not sure if you're using the MachineScheduler pass or PostRAScheduler. 
>  
> For MachineScheduler, use -view-misched-dags and -debug-only=misched.
>  
> For PostRA: -debug-only=post-RA-sched.
>  
> Modifying the DAG edges is tricky to get right. But removePred() and addPred() are the basic primitives...
>  
> -Andy

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