Loop safepoints and backedge copies

[Tom Rodriguez]

> I'm writing a compiler that uses the HotSpot's typeflow pass.
>
> Is it safe to say that for backwards branches (target bci <= current
> bci) you only need to insert a safepoint check when the block you are
> in is a backedge copy?

The specific requirement is that any cycle in the control flow graph  
has to have a safepoint somewhere in it.  The rule we use is that we  
emit a safepoint on any branch that may go backwards.  A block in  
ciTypeFlow that is marked as a backedge_copy is actually the loop  
backedge of a zero trip loop so it should be possible to place your  
safepoint in that block.  Unfortunately not all backedges are cloned  
so you can't simply restrict safepoint insertion to backedge_copy  
blocks.  If you are worried about inserting useless safepoints you are  
probably better off having an optimization that removes useless ones.   
If a loop has a call in it then it doesn't need a separate safepoint  
poll and eliminating that is a good idea.  Also any safepoint poll  
outside of a loop is useless.

tom

>
>
> Cheers,
> Gary
>
> -- 
> http://gbenson.net/