[LLVMdev] fix for loop scale limiting in BFI

Xinliang David Li xinliangli at gmail.com
Fri Mar 27 13:36:16 PDT 2015

On Fri, Mar 27, 2015 at 1:12 PM, Diego Novillo <dnovillo at google.com> wrote:

> On Fri, Mar 27, 2015 at 4:09 PM, Xinliang David Li <xinliangli at gmail.com>
> wrote:
> > How about only removing the scaling limit when PGO is on? I don't see the
> > need for this change without PGO.
> This is what my patch does, and it's getting into issues.  With the
> scaling limit gone, the frequencies propagated overflow 64bits, so
> they need to be scaled down to a 64bit space.

What I mean is only do this when real profile data is used? I don't see the
patch has that check ?

> To be on the safe side, my patch is mapping them down to a 32bit
> space, but I am squishing them too much on  the lower end. So regions
> of the CFG that before had distinct temperatures are now showing up
> with frequency == 1.
> I need a better smoother for the mapping from the Scale64 floats down
> to 64bit (or 32bit) integers.

This seems to show another weakness of the block frequency propagation --
 it can not handle infinite loops.  We need to think about how to handle it


> Diego.
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