[LLVMdev] Loss of precision with very large branch weights

[Diego Novillo]

On Fri, Apr 24, 2015 at 3:44 PM, Xinliang David Li <davidxl at google.com>
wrote:

> On Fri, Apr 24, 2015 at 12:29 PM, Diego Novillo <dnovillo at google.com>
> wrote:
> >
> >
> > On Fri, Apr 24, 2015 at 3:28 PM, Xinliang David Li <davidxl at google.com>
> > wrote:
> >>
> >> yes -- for count representation, 64 bit is needed. The branch weight
> >> here is different and does not needs to be 64bit to represent branch
> >> probability precisely.
> >
> >
> > Actually, the branch weights are really counts.
>
> No -- I think that is our original proposal (including changing the
> meaning of MD_prof meta data) :).


Sure.  Though they kind of are. They get massaged and smoothed when
branch_weights are placed from the raw counts, but for sufficiently small
values they are very close to counts.


>
> >They get converted to
> > frequencies.  For frequencies, we don't really need 64bits, as they're
> just
> > comparative values that can be squished into 32bits.  It's the branch
> > weights being 32 bit quantities that are throwing off the calculations.
>
> Do you still see the issue after fixing bhe bug (limit without scaling) in
>  BranchProbabilityInfo::calcMetadataWeights ?
>

That's the fix I was contemplating initially. I was curious at whether
moving to 64bit would make this easier.


Diego.
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