Re: [patch] Fix node weight updates during ipa-cp (issue7812053)

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Fri, Apr 5, 2013 at 7:18 AM, Teresa Johnson <tejohnson@google.com> wrote:
> On Thu, Mar 28, 2013 at 2:27 AM, Richard Biener
> <richard.guenther@gmail.com> wrote:
>> On Wed, Mar 27, 2013 at 6:22 PM, Teresa Johnson <tejohnson@google.com> wrote:
>>> I found that the node weight updates on cloned nodes during ipa-cp were
>>> leading to incorrect/insane weights. Both the original and new node weight
>>> computations used truncating divides, leading to a loss of total node weight.
>>> I have fixed this by making both rounding integer divides.
>>>
>>> Bootstrapped and tested on x86-64-unknown-linux-gnu. Ok for trunk?
>>
>> I'm sure we can outline a rounding integer divide inline function on
>> gcov_type.  To gcov-io.h, I suppose.
>>
>> Otherwise this looks ok to me.
>
> Thanks. I went ahead and worked on outlining this functionality. In
> the process of doing so, I discovered that there was already a method
> in basic-block.h to do part of this: apply_probability(), which does
> the rounding divide by REG_BR_PROB_BASE. There is a related function
> combine_probabilities() that takes 2 int probabilities instead of a
> gcov_type and an int probability. I decided to use apply_probability()
> in ipa-cp, and add a new macro GCOV_COMPUTE_SCALE to basic-block.h to
> compute the scale factor/probability via a rounding divide. So the
> ipa-cp changes I made use both GCOV_COMPUTE_SCALE and
> apply_probability.
>
> I then went through all the code to look for instances where we were
> computing scale factors/probabilities and performing scaling. I found
> a mix of existing uses of apply/combine_probabilities, uses of RDIV,
> inlined rounding divides, and truncating divides. I think it would be
> good to unify all of this. As a first step, I replaced all inline code
> sequences that were already doing rounding divides to compute scale
> factors/probabilities or do the scaling, to instead use the
> appropriate helper function/macro described above. For these
> locations, there should be no change to behavior.
>
> There are a number of places where there are truncating divides right
> now. Since changing those may impact the resulting behavior, for this
> patch I simply added a comment as to which helper they should use. As
> soon as this patch goes in I am planning to change those to use the
> appropriate helper and test performance, and then will send that patch
> for review. So for this patch, the only place where behavior is
> changed is in ipa-cp which was my original change.
>
> New patch is attached. Bootstrapped (both bootstrap and
> profiledbootstrap) and tested on x86-64-unknown-linux-gnu. Ok for
> trunk?
>

This caused:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57077


H.J.