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

[Teresa Johnson <tejohnson at google dot com>]

I'll take a look. Teresa

On Thu, Apr 25, 2013 at 3:29 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> 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.



-- 
Teresa Johnson | Software Engineer | tejohnson@google.com | 408-460-2413