[LLVMdev] RFC - Improvements to PGO profile support

[Xinliang David Li]

On Tue, Mar 24, 2015 at 10:54 AM, Bob Wilson <bob.wilson at apple.com> wrote:
>
>> On Mar 24, 2015, at 10:53 AM, Diego Novillo <dnovillo at google.com> wrote:
>>
>> On Tue, Mar 24, 2015 at 1:48 PM, Bob Wilson <bob.wilson at apple.com> wrote:
>>>
>>>> On Mar 24, 2015, at 10:27 AM, Xinliang David Li <davidxl at google.com> wrote:
>>>>
>>>>  2.3) remove the 'laplace rule of succession' which can be very harmful
>>>
>>> I have not seen any consensus on this. I’m not at all convinced this would be a good idea. From what I’ve seen, using LaPlace’s rule avoids really bad behavior in cases where the counts are very small and has almost no impact when the counts are large.
>>
>> Duncan and I chatted briefly about this on IRC. I'm going to
>> experiment with only applying laplace smoothing if any scaled weights
>> are 0. AFAIU, usage of this rule is really just trying to avoid having
>> downstream users deal with 0 weights. Duncan, please whack me with a
>> clue stick if I totally misrepresented our conclusions.
>
> Zero is the extreme case, but it’s also important for other small counts. I’d like to see some specific examples of why you think the current behavior is harmful.

Using the rule has at least the following bad side effects:
1) wrong estimation of average loop trip count -- possibly leading to
wrong unroll decisions
2) result in bad inlining decisions. For instance:
    for (...)
        bar();  // (1)

 where (1) is the only callsite to bar().   Using the rule, BB count
enclosing the call to bar() can be as low as half of the entry count
of bar().  Inliner will get confused and think there are more hot
callsites to 'bar' and  make suboptimal decisions ..

Also if bar has calls to other functions, those callsites will look
hotter than the call to 'bar' ...

The attached are two cases as well as the frequency graph computed
today (with the laplace rule) and the correct frequency expected.

David
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