[PATCH] Indirect call target profiling related profile reader/writer changes

[Xinliang David Li]

On Wed, May 6, 2015 at 5:38 PM, Philip Reames <listmail at philipreames.com>
wrote:

>
>
> On 04/14/2015 11:56 AM, betulb at codeaurora.org wrote:
>
>> On 04/10/2015 09:25 AM, betulb at codeaurora.org wrote:
>>>
>>>> On 04/09/2015 11:06 AM, Betul Buyukkurt wrote:
>>>>>
>>>>>> In http://reviews.llvm.org/D8908#153838, @reames wrote:
>>>>>>
>>>>>>  Have the IR level construct patches made it up for review?  If so,
>>>>>>> can
>>>>>>>
>>>>>> So far I've posted two patches. These two patches should apply cleanly
>>>>>> to the tip, working with the present profile infrastructure. The next
>>>>>> set of patches will be the enabler ones: i.e. three more patches one
>>>>>> for
>>>>>> each of clang, llvm and compiler-rt. Clang patch will be up for review
>>>>>> later today.
>>>>>>
>>>>>>  you send me a link?  I managed to miss them.
>>>>>>>
>>>>>> So far there is this patch and the instrinsic instruction definitions:
>>>>>> http://reviews.llvm.org/D8877. All patches are necessary for getting
>>>>>> the
>>>>>> IC targets and having them displayed by the llvm-profdata.
>>>>>>
>>>>> Ok, I'm really not convinced that the instrumentation code needs to be
>>>>> or should be an intrinsic.  This seems like something which should be
>>>>> emitted by the frontend and optimized like any other code.  To say this
>>>>> a different way, my instrumentation is going to be entirely different
>>>>> than your instrumentation.
>>>>>
>>>>> Having said that, I really don't care about this part of the proposed
>>>>> changes since they aren't going to impact me at all.  I'm am
>>>>> specifically not objecting to the changes, just commenting.  :)
>>>>>
>>>>>> I'm assuming this will be some type of per call site metadata?
>>>>>>>
>>>>>> We do assign metadata at the indirect call sites. Format looks like as
>>>>>> follows:
>>>>>>
>>>>>> !33 = metadata !{metadata !"indirect_call_targets", i64
>>>>>> <total_exec_count>, metadata !"target_fn1†, i64 <target_fn1_count>,
>>>>>> metadata !"target_fn2†, i64 <target_fn2_count>, ….}
>>>>>>
>>>>>> Currently, we're recording only the top most called five function
>>>>>> names
>>>>>> at each indirect call site. Following the string literal
>>>>>> “indirect_call_targets† are the fields  <total_exec_count> i.e. a
>>>>>> 64
>>>>>> bit value for the total number of times the indirect call is executed
>>>>>> followed by the function names and execution counts of each target.
>>>>>>
>>>>> This was the part I was trying to ask about.  I really want to see
>>>>> where
>>>>> you're going with this optimization wise.  My naive guess is that this
>>>>> is going to be slightly off for what you actually want.
>>>>>
>>>>> Assuming you're going for profile guided devirtualization (and thus
>>>>> inlining), being able to check the type of the receiver (as opposed to
>>>>> the result of the virtual lookup) might be advantageous.  (Or, to say
>>>>> it
>>>>> differently, that's what I'm used to seeing.  Your approach might be
>>>>> completely reasonable, it's just not what I'm used to seeing.)  Have
>>>>> you
>>>>> thought about the tradeoffs here?
>>>>>
>>>> Not sure if I understood the problem here,
>>>>
>>> First, I am not trying to say there is a problem with your approach; I
>>> am only saying that it's not what I would have expected based on past
>>> experience.  You may be entirely correct in your approach, you just need
>>> to convince me of that.  :)
>>>
>>>> however, we're recording both
>>>> the target address and the addresses/names of the instrumented functions
>>>> during the execution of the instrumented binary. During profile reading
>>>> these addresses are used to match the target addresses to corresponding
>>>> functions.
>>>>
>>> Ok, let's start from the basics.  For profile guided devirtualization,
>>> you're constructing a cache from (something) to function pointer and
>>> using that cache lookup to enable inlining of the hot target.  You have
>>> two standard choices on what to use as your cache key: the result of the
>>> virtual lookup and the inputs to the virtual lookup.
>>>
>>> Option 1 - Inputs to virtual lookup
>>> if ((receiver, vtable index) == what I predicted)
>>>     tartget_I_predicted(); // inline me!!
>>> else {
>>>     target = full virtual dispatch();
>>>     target();
>>> }
>>>
>>> Option 2 - result of virtual lookup
>>> target = full virtual dispatch();
>>> if ('target' == what I predicted)
>>>     tartget_I_predicted(); // inline me!!
>>> else {
>>>     target();
>>> }
>>>
>>> You seem to be proposing option 2.  I'm saying that I'm used to seeing
>>> option 1 used.  Both approaches have their appeal, I'm just asking you
>>> to explain *why* you've chosen the one you apparently have.
>>>
>> Not all indirect calls occur from C++ like codes. We're profiling and
>> optimizing out indirect calls from C codes as well. We're seeing up to 8%
>> gains on individual benchmarks in spec. This was measured on our platform.
>>
> This answers my original question and is a good point.  I was definitely
> looking at this through a virtual dispatch centric view.
>
> I have no objection to supporting profiling/predication of arbitrary
> indirect call sites.  As someone else pointed out in the discussion,
> there's no reason we can't specialize for virtual call profiling and
> support the generic profiling as a fallback.  That seems like a good way
> forward.  The details still need to worked out, but the general approach
> makes sense.
>

Right. Besides the runtime support has been enhanced to be more generic,
which makes type/vtable profiling just as easy in the future.

David



> p.s. My bandwidth to devote to this topic has pretty much disappeared.  I
> probably won't be responding further unless specifically requested to.
>
>
>>  During optimization, target function name should be adequate to
>>>> retrieve the function type information which then can be used to compare
>>>> the types of the arguments as well as the return type w/ the call site
>>>> to
>>>> ensure the match.
>>>>
>>> Er, I'm now really confused.  With a properly constructed check, there
>>> should be no runtime checking of types required.  Can you explain why
>>> you think this is needed?
>>>
>>>> To clarify, our interest at this time is to upstream all the
>>>> infrastructure related changes. The optimizations using the profile data
>>>> will not be part of our initial set of patches. On the other hand,
>>>> agreeing on the metadata format at the call sites would be advantageous
>>>> in
>>>> terms of supporting multiple profile methods.
>>>>
>>> See my other response to this thread.
>>>
>>>> Thanks,
>>>> -Betul
>>>>
>>>>
>>>>
>>
>
>
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