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

[betulb at codeaurora.org]

>
>> On Apr 10, 2015, at 9:25 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, 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. 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.
>>
>> 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.
>>
>> Thanks,
>> -Betul
>
> I have the same questions and concerns as Philip. If the goal of this is
> to support devirtualization, this is not how I would expect it to be done.
> If you really want something more general, can you provide data to show
> that it is worth the cost? Recording all that information when profiling
> will not be cheap, and it adds significant complexity as well.

There was an earlier email of mine that went to the llvm-dev mailing list
discussing about the implementation dependencies and gains/costs.
http://lists.cs.uiuc.edu/pipermail/llvmdev/2015-April/084271.html

> I also don’t think we should consider taking all of your infrastructure
> changes without first seeing at least preliminary versions of the
> optimization patches that demonstrate the benefits.
>
>>
>>>
>>>>
>>>> Thanks.
>>>> -Betul
>>>>
>>>>
>>>> http://reviews.llvm.org/D8908 <http://reviews.llvm.org/D8908>
>>>>
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>