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

[betulb at codeaurora.org]

>
> 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.

>> 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
>>
>>
>