[LLVMdev] IC profiling infrastructure

[Justin Bogner]

I still have some issues with what we're doing with value_kind here. Let
me summarize the arguments I've heard for including the value_kind in
various places in this set of changes:

1. We would like to be able to enable multiple types of value profiling
   at the same time.

1a. We would like to be able to selectively use only particular value
    kinds from a particular profile.

1b. We would like to be able to combine profiles with different sets of
    profiling options enabled.

2. We will need to know the value kind in order to parse the raw profile
   data.

3. We'd like to avoid needing to change the .profdata format when we add
   new value kinds, since it needs to stay backwards compatible.

4. The frontend work is simpler if the value kind is explicitly encoded.

There are a couple of these points that certainly need to be addressed,
and some that are a little bit less clear.

A. We do need to handle (1) in some way, but the issue can be stated a
   little bit more generally. If there are optional parts of the profile
   data, we definitely need to know which options are enabled.

B. (1a) is only an issue if you believe (4). This might be true or it might
   not, and I don't think we can make an informed decision until we're
   actually using multiple value kinds. For now there's just a bunch of
   code that's effectively useless - "if value kind is 2, do nothing".  I'm
   opposed to adding this kind of practically unreachable code "just in
   case". It adds a kind of complexity and premature generality that, in my
   experience, will have to be completely re-done when we get to actually
   generalizing the code.

C. (1b) is an interesting one. Is this actually a useful feature? If we
   want to combine multiple profiles with different subsets of profiling
   turned on, then we definitely need the information recorded
   somewhere.

D. I have issues with (2). If the different value kinds actually have
   different structures and need to be read in / interpreted
   differently, then writing the support for them now simply won't
   work. We're reading them in as if they're interpreted in the exact
   same way as the indirect call profiling - if they aren't, then we
   have to rewrite this code when we add a new one anyway. What's the
   point of writing code that won't actually work when we try to use it?

E. There is value in dealing with (3) as long as we're confident that we
   can get it right. If we're pretty confident that the value profile
   data structure we're encoding in the .profdata file is going to work
   for multiple value profile kinds, then it makes sense to allow
   multiple of them and encode which kinds they are. David, is
   "InstrProfValueRecord" in Betul's current patches going to work to
   record the other types of value profiling information you're planning
   on looking at?

I'd also like to point out that the instrprof parts of LLVM try to be
relatively frontend agnostic. We should strive to keep it such that the
frontend is the piece that decides what a particular profiling kind
means if possible, as long as the structure and way the data is laid out
is suitable. This doesn't affect the current work very much, but it's a
good idea to keep it in mind.

So, with all of that said, I have a couple of suggestions on how to move
forward.

- I don't think we should store the kind in the raw profile yet. Let's
  keep this simple, since it's easy to change and will be easier to get
  right when we're implementing or experimenting with a second kind
  (based on (D)).

- Assuming the answer to my question in (E) is "yes", let's go ahead and
  include a value kind for the value records in the .profdata format. I
  don't really like the current approach of an array of std::vectors,
  because I think we can do this more simply. I'll bring that up on the
  review thread.

- In clang, we should error if we see value kinds we don't understand
  yet.

I think that this addresses the short term concerns without forcing us
to write unreachable/untestable code that may or may not work for the
longer term concerns. It minimally handles (A ) in that the data that is
there implies which features are being used, puts off (B) without making
it harder to deal with when the time comes, allows us to implement (C)
without much trouble if we want it, avoids doing work that could cause
problems with (D), and handles (E) since avoiding it would mean more
work later.

What do you think?

Xinliang David Li <davidxl at google.com> writes:
> Justin, do you have more concerns on keeping value_kind?
>
> If there is still disagreement, can we agree to move on with it for
> now ?  After the initial version of the patches checked in, we can do
> more serious testings with large apps and revisit this if there are
> problems discovered.
>
> thanks,
>
> David
>
>
>
> On Mon, Jun 15, 2015 at 10:47 PM, Xinliang David Li <davidxl at google.com> wrote:
>>> But since the consumer is the frontend, and it knows which counts are
>>> which, it knows the kind, no? I don't understand how storing the kind
>>> info helps or hurts - it's just redundant.
>>
>> Yes, the frontend consumer knows, but the raw reader does not. It is
>> natural to do those processing when processing the raw profile data
>> (for instance when function pointer to name mapping still exists).
>>
>>>
>>>>>
>>>>> The other thing we should do is store which profiling options are
>>>>> enabled, in both formats. When we specify profile-instr-use it'll be
>>>>> less error prone if we can detect whether or not this includes indirect
>>>>> call profiling without checking other options, and it's probably a good
>>>>> idea for "llvm-profdata merge" to disallow merging profiles that are
>>>>> gathering different sets of data. A bitfield seems suitable for this.
>>>>
>>>> For value profiling, allowing profile-gen and profile-use passes using
>>>> different options is a useful feature. Consider the following
>>>> scenarios:
>>>> 1) collect the same profile data once with all kinds of value profile
>>>>    data collected. The exact same profile data can be used in performance
>>>>    experiments with different kinds of value profiling enabled/disabled
>>>
>>> This use case is pretty easy to accomodate for with the model where the
>>> profile stores its type. We could autodetect the type to decide how to
>>> interpret the profile, and if more specific profile flags are set simply
>>> ignore some of the data. The thing that the file format storing the type
>>> gives us is that it's harder to misinterpret the data by supplying the
>>> mismatching flags between reading and writing.
>>
>> Yes, this is achievable with some extra effort -- for instance by
>> collecting the profile sites of all kinds regardless of the flags.
>> After profiling matching, simply drop selected sites according to the
>> flags.
>>
>> This adds complexity of the code. With value kinds encoded in profile
>> data, the handling is much simpler -- each value profiler only needs
>> to deal with its own kind.
>>
>>>
>>>> 2) work around profile-use/value transformation bug selectively for
>>>>    some file without the need to change anything in instrumentation pass
>>>
>>> I'm not sure I understand what you mean here.
>>
>> Similar to the above, but for correctness.
>>
>>>
>>>> Besides, with the latest patch, the value_kind is not recorded in each
>>>> profile value thus the overhead is minimized.
>>>
>>> The overhead is low, sure, but the code complexity of dealing with the
>>> multiple kinds in this way is quite real.
>>
>> I actually believe having value-kind can help reduce code complexity
>> instead of the other way around. What is the extra complexity?
>>
>> thanks,
>>
>> David
>>
>>
>>>Since I'm not convinced we're
>>> getting real benefit from storing the kind, I don't believe this
>>> trade-off is worth it.
>>>
>>>> thanks,
>>>>
>>>> David