[PATCH] D15003: Interface to attach maximum function count from PGO to module as module flags.

[Xinliang David Li via llvm-commits]

On Wed, Dec 2, 2015 at 11:08 AM, Easwaran Raman <eraman at google.com> wrote:
> To further add to David's replies, without something like this, it is
> difficult to say whether a function 'foo' with an entry count of 1000
> corresponds to a hot function or not: it is obviously hot if the max
> function entry count itself is 1000 (then foo is the hottest function), but
> perhaps not hot if the max function entry count is 1 million. The frontend
> uses this value to set cold and inline-hint attributes, and the purpose of
> this patch is to make this available to the optmizer.
>
> I think we eventually want to use this (or max BB count) to find
> per-callsite hotness and inline hot callsites, but perhaps an intermediate
> step is to continue use the current heuristic of boosting hot callees, but
> make that determination in the inliner instead of relying on attribute set
> by the frontend.
>

Yes -- lets make the changes in stages. First migrate the max-entry
count setting and move the callee hotness based logic to inliner.
After than, add another interface to pass max BB count ...

David


>
>
> On Fri, Nov 27, 2015 at 9:42 AM, Xinliang David Li <davidxl at google.com>
> wrote:
>>
>>
>>
>> On Fri, Nov 27, 2015 at 5:13 AM, Diego Novillo <dnovillo at google.com>
>> wrote:
>>>
>>>
>>>
>>> On Fri, Nov 27, 2015 at 12:44 AM, Xinliang David Li <davidxl at google.com>
>>> wrote:
>>>>
>>>>
>>>>
>>>> On Thu, Nov 26, 2015 at 1:34 PM, Diego Novillo <dnovillo at google.com>
>>>> wrote:
>>>>>
>>>>> dnovillo added a comment.
>>>>>
>>>>> How is this going to interact with SamplePGO?  Currently, I'm testing a
>>>>> change that adds the InlineHint attribute for functions that globally
>>>>> collected a fraction of samples greater than a given threshold.  Likewise,
>>>>> for functions that fall below another threshold, they receive the Cold
>>>>> attribute.
>>>>
>>>>
>>>> Setting global max count and setting inlinehint are two different things
>>>> to do. The former helps per-callsite inline decision while the later is for
>>>> per-callee inline decision. Per-callee decision is not always good -- for
>>>> instance you don't want to inline a function just because it is hot -- the
>>>> callsite may be really cold.
>>>
>>>
>>> Oh, agreed.  Setting inline hints is the only thing the profiler can do
>>> at this time, this is why I'm interested in a per-callsite solution.  I want
>>> to make sure that this change is taking us in that direction.  We will also
>>> need to undo the inline hint setting done in CodeGenPGO.cpp.
>>>
>>
>>
>> right -- you really don't need inlinehint -- which should only be needed
>> to pass user's hint. By using entry count and max count passed, the inliner
>> can get the same information. More importantly, inliner will also update the
>> entry count of the out of line instance as inlining is happening, so it will
>> always see the most up to date info. For instance, after the most dominating
>> callsite is inlined, the callee may become super cold, and this will be
>> reflected during inline update.
>>
>>
>>>>
>>>>
>>>>>
>>>>>
>>>>> The API talks in terms of function counts, how is this related to the
>>>>> entry count I added earlier? What would SamplePGO have to do here?
>>>>>
>>>>
>>>> The inline decision is based on the new information provided by the API
>>>> and the entry count info -- from the entry count and block relative
>>>> frequency, the inliner can find out the profile count of the callsite -- the
>>>> callsite count is then compared with a threshold determined by the global
>>>> hotness information provided by the API.
>>>
>>>
>>> So, the units do not matter.  This is just a number.  Higher values mean
>>> hotter functions.  Easwaran, could you add documentation to this effect?
>>> Thanks.
>>
>>
>> Right -- the requirement is that the entry count should be in same unit as
>> this.
>>
>>>
>>>
>>> This API only sets the max entry count for the function.  How do we set
>>> the per-callsite counts?  SamplePGO has this information now but does not
>>> know what to do with it.
>>
>>
>> To be precise, it  sets the count of the hottest function entry in whole
>> program. This API allows the compiler to migrate the inline hint logic
>> currently done in FE to inliner (so that inline hint does not need to be
>> passed as you mentioned).  This is the first step.   As I mentioned, we also
>> need max BB count of the program for inliner to decide per-callsite hotness.
>> That can either be done in a follow up or together in this patch.
>>
>>
>> David
>>
>>
>