[llvm-dev] [RFC] Target-specific parametrization of function inliner

Wed Apr 6 10:42:21 PDT 2016

On Fri, Apr 1, 2016 at 12:10 PM, Hal Finkel <hfinkel at anl.gov> wrote:

>
> ------------------------------
>
> *From: *"Xinliang David Li" <davidxl at google.com>
> *To: *"Hal Finkel" <hfinkel at anl.gov>
> *Cc: *"Artem Belevich" <tra at google.com>, "llvm-dev" <
> llvm-dev at lists.llvm.org>, "chandlerc" <chandlerc at gmail.com>, "Easwaran
> Raman" <eraman at google.com>
> *Sent: *Thursday, March 10, 2016 11:00:30 AM
> *Subject: *Re: [llvm-dev] [RFC] Target-specific parametrization of
> function inliner
>
> IMO, a good inliner with a precise cost/benefit model will eventually need
> what Art is proposing here.
>
> Giving the function call overhead as an example. It depends on a couple of
> factors: 1) call/return instruction latency; 2) function epilogue/prologue;
> 3) calling convention (argument parsing, using registers or not, what
> register classes etc).  All these factors depend on target information.  If
> we want go deeper, we know certain micro architectures uses a stack of
> call/return pairs to help branch prediction of ret instructions -- such
> stack has a target specific limit which can be triggered when a callsite is
> deep in the callchain.   Register file size and register pressure increase
> due to inline comes as another example.
>
> Another relevant example is the icache/itlb sizes. To do a more precise
> analysis of the cost to 'speed' due to icache/itlb pressure increase
> requires target information, profile information as well as some global
> analysis. Easwaran has done some research in this area in the past and can
> share the analysis design when other things are ready.
>
>
> I don't know what you mean by "when other things are ready", but what you
> say above sounds exactly right. I'm certainly curious what Easwaran has
> found.
>

By readiness, I mean the basic infrastructure support (such as making
profile data available for inliner) and related tuning based on simple
heuristics. The former will be revisited very soon.  Those work will be
useful to setup a good baseline before we start engaging in more
sophisticated analysis.

>
> Generally, there seem to be two categories here:
>
>  1. Locally decidable issues, for which there are (or can be) good static
> heuristics (call latencies, costs associated with parameter passing, stack
> spilling, etc.)
>  2. Globally decidable issues, like reducing the number of pages consumed
> by temporally-correlated hot code regions - profiling data likely necessary
> for good decision-making (although it might be possible to make a
> reasonable function-local threshold based on page size without it)
>
>
 Program level static analysis needs to be combined with profile data to
form independent or nested hot regions (with cache/tlb reuse). For global
inlining decisions,  there won't be a single budget to be used. The
decision will highly depend on the region nest where the callsite sits in.

Another side effect is that we may need more flexible inlining order (based
on net benefit of inlining a callsite) than the current bottom up order
scheme.

> and then there are things like icache/itlb effects due to multiple
> applications running simultaneously, for which profiling might help, but
> are also policy-level decisions over which users may need more-direct
> control.
>

Some thread level profiling may also be useful in guiding the decision --
i.e., use information about the shared instruction footprint across
different threads - e.g. Programs running heterogeneous threads vs programs
with homogeneous worker threads running identical code.

thanks,

David

>
>
>
>
>>
>> Hi Art,
>>
>> I've long thought that we should have a more principled way of doing
>> inline profitability. There is obviously some cost to executing a function
>> body, some call site overhead, and some cost reduction associated with any
>> post-inlining simplifications. If inlining reduces the overall call site
>> cost by more than some factor, say 1% (this should probably depend on the
>> optimization level), then we should inline. With profiling information, we
>> might even use global speedup instead of local speedup.
>>
>
> yes -- with target specific cost information, global speedup analysis can
> be more precise :)
>
>
>>
>> Whether we need a target customization of this threshold, or just a way
>> for a target to supplement the fine inlining decision, is unclear to me. It
>> is also true that a the result of a bunch of locally-optimal decisions
>> might be far from the global optimum. Maybe the target has something to say
>> about that?
>>
>
>
> The concept of threshold can be a topic of another discussion.  In current
> design, I think the threshold should remain target independent.  It is the
> cost that is target specific.
>
> That's fine, but the units are important here. Having a target independent
> threshold in terms of, roughly, instruction count makes little sense. How
> instruction count is correlated with either performance or code size is
> highly target specific (although it is certainly closer for code size).
> That, however, is, roughly what our TTI.getUserCost gives us. Having
> target-independent thresholds like % speedup (e.g. inlining should be done
> when the speedup is > some %) or code-size thresholds (e.g. functions
> spanning more than a 4 KB are bad) makes sense.
>
>  -Hal
>
>
> thanks,
>
> David
>
>
>
>>
>> In short, I'm fine with what you're proposing, but to the extent
>> possible, I want the numbers provided by the target to mean something.
>> Replacing a global set of somewhat-arbitrary magic numbers, with
>> target-specific sets of somewhat-arbitrary magic numbers should be our last
>> choice.
>>
>> Thanks again,
>> Hal
>>
>>
>> >
>> > Thanks,
>> > --
>> >
>> >
>> > --Artem Belevich
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > llvm-dev at lists.llvm.org
>> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> >
>>
>> --
>> Hal Finkel
>> Assistant Computational Scientist
>> Leadership Computing Facility
>> Argonne National Laboratory
>>
>
>
>
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
>
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