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

[Hal Finkel via llvm-dev]

----- Original Message -----

> 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. 

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) 

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. 

> > 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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