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

Xinliang David Li via llvm-dev
Thu, 10 Mar 2016 17:00:30 -0000

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.


>
> 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.

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
>
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