[llvm-dev] RFC: PGO Late instrumentation for LLVM

[Xinliang David Li via llvm-dev]

On Tue, Sep 1, 2015 at 7:21 PM, Ivan Baev via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> > Date: Tue, 1 Sep 2015 14:21:16 -0700
> > From: Rong Xu via llvm-dev <llvm-dev at lists.llvm.org>
> > Cc: llvm-dev <llvm-dev at lists.llvm.org>, David Li <davidxl at google.com>
> Subject: Re: [llvm-dev] RFC: PGO Late instrumentation for LLVM
>
> >>>> *(2) Performance impact of context sensitivity*
> >>>> LLVM does not use the profile information fully in the back-end
> optimizations, for instance, inlining does not fully use the profile counts
> >>>> -- it only marks hot/cold function attribute based on function entry
> counts. To evaluate the impact of profile context sensitivity, GCC is used
> >>>> in the experiment. Note that GCC PGO improves clang performance a lot
> more
> >>>> than clang PGO.
> >>>> First we summarize the methodology used in the experiment:
> >>>> 0)  build clang with GCC O2 without early inlining and measure
> clang's
> >>>> performance. GCC early inlining (einline) is similar to pre-inline
> used by
> >>>> late instrumentation.
> >>>> 1) build clang with GCC O2 with early inlining and measure
> >>>> performance.
> >>>> The performance difference of 1) and 0) is denoted as E which
> measures
> >>>> the contribution of early inlining.
> >>>> 2) build clang with GCC O2 + PGO without early inlining.
> >>>> 3) build clang with GCC O2 + PGO with early inlining.
> >>>> The performance difference of 3) and 2) is denoted as EC. It
> >>>> constitutes
> >>>> roughly two parts a) early inlining contribution b) context sensitive
> profiling enabled with early inlining.
> >>>> The contribution of context sensitive profiling can be estimated by
> EC
> >>>> -
> >>>> E above.
> >>>>
> -------------------------------------------------------------------------------
> Config                        wall_time_for_use  speedup_vs_(0)
> >>>>  speedup_vs_(1)
> >>>> (0) base w/o einline             84.946            1.000
> 0.934
> >>>> (1) base O2                      79.310            1.071
> 1.000
> >>>> (2) profile-arcs w/o einline     63.518            1.337
> 1.249
> >>>> (3) profile-arcs                 48.364            1.756
> 1.640
> >>>> We see the following:
> >>>> 1) GCC PGO with early inlining improves clang performance by 64.0%
> (v.s.
> >>>> base O2 w/ early inline).
> >>>> 2) GCC PGO w/o early inlining improves clang performance by 33.7%
> (v.s.
> >>>> base O2 w/o early inline).
> >>>> 3) Early inlining performance contribution is about 7.1%.
> >>>> 4) Profile context sensitivity contribution is estimated to be 22.2%
> (i.e. 64.0% -33.7% - 7.1%), which is pretty significant.
>
> Rong,
> Sorry for the late response. Just wanted to clarify my understanding of
> data in (2) Performance impact of context sensitivity.
>
> On clang as an application:
> 3) Early inlining contribution is about 7.1%,
>

This is the effect of pre-inlining without profile guidance.


> 2) PGO w/o early inlining contribution is about 33.7%,
>
> 4) so the additional combined effect of 2 and 3 is about 22.2%, correct?
>

Not combined effect -- but remaining effect (by excluding 2 and 3)


> In other words, just avoiding inlining small/simple callees and updating
> their profile counts in the call graph by the main inliner - all through
> the use of early inlining - improves clang performance by 22.2%.
>

Not sure what you mean here. 22% is the estimate of the effect of CS
profile due to clones of profile counters during instrumentation (through
pre-inlining). Profile update with inlining always exist including in 2).

David


>
> Thanks,
> Ivan
>
>
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