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Re: GCC missing -flto optimizations? SPEC lbm benchmark

Bin.Cheng <> 于2019年2月15日周五 下午5:12写道:

> On Fri, Feb 15, 2019 at 3:30 AM Steve Ellcey <> wrote:
> >
> > I have a question about SPEC CPU 2017 and what GCC can and cannot do
> > with -flto.  As part of some SPEC analysis I am doing I found that with
> > -Ofast, ICC and GCC were not that far apart (especially spec int rate,
> > spec fp rate was a slightly larger difference).
> >
> > But when I added -ipo to the ICC command and -flto to the GCC command,
> > the difference got larger.  In particular the 519.lbm_r was more than
> > twice as fast with ICC and -ipo, but -flto did not help GCC at all.
> >
> > There are other tests that also show this type of improvement with -ipo
> > like 538.imagick_r, 544.nab_r, 525.x264_r, 531.deepsjeng_r, and
> > 548.exchange2_r, but none are as dramatic as 519.lbm_r.  Anyone have
> > any idea on what ICC is doing that GCC is missing?  Is GCC just not
> > agressive enough with its inlining?
> IIRC Jun did some investigation before? CCing.
> Thanks,
> bin
> >
> > Steve Ellcey
> >

ICC is doing much more than GCC in ipo, especially memory layout
optimizations. See
ICC is more aggressive in array transposition/structure splitting
/field reordering. However, these optimizations have been removed
from GCC long time ago.
As for case lbm_r, IIRC a loop with memory access which stride is 20 is
most time-consuming.  ICC will optimize the array(maybe structure?)
and vectorize the loop under ipo.


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