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Re: GCC missing -flto optimizations? SPEC lbm benchmark
On February 15, 2019 1:45:10 PM GMT+01:00, Hi-Angel <firstname.lastname@example.org> wrote:
>I never could understand, why field reordering was removed from GCC?
The implementation simply was seriously broken, bitrotten and unmaintained.
>mean, I know that it's prohibited in C and C++, but, sure, GCC can
>detect whether it possibly can influence application behavior, and if
>not, just do the reorder.
>The veto is important to C/C++ as programming languages, but not to
>machine code that is being generated from them. As long as app can't
>detect that its fields were reordered through means defined by C/C++,
>field reordering by compiler is fine, isn't it?
>On Fri, 15 Feb 2019 at 12:49, Jun Ma <email@example.com> wrote:
>> Bin.Cheng <firstname.lastname@example.org> 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 <email@example.com>
>> > >
>> > > I have a question about SPEC CPU 2017 and what GCC can and cannot
>> > > with -flto. As part of some SPEC analysis I am doing I found
>> > > -Ofast, ICC and GCC were not that far apart (especially spec int
>> > > spec fp rate was a slightly larger difference).
>> > >
>> > > But when I added -ipo to the ICC command and -flto to the GCC
>> > > the difference got larger. In particular the 519.lbm_r was more
>> > > twice as fast with ICC and -ipo, but -flto did not help GCC at
>> > >
>> > > There are other tests that also show this type of improvement
>> > > 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
>> > > any idea on what ICC is doing that GCC is missing? Is GCC just
>> > > agressive enough with its inlining?
>> > IIRC Jun did some investigation before? CCing.
>> > Thanks,
>> > bin
>> > >
>> > > Steve Ellcey
>> > > firstname.lastname@example.org
>> ICC is doing much more than GCC in ipo, especially memory layout
>> optimizations. See https://software.intel.com/en-us/node/522667.
>> 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
>> most time-consuming. ICC will optimize the array(maybe structure?)
>> and vectorize the loop under ipo.