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Re: GCC missing -flto optimizations? SPEC lbm benchmark


On Fri, Feb 15, 2019 at 3:30 AM Steve Ellcey <sellcey@marvell.com> 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
> sellcey@marvell.com


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