Bin.Cheng <amker.ch...@gmail.com> 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 <sell...@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 > > sell...@marvell.com 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 is most time-consuming. ICC will optimize the array(maybe structure?) and vectorize the loop under ipo. Thanks Jun
