On Fri, Feb 15, 2019 at 4:46 AM Hi-Angel <hiangel...@gmail.com> wrote: > > I never could understand, why field reordering was removed from GCC? I > 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?
In my opinion field reordering is very hard for the compiler to do correctly and trivial for a human programmer to do correctly. So in practice the best approach is for the compiler, or some other tool, to say "you should reorder the fields here." As far as I can see, the only real reason to implement field reordering in a compiler is for benchmark cracking, since benchmarks typically don't let you modify the source code. It's not a useful optimization in practice other than for benchmarks. (Array transformations and struct splitting, on the other hand, can be useful.) Ian > On Fri, 15 Feb 2019 at 12:49, Jun Ma <majun4950...@gmail.com> wrote: > > > > 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 p> > > > 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
