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Re: [EXT] Re: GCC missing -flto optimizations? SPEC lbm benchmark

On Fri, 2019-02-15 at 17:48 +0800, Jun Ma wrote:
> 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.
> Thanks
> Jun

Interesting.  I tried using '-qno-opt-mem-layout-trans' on ICC
along with '-Ofast -ipo' and that had no affect on the speed.  I also
tried '-no-vec' and that had no affect either.  The only thing that 
slowed down ICC was '-ip-no-inlining' or '-fno-inline'.  I see that
'-Ofast -ipo' resulted in everything (except libc functions) getting
inlined into the main program when using ICC.  GCC did not do that, but
if I forced it to by using the always_inline attribute, GCC could
inline everything into main the way ICC does.  But that did not speed
up the GCC executable.

Steve Ellcey

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