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Re: Comparison of GCC-4.9 and LLVM-3.4 performance on SPECInt2000 for x86-64 and ARM


> On 25 June 2014 10:26, Bingfeng Mei <bmei@broadcom.com> wrote:
> > Why is GCC code size so much bigger than LLVM? Does -Ofast have more unrolling
> > on GCC? It doesn't seem increasing code size help performance (164.gzip & 197.parser)
> > Is there comparisons for O2? I guess that is more useful for typical
> > mobile/embedded programmers.
> 
> Hi Bingfeng,
> 
> My analysis wasn't as thorough as Vladimir's, but I found that GCC
> wasn't eliminating some large blocks of dead code or inlining as much
> as LLVM was. I haven't dug deeper, though. Some of the differences

I would be defnitely interested in such testcases.. I do not think we should be
just missing DCE oppurtunities. As for inlining, GCC often disables inlining based
on profile (by concluding that given call is cold), but that should not result
in larger code of course.

I also noticed that GCC code size is bigger for both firefox and libreoffice.
There was some extra bloat in 4.9 compared to 4.8.
Martin did some tests with -O2 and various flags, perhaps we could trottle
some of -O2 optimizations.

Honza
> were quite big, I'd be surprised if it all can be explained by
> unrolling loops and vectorization...

> 
> cheers,
> --renato


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