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


On 2014-06-25, 5:32 AM, Renato Golin wrote:
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.

That might be a consequence of difference in aliasing I wrote about. I looked at the code generated by LLVM and GCC of an interpreter and saw bigger code generated by GCC too.

A sequence of bytecodes execution and each bytecode checks types of variables (small structure in memory) and set up values and types of results variables. So GCC was worse to propagate the variable type info (e.g. int) in the bytecode sequence execution where it would be possible and remove unnecessary code (cases where other types, e.g. fp, is processed). LLVM was more successful with this task.

 I haven't dug deeper, though. Some of the differences
were quite big, I'd be surprised if it all can be explained by
unrolling loops and vectorization...



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