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Re: Comparison of GCC-4.9 and LLVM-3.4 performance on SPECInt2000 for x86-64 and ARM
- From: Richard Biener <richard dot guenther at gmail dot com>
- To: Vladimir Makarov <vmakarov at redhat dot com>
- Cc: Renato Golin <renato dot golin at linaro dot org>, Bingfeng Mei <bmei at broadcom dot com>, Ramana Radhakrishnan <ramana dot radhakrishnan at arm dot com>, "gcc.gcc.gnu.org" <gcc at gcc dot gnu dot org>
- Date: Wed, 25 Jun 2014 16:02:49 +0200
- Subject: Re: Comparison of GCC-4.9 and LLVM-3.4 performance on SPECInt2000 for x86-64 and ARM
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- References: <53A98705 dot 10909 at redhat dot com> <53A98CE2 dot 9080108 at arm dot com> <53A98F0A dot 7000802 at redhat dot com> <53A991D7 dot 6070709 at arm dot com> <53A993F8 dot 7030101 at redhat dot com> <B71DF1153024A14EABB94E39368E44A6042CE185 at SJEXCHMB13 dot corp dot ad dot broadcom dot com> <CAMSE1kdDrF1urKf5yeoou3Y_o2gf8Gk5CTWw4_i3_sCjaA54gg at mail dot gmail dot com> <53AAD61A dot 6010201 at redhat dot com>
On Wed, Jun 25, 2014 at 4:00 PM, Vladimir Makarov <firstname.lastname@example.org> wrote:
> On 2014-06-25, 5:32 AM, Renato Golin wrote:
>> On 25 June 2014 10:26, Bingfeng Mei <email@example.com> wrote:
>>> Why is GCC code size so much bigger than LLVM? Does -Ofast have more
>>> on GCC? It doesn't seem increasing code size help performance (164.gzip &
>>> 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.
Maybe LLVM is just too aggressive here (but is lucky to not miscompile
> 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...