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wrc1944
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Joined: 15 Aug 2002
Posts: 3435
Location: Gainesville, Florida
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 Posted: Mon Jan 07, 2019 4:04 pm Post subject: Enabling vectorization for -O2 opts for Intel, AMD Zen |
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Just saw this post on Phoronix: "Enabling vectorization for -O2 optimization for Intel, AMD Zen, and generic x86_64 CPU targets"
https://www.phoronix.com/scan.php?page=news_item&px=GCC-O2-x86_64-Vectorization
One comment says: | Quote: | Compiler autovectorization is absolutely essential these days if you intend to make your cpus work at least a bit as how they were intended to be used. Otherwise go back to 2008 era nehalem and westmere, they run just fine. But if you want to make use of avx and don't want to hand-code asm, compiler can help you. ICC for example does vectorization at -O2, so does clang. Holding it back on gcc is not helpful.
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Years ago, after reading about ICC with vectorization benefits on Intel cpus, I had enabled vectorization at -O2 on several amd cpu versions (eventually including bulldozers), but lately haven't done so, even though I never ran into problems I could trace to enabling it..
Considering that GCC seems to be contemplating enabling it for default -O2 at some future date, I was wondering if this might rate a Gentoo discussion about pros/cons, and other personal experiences.
Specifically, if someone with more knowledge than me could offer a few thoughts on the meaning of the benchmark info posted on a gcc mailing list, here. I'm a long-time AMD user, so my particular interest is Zen cpus.
https://gcc.gnu.org/ml/gcc/2019-01/msg00019.html
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Last edited by wrc1944 on Mon Jan 07, 2019 4:43 pm; edited 1 time in total |
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Perfect Gentleman
Veteran
Joined: 18 May 2014
Posts: 1249
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| Posted: Mon Jan 07, 2019 4:15 pm Post subject: |
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| Code: | CFLAGS="-march=native -mtune=native -O2 -pipe -fomit-frame-pointer -fno-stack-protector -ftree-vectorize -finline-functions -funswitch-loops -s"
CXXFLAGS=${CFLAGS} |
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est921
n00b
Joined: 20 Dec 2015
Posts: 28
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| Posted: Tue Jan 15, 2019 11:24 am Post subject: |
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It looks like phoronix uses | Code: | | -ftree-vectorize -ftree-slp-vectorize |
but as far as I understand it | Code: | | -ftree-slp-vectorize |
should already be enable by . Does anyone know why they do that? |
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Ant P.
Watchman
Joined: 18 Apr 2009
Posts: 6920
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| Posted: Tue Jan 15, 2019 11:49 am Post subject: |
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| Phoronix is notorious for not bothering to RTFM for the sake of puffing up their badly-written clickbait. |
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