Re: Linux-5.2.8 configuration: Intel Atom x5-Z8330 and CONFIG_MATOM

On February 11, 2020, at 16:23, Hans Malissa <[email protected]> wrote:

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>On February 9, 2020 at 1:53 PM, [email protected] wrote:
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>From: Hans Malissa
>Sent: Sunday, February 9, 2020 12:34
>To: [email protected]
>Subject: [lfs-chat] Linux-5.2.8 configuration: Intel Atom x5-Z8330 and CONFIG_MATOM
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>This is not a support request; I'm mainly asking out of curiosity (that's why I'm posting this in lfs-chat).
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>My CPU is an Intel Atom x5-Z8330 (06-4c-04) (https://www.intel.com/content/www/us/en/products/boards-kits/compute-stick/stk1a32sc.html), and during the configuration of Linux-5.2.8 there is an item in 'Processor type and features' under 'Processor family' that says 'Intel Atom' (CONFIG_MATOM). The description says: 'Select this for the Intel Atom platform. Intel Atom CPUs have an in-order pipelining architecture and thus can benefit from accordingly optimized code. Use a recent GCC with specific Atom support in order to fully benefit from selecting this option.'. I'm not sure what that means. Does the GCC that's built during LFS/BLFS have such a 'specific Atom support' or what would it take to get that? LFS runs fine with the 'Generic-x86-64' option; I'm just wondering if I can get more out of this CPU.
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>Thanks a lot!
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>For GCC you have to set “march” and/or related settings under CFLAGS/CXXFLAGS as you compile to get the runtime support. It mostly means the processor will be used more efficiently if you use the specific code instead of the generic code. If you want to build a new LFS with CFLAGS set to enable Atom support, have fun!
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>Thanks for the explanation: Looking at gcc -dumpspecs, it says:
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>...
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>*cc1_cpu:
>%{march=native:%>march=native %:local_cpu_detect(arch) %{!mtune=*:%>mtune=native %:local_cpu_detect(tune)}} %{mtune=native:%>mtune=native %:local_cpu_detect(tune)}
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>...
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>The fact that it says 'march=native', wouldn't that mean that gcc will figure out by itself that I'm working on Atom? If I'd start building LFS on a system where the kernel was built with CONFIG_MATOM, would I then end up with a LFS system that has been optimized for Atom support? Or do I manually have to change the CFLAGS/CXXFLAGS?
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>Thanks a lot,
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>Hans
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Native means it will attempt to guess the arch currently in use. It should pick up the Atom processor by itself. If you want to be absolutely sure, then you can set CFLAGS and CXXFLAGS manually, but it's not required. I'm sure I'll be corrected if I miss anything.

Flareon

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