linux-kernel - Re: [PATCH] arch/x86: Add an option to build the kernel with '-march=native' on x86-64

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Message-Id: <106aacc5-e863-4e74-b223-34b7a0be60a0@app.fastmail.com>
Date: Mon, 24 Mar 2025 09:45:14 +0100
From: "Arnd Bergmann" <arnd@...nel.org>
To: "Tor Vic" <torvic9@...lbox.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Cc: "Borislav Petkov" <bp@...en8.de>, "Thomas Gleixner" <tglx@...utronix.de>,
 "Ingo Molnar" <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] arch/x86: Add an option to build the kernel with '-march=native'
 on x86-64

On Sun, Mar 23, 2025, at 16:14, Tor Vic wrote:
> On 3/22/25 12:40, Arnd Bergmann wrote:
>> On Fri, Mar 21, 2025, at 15:28, Tor Vic wrote:
>>
>> Are you able to quickly run the same tests again using
>> just one of the two?
>> 
>> a) -march=x86-64 -mtune=skylake
>> b) -march=skylake -mtune=generic
>> 
>
> I ran the tests on Zen2 (znver2), but this time the kernel was built 
> with clang-20+lto (the skylake tests were with gcc-14).
>
> It turned out that with '-march=native', there is almost no difference 
> compared to '-march=x86-64'.
> All results are within +0.8% and -0.6%, most of which are probably 
> noise. Hackbench, stress-ng fork and xz compression seem to profit the 
> most from 'native'.
>
> The vmlinux image is 0.03% bigger with 'native'.
>
> I guess that 'native' can be somewhat useful on some architectures, but 
> not on all...

It certainly depends on configuration, toolchain and target machine,
so it's very hard to say anything general about -march=native.
znver2 and skylake are quite similar in the supported instructions,
so my guess would be that the clang-20+lto doesn't behave that differently
between march=x86-64 and march=znver2.

Of course if you don't have the same kernel version and configuration
that you are testing, the results between the two machines, the
results would be fairly random as well.

     Arnd