Re: [PATCH 0/2] x86: Remove ideal_nops[]

[Sedat Dilek <sedat.dilek@gmail.com>]

On Fri, Mar 12, 2021 at 1:00 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> Hi!
>
> A while ago Steve complained about x86 being weird for having different NOPs [1]
>
> Having cursed the same thing before, I figured it was time to look at the NOP
> situation.
>
> 32bit simply isn't a performance target anymore, so all we need is a set of
> NOPs that works on all.
>
> x86_64 has two main NOP variants, NOPL and prefix NOP. NOPL was introduced by
> P6 and is architecturally mandated for x86_64. However, some uarchs made the
> choice to limit NOPL decoding to a single port, which obviously limits NOPL
> throughput. Other uarchs have (severe) decoding penalties for excessive (>~3)
> prefixes, hobbling prefix NOP throughput.
>
> But the thing is, all the modern uarchs can handle both without issue; that is
> AMD K10 (2007) and later and Intel Ivy Bridge (2012) and later. The only
> exception is Atom, which has the prefix penalty.
>
> Since ultimate performance of a 10 year old chip (Intel Sandy Bridge, 2011) is
> simply irrelevant today, remove variable NOPs and use NOPL.
>

Hi Peter,

I am an Intel SandyBridge power user and want the ultimate performance
on my hardware.

What does this change exactly mean to/for me?

I got this laptop as the last gift for my birthday in 2012 from my mother.
She died the same year.
So, this is a bit sentimental hardware for me.

It's amazing what this laptop all was involved in.
10+ years of LLVM/Clang for Linux-kernel and Linux graphics stack.
Worked in a Ubuntu/precise 12.04 LTS WUBI (installation) environment -
5 years (full LTS period) long!
How many Linux-kernel bugs got reported and/or fixed...
Debian/stretch...Debian/bullseye with no fresh installation. Rolling release.

I remember my decision in March 2012 not to choose that Asus notebook
with the first hardware-revision of IvyBridge and bought
conservatively a SandyBridge Gen. 2 Samsung notebook.

It's a pity to see no or restricted/limited Vulkan support.

If you are not concerned - life goes on for you.

It's like being white colored not understanding what "Black Lives
Matter" really means.
If people use or talk about white/black listings then allow/deny lists.
Or being a female software developer having a 10-15% less salary
because you are not male - in the same department!
This week we had our 100th anniversary of International Women's Day.
I am not black - I am male - I am not concerned - Live goes on?

Again, this machine is able to do fast Linux-kernel builds with an
adapted Debian Linux v5.10 kernel-config.
If you do NOT use Debian's LLVM/Clang - means build a selfmade
stage1-only LLVM toolchain (saves ~1 hour of build-time) - or a
ThinLTO+PGO optimized LLVM toolchain (saves again ~1 hour of
build-time).
Latest Linus Git plus With Clang-CFI took me today approx. 04:20
[hh:mm] with a selfmade stage1-only LLVM toolchain version 12.0.0-rc3.
Again, this is amazing.

What I wanna try to say is:
This is old hardware but you can - if you are a smart enough -
optimize your builds.

On the other hand I can understand dropping support for XXX whatever hardware...
Where is the limit(ation):
Support 10 years or 7 years old hardware?

Sorry, I am a bit concerned that this is the beginning - or a backdoor
? - to drop (optimized) Intel SandyBridge support.

So, what do I need to do - to have "ultimate performance" back for
SandyBridge with your patchset :-)?

Yes, you are right: Life goes on.

Regards,
- Sedat -


> This gives us deterministic NOPs and restores sanity.
>
>
>
> [1] https://lkml.kernel.org/r/20210302105827.3403656c@gandalf.local.home
>