Date: Tue, 7 Jul 2020 14:06:41 +0200
From: Natanael Copa <ncopa@alpinelinux.org>
To: "Alex Xu (Hello71)" <alex_y_xu@yahoo.ca>
Cc: ~alpine/devel@lists.alpinelinux.org
Subject: Re: Distro optimization flags
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Hi!

On Wed, 01 Jul 2020 23:10:30 -0400
"Alex Xu (Hello71)" <alex_y_xu@yahoo.ca> wrote:

> Recently there was some discussion on #alpine-devel about optimization 
> flags. I think it's worth looking at this issue more closely.
> 
> === Rationale ===
> 
> -Os is much slower than -O2. I recompiled gcc 9.3.0-r3 from head with 
> arguments {3} from below and tested compiling Linux 5.7.7 allnoconfig from 
> tmpfs on edge with make -j4. On my Intel laptop, edge gcc takes about 45 
> seconds, O2 gcc takes 39 seconds, and Debian sid takes only 30 seconds. 
> On my Ryzen desktop, edge takes 38 seconds, O2 takes 33 seconds, and 
> Debian takes only 22 seconds. In other words, O2 is about a 15% speedup, 
> and LTO is another 30-50% on top of that.
> 
> https://lore.kernel.org/lkml/20110323211415.GA8791@elte.hu/ from 2011 
> says that the kernel ran 'hackbench 15' 10% faster using -O2.
> 
> http://web.archive.org/web/20200408145313/https://rv8.io/bench from 2017 
> appears to say that rv8 ran about 25% faster using -O2 compared to -Os.

Thank you for doing those tests. Results are interesting.

...
 
> There is also a third option: we can use -O2 in some common CPU-heavy 
> programs and libraries, such as gcc and openssl. Alpine already uses 
> default optimization for musl, which I think works out to -Os for most 
> components and -O3 for performance-sensitive areas. It would be great if 
> all packages could do this, but it also sounds like way too much work to 
> patch every single package (and probably PGO is the right answer there 
> anyways).

We already do this. We set -O2 for zlib for example.

...
 
> == Conclusions ==
> 
> Personally, I think a 15% speedup is very much worth a 15% increase in 
> the small portion of my storage used for storing programs. I definitely 
> think that the optimization level for gcc itself should be changed, and 
> building it with LTO should be fixed/implemented as soon as possible. I 
> certainly hope that nobody is installing gcc on their minimal IoT 
> systems or whatever that cannot spare 10 MB of space. (Also, those 
> people are wasting space already on Obj-C and D support.)
> 
> In my opinion, anybody that doesn't want to use an extra few dozen 
> megabytes of space either should care more about the extra power 
> consumption, or should be using a custom OpenWRT or Buildroot anyways, 
> where they can customize everything.

I think we should keep -Os as the default and enable -O2 on few
packages where it makes sense. Alpine Linux is "Small. Simple. Secure"
after all.

Those who really want an -O2 distro has a lot of other distros to chose
between.

That said, I agree that it makes sense to build gcc with -O2.

> 
> [1] https://lists.alpinelinux.org/~alpine/devel/%3C2896c13070c508a49cbaa72c8fb7f34ea947358b.camel%40cogitri.dev%3E
> [2] https://github.com/richfelker/mallocng-draft/commit/a9187f0387dcbb77f1f7e4d7774602fd394fb27b
> 
> Cheers,
> Alex.