[llvm-dev] LTO'd / PGO'd Pre-built Release Binary

Mon Sep 7 07:10:39 PDT 2020

> I emailed llvm-dev a while ago suggesting that we at least turn on LTO.

Ah, sorry, missed that. Otherwise I would have replied to/revived that thread.

> I really didn't get much feedback on that unfortunately, I think most people get clang via their distro or compile it themselves.

Well, that's what I was wondering, wasn't sure if it is worth doing....but I guess we upload these binaries for a reason and guess these binaries are picked up by some people? Even if not, it would be good to set an example (and for testing)?

> I think the most complicated part is that the release testers are the one responsible for building the final binaries and there is a lot of different systems that might require some tweaking.

Oh yeah, definitely. I see that it's easy to suggest this, and that it's a lot of work for the people who have to do this.

Cheers.
________________________________
From: Tobias Hieta <tobias at plexapp.com>
Sent: 07 September 2020 14:56
To: Sjoerd Meijer <Sjoerd.Meijer at arm.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] LTO'd / PGO'd Pre-built Release Binary

Sjoerd,

I emailed llvm-dev a while ago suggesting that we at least turn on LTO. I really didn't get much feedback on that unfortunately, I think most people get clang via their distro or compile it themselves.

I did look into what's needed: the release script need to build the first and second stage as it does now but for the final stage we would need to adapt it to send in a compatible linker (gold or lld) and fix ranlib/ar invocations.

I think the most complicated part is that the release testers are the one responsible for building the final binaries and there is a lot of different systems that might require some tweaking.

I have had a dream of some CI service being able to build PGO/LTO optimized binaries (at least for Linux, windows and Mac) for each release - but I think we would need a cloud vendor sponsoring that CPU time (as a reference our toolchain build takes ~5 hrs for something similar).

Thanks,
Tobias

On Mon, Sep 7, 2020, 14:50 Sjoerd Meijer via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
A compiler built with LTO / PGO can significantly improve build times. Locally I see ~10% reduction in compilation times of Debug or Release versions of clang/llvm using an LTO'd clang compiler. This is not really news of course (e.g. recently also reconfirmed in https://linuxplumbersconf.org/event/7/contributions/802/), but that made me curious if we can and/or should enable LTO and/or PGO to create our prebuilt binaries?

The disadvantage for our release people will be obvious: that will take a lot more time and resources. But would it be worth to make this investment?

Cheers,
Sjoerd.

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