[llvm-dev] Consider making build directories of the buildbots available

Wed Apr 14 13:53:05 PDT 2021

That's an interesting idea. There are several issues to consider:

  1.  The build needs to be deterministic, if we want to share .OBJ files among LLVM developers. In essence, that means fully implementing [1]. I'm not sure how much of the LLVM codebase supports that.
Just the toolchain part is challenging to implement. We would need to ensure all users are using precisely the same compiler, the same linker, same cmake, same platform SDKs, etc. in order to expect good cache hits.
  2.  There's the security consideration. If anyone is to pull on the cached .OBJ files, you need to "trust" these .OBJ files in the first place. That means maybe restraining the list of cache "publishers" to the LLVM github group, and signing the .OBJs with a private key or something along those lines.
  3.  There's a file size consideration. My build folder for only { llvm, clang, lld } is about 40 GB. When using the ThinLTO cache, that goes over 100 GB. Still, build artefacts compress quite well (3:1 at least), and you'd probably pay the network price only once, then cache hits would be incremental.

I think the same idea could apply for distributing the compilation. It'd be interesting to have a public LLVM distributed compilation service. But how can we trust it? Even if we only compile on "trusted" machines, there's still a risk of attack. That's probably why these caching/remote compilation systems are only used inside an organization, which can guarantee trust (somehow). Since you're at Sony, perhaps you have access to a internal SN-DBS pool? [2]

[1] https://blog.llvm.org/2019/11/deterministic-builds-with-clang-and-lld.html
[2] https://www.snsystems.com/tech-blog/2014/01/06/building-with-the-network/

De : llvm-dev <llvm-dev-bounces at lists.llvm.org> De la part de via llvm-dev
Envoyé : April 14, 2021 3:31 PM
À : llvm-dev at lists.llvm.org
Objet : Re: [llvm-dev] Consider making build directories of the buildbots available

Ah, apologies,  I think I wasn't clear. I am talking about making object files available in this manner so that people can download them and compile just their changes without having to compile all of LLVM, thus reducing the barrier to entry. As far as I am aware, the releases on Github do not contain object files.

Nabeel Omer

From: llvm-dev <llvm-dev-bounces at lists.llvm.org<mailto:llvm-dev-bounces at lists.llvm.org>> On Behalf Of Neil Nelson via llvm-dev
Sent: 14 April 2021 20:05
To: llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] Consider making build directories of the buildbots available

Perhaps these pages may help.

https://github.com/llvm/llvm-project/releases

https://github.com/llvm/llvm-project/releases/tag/llvmorg-12.0.0-rc5

Neil Nelson
On 4/14/21 12:53 PM, via llvm-dev wrote:
Hi LLVM devs,

As you are already aware, performing a clean build of LLVM requires considerable computing resources. This presents a barrier to entry for people who do not have access to large computers. Since the buildbots already regularly compile the LLVM codebase, making tarballs of their build directories available on a public facing server will dramatically reduce the barrier to entry. Is this something that the community is willing to consider?

Thanks,
Nabeel Omer

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