codegen-units=16 doubles compile times for Firefox

[Manishearth]

https://bugzilla.mozilla.org/show_bug.cgi?id=1436251

The other day I was experimenting with codegen-units, and realized that Firefox's build gets really slow with codgen-units=16 (the default):

with codegen-units=1
real    20m35.218s
user    135m46.044s
sys     5m49.332s

with default codegen-units (16)
real    38m3.751s
user    156m53.384s
sys     6m22.100s

I wasn't able to reduce the testcase.

If you wish to do it, use nightly to build Firefox with the following mozconfig file:

ac_add_options --enable-debug
ac_add_options --enable-optimize
ac_add_options --enable-release

and revert this patch: https://hg.mozilla.org/mozilla-central/rev/fe90c7d53a7c

A useful bit of information may be that we pass -Clto only to the toplevel crate, i.e. we build with cargo rustc -- -Clto. I don't remember but it might have been only the last crate ("gkrust") taking forever to build (I can retest this if you wish) so it might be some pathological case with LTO.

cc @michaelwoerister

[matthiaskrgr]

Sounds like #48025

[Manishearth]

cc @alexcrichton does this sound like a dupe of #48163 ?

I'm going to test with/without that PR soon.

[michaelwoerister]

Depending on what Rust version you are using, it might also be related to #48226?

[michaelwoerister]

What's the compile time without the ac_add_options --enable-debug setting?

[Manishearth]

Will try and report back

…

On Feb 15, 2018 7:54 AM, "Michael Woerister" ***@***.***> wrote: What's the compile time without the ac_add_options --enable-debug? — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#48233 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ABivSN33HexQGqctOV85IRvk5stO0-c0ks5tVFMdgaJpZM4SHACu> .

[nikomatsakis]

Nominating for prioritization and discussion (presuming it's not resolved before then).

[nikomatsakis]

triage: P-medium

Marking as P-medium for now until we get some more heat from the FF folks. ;)

[nikomatsakis]

In particular, nobody is using CGU=16 yet, right? (But also, it's not entirely obvious to me that it would be desirable, given #47745)

[Manishearth]

I tried it out again and it seems to be fixed.

CGU=16 is the default, yeah? So everyone is using it.

[michaelwoerister]

@Manishearth, I thought that FF is compiled with codegen-units=4?

[Manishearth]

Release mode uses the default.

[nikomatsakis]

Mmmmm, that might bump the priority somewhat. =)

[michaelwoerister]

Release mode uses the default.

Is this with full LTO?

[Manishearth]

I'm not sure, eddyb said that LTO turns it off.

However, Firefox builds with LTO turned on only for the last crate.

[alexcrichton]

To clarify, rustc is using 16 codegen units by default in all modes, regardless of optimization level, LTO settings, etc. @Manishearth you mention this is fixed now though? If that's the case this was indeed likely fixed by #48163

[Mark-Simulacrum]

Based on "I tried it out again and it seems to be fixed." from #48233 (comment), I'm going to close this issue.