rustbuild: Disable multiple CGUs in stable release builds

[petrochenkov]

#45400 enabled multiple CGUs + ThinLTO in rustc builds to speedup bootstrap, however it made the produced compiler slower (see #45400 (comment) and below).

This is desirable for development builds, where build times are priority, but not for stable release builds that happen rarely (so build times don't matter much) and produce binaries used by majority of Rust users.

Enabling full LTO (#45400 (comment)) in stable release builds is also worth investigating.

[Mark-Simulacrum]

Responding to @michaelwoerister from this comment:

I agree that we should build stable and beta releases with just one CGU and whatever other performance optimization we can do. I think the only reason we don't use regular LTO for stable releases is that rustc is built out of Rust dylibs which are incompatible with LTO.

I'm personally not opposed to shipping faster stable/beta artifacts -- we already do this by disabling LLVM assertions for example, so I don't see why disabling ThinLTO is a bad thing here. Seems like a fairly clear win, at least until ThinLTO is no longer a loss.

@alexcrichton, @Mark-Simulacrum, do we have numbers on how this influences build times in CI?

Not that I know of. Travis is problematic due to running our builds always at least once extra and often more than that, but we may be able to get data from @aidanhs. I'd also be interested in relative performance increases locally (building the compiler itself) but that just needs someone to be patient enough to rebuild the compiler with and without this patch.

[michaelwoerister]

I'm personally not opposed to shipping faster stable/beta artifacts -- we already do this by disabling LLVM assertions for example

That would have been my line of argument too.

[alexcrichton]

Yes as @michaelwoerister mentioned the current design with dylibs inhibits full LTO (in general procedural macros throw a wrench into full LTO).

For disabling multiple CGUs/ThinLTO it seems fine to do that on release builds which as mentioned already have other tweaks anyway.

[aidanhs]

I might be able to get some stats if you let me know what you'd like to compare - before and after a particular commit I assume? There's a lot of noise in Travis (particularly at the moment) so there may not be very high quality results.