build: adopt Go 1.16 as bootstrap toolchain for Go 1.18 #44505
I propose that starting with the Go 1.18 dev cycle, we require Go 1.16 as the Go bootstrap version (instead of Go 1.4).
When we switched to writing the Go compiler in itself, I wrote two proposals related to the bootstrap process.
The original proposal, https://golang.org/s/go13compiler (December 2013), was about the overall process of converting the compiler to Go.
This was all fairly hypothetical, and I certainly no longer believe it makes any sense to write a C back end for the Go compiler. (In fact, for the windows/arm64 port I did, I now have a working Go toolchain but still don't know what C compiler I'm supposed to use!)
The final proposal, https://golang.org/s/go15bootstrap (January 2015), simplified the process from an iterative one to hard-coding Go 1.4 as the bootstrap toolchain:
This was an important simplification, especially for people packaging Go for other systems. That decision has served us well.
But it has now been over six years since Go 1.4. Much has happened in the world of Go, and many bugs have been fixed. Many of the systems Go runs on today aren't supported by Go 1.4 (including darwin/arm64 for M1 Macs). Those are using newer toolchains to bootstrap, and the other systems could too. At a higher level, Go is far more mature and widely available now than it was in the Go 1.4 era. There are tons of available binary distributions to use for bootstrapping.
I propose that Go 1.17 be the last version of Go requiring Go 1.4 for bootstrapping, and that Go 1.18 require Go 1.16 for bootstrapping.
Why not Go 1.15?
Why not Go 1.17?
Why not Go 1.18?
Why not a quickly rolling version?
As noted above, I think it has served us well to have a fixed version required to build Go, as opposed to an automatically sliding version as originally envisioned. Packagers benefit from not having to update their package-building scripts to provide a different environment to each new Go release.
At the same time, bumping the version forward every five years or so lets us take advantage of newer Go capabilities and ports and to let us retire old compatibility shims (standard library packages like sort are carrying various +build'ed files to keep them building with Go 1.4). Modern C compilers are not written in pre-ANSI C.
What about a slow-rolling version?
That's essentially what this proposal would establish as our practice, although without a specific timeline.
The next obvious entry in the sequence after Go 1.4 and Go 1.16 is Go 1.256, followed by Go 1.65536.
Using dates instead, assuming we switch to Go 1.16 in Go 1.18 (Feb 2022), it seems reasonable to me to revisit the bootstrap version four years later, which at our current release cycle would mean using Go 1.24 (Feb 2025) for Go 1.26 (Feb 2026).
But this proposal is not about establishing the sequence, which would depend on many other factors.
It is only about picking Go 1.16 as the bootstrap version starting in Go 1.18.
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Actually, I confused myself while writing this.
Being able to assume a recent compiler for bootstrapping would be really nice for not having to remember all the quirks of Go 1.4, or needing to worry about working around long-fixed issues.
FWIW, OpenJDK appears to require the immediately previous release for bootstrapping (see https://openjdk.java.net/groups/build/doc/building.html, "Build JDK Requirements"), and also has a 6-month release cycle (https://en.wikipedia.org/wiki/OpenJDK#OpenJDK_versions), but building OpenJDK takes way longer than running make.bash does. I don't think we need to be nearly as aggressive at uprev'ing Go bootstrap versions, but I think it at least suggests end users could tolerate another uprev before 1.256. :)
This is a good idea, however, perhaps it would make sense to additionaly declare that the bootstrap version of Go is also going to be the long term support or "LTS" version, which keeps receiving backports of bugfixes for 4 years (or whatever the support period for the bootstrap version is going to be). In some organizations and for some developments, having a stable LTS version that keeps receiving bug fixes is extremely valuable.
FWIW, the level of aggressive bootstrap version bumps OpenJDK does are a big hurdle towards having recent OpenJDK packaged for backported distribution releases (and require a lot of extra coordination / "fancy footwork" from maintainers to coordinate or "hack" around).
As a concrete example for Go, this could make adding new architectures more difficult for a distribution until/unless
IMO this would be easier to understand/stomach if the proposed version were one that's already supported by at least the most recent