build: move GOEXPERIMENT knob from make.bash to cmd/go

[mdempsky]

GOEXPERIMENT currently allows trying out a handful of different experimental toolchain features, none of which need to be decided at make.bash time. We can just make the rest of the toolchain GOEXPERIMENT-aware. (If you really want the toolchain itself to be built with an experiment enabled, "GOEXPERIMENT=foo go install -a cmd" would work.)

[bcmills]

How many of the GOEXPERIMENT values could instead be -gcflags or build constraints, both of which cmd/go already understands?

I'd rather we reduce than add to the number of ways to vary the build configuration.

[mdempsky]

I don't feel strongly about whether GOEXPERIMENT should be the knob used by cmd/compile and other tools. I think it would be a simpler incremental step to continue using that, but I'm fine switching to normal flags too.

I do think cmd/go should probably be in charge of orchestrating everything still somehow. I've seen users too frequently use -gcflags=foo instead of -gcflags=all=foo; use just -gcflags without realizing they need to pass linker flags too; etc.

[gopherbot]

Change https://golang.org/cl/271217 mentions this issue: cmd/compile: fix panic in field tracking logic

[bcmills]

I think this kind of interacts with the various proposals for build configurations (#39005, #42343, and perhaps others; CC @mvdan @dominikh @jayconrod @matloob).

In particular, you can think of a GOEXPERIMENT value as essentially just a predefined shorthand for a set of build flags. If we add some mechanism for users to define their own shorthand for sets of build flags, we should try to get GOEXPERIMENT to converge (or at least harmonize) with that.

[gopherbot]

Change https://golang.org/cl/280113 mentions this issue: [dev.regabi] cmd: move GOEXPERIMENT knob from make.bash to cmd/go

[mdempsky]

CL 280113 demonstrates what I had in mind for this. I've tested and confirmed it works for fieldtrack. I don't see any reason it shouldn't work for others.

I think there are some rough edges still (e.g., I suspect GOENV doesn't work to set GOEXPERIMENT, and setting GOEXPERIMENT should probably update the package suffix), but I'll clean those up if this is a worthwhile direction to pursue. I think it is.

/cc @rsc @aclements @cherrymui @thanm

[aclements]

We're definitely running into this with the register ABI work. We currently use GOEXPERIMENT as a way to control the compiler, assembler, and various runtime packages all together, but that only has to be done on a whole-build basis, not a whole make.bash-basis. And the current behavior of GOEXPERIMENT means that we can't easily build a working tool chain and then do individual, more targeted builds with the option enabled.

We don't necessarily need GOEXPERIMENT to change, but some coherent whole-build option would make this much easier. We already have some things that work like this, like go build's -race option, but not anything that's easily extensible like GOEXPERIMENT.

[aclements]

Here's a variation on @mdempsky 's proposal that would be backwards compatible while satisfying our needs as I understand them:

Add a flag to go build, say, -goexperiment, that allows for setting the experiments in a single build. These always apply to the whole build, and would be plumbed into the compiler, assembler, and runtime by the go tool (using the same conventions GOEXPERIMENT uses today). The value of GOEXPERIMENT at make.bash time would set the default value of this flag; hence this would behave exactly as today if you don't pass -goexperiment to the go tool. (Alternatively, GOEXPERIMENT at go build time could override the make.bash setting, but I think giving it meaning both for make.bash and go build is potentially confusing and also I like fewer environment variables.)

If we eventually have some broader build configuration mechanism, I'd be more than happy to switch to that, but I think this is something we could do today (@mdempsky already did most of the work). And since this is meant for tool chain development, it's something we could back out or replace with another mechanism later.

[aclements]

I hadn't appreciated that @mdempsky 's proposal was that the GOEXPERIMENT set at make.bash time becomes the default, but can be overridden at go build time by setting GOEXPERIMENT then (so it is mostly backwards compatible).

A bunch of us from the runtime/compiler team chatted about this today and everybody seems to think this is a good idea. There wasn't strong consensus on whether the go build override should be the GOEXPERIMENT environment variable or a flag, but people generally favored the environment variable because environment variables tend to more obviously flow to the whole process.

[rsc]

It looks like the four active experiments, from cmd/internal/objabi/util.go, are:

{"fieldtrack", &Fieldtrack_enabled},
{"preemptibleloops", &Preemptibleloops_enabled},
{"staticlockranking", &Staticlockranking_enabled},
{"regabi", &Regabi_enabled},

preemptibleloops sounds like it is dead code that should be removed.
fieldtrack is Google-internal but not a big deal for others to have easier access to.

staticlockranking looks like it could just as easily be a build tag so it's fine to keep as an experiment instead.

So really this looks like it is about exposing regabi, which seems fine as well.

I wondered a bit what the distinction between this and GODEBUG is. GODEBUG is purely runtime, I suppose.
But there are definitely some things we control with GODEBUG that feel like the above (GODEBUG=asyncpreemptoff=1 vs GOEXPERIMENT=preemptibleloops for example).

Overall seems reasonable. I think this is something we all working on the go command and compiler need to agree on, but not something that needs community buy-in. This is an internal detail of how Go is built, more than anything. And we all seem to agree.

Marking accepted.

[rsc]

No change in consensus, so accepted. 🎉
This issue now tracks the work of implementing the proposal.
— rsc for the proposal review group

[gopherbot]

Change https://golang.org/cl/300991 mentions this issue: cmd: move GOEXPERIMENT knob from make.bash to cmd/go

[gopherbot]

Change https://golang.org/cl/302969 mentions this issue: test: switch fieldtrack test to use GOEXPERIMENT

[rsc]

This seems to have been committed before being accepted. Please hold off in the future.

[aclements]

@rsc, I'm not sure what you mean. It looks to me like it was accepted on March 10th and committed on March 11th.

[rsc]

@aclements, indeed, my bad. Many apologies!

[Looking back I see what I did wrong during the meeting - I did something slightly different from my normal routine and ended up opening this issue in the middle of a batch of "likely accept" tabs. When I got to it, I completely missed that it was out of place and read it as a "likely accept". Entirely my fault. Then I placed it on the "accept" list for the minutes bot, and the bot correctly found no state updates to apply, although it did dutifully include the entry in the minutes. So that's why this issue is listed as accepted again this week in the minutes. Again, my bad. Sorry!]