cmd/compile: add nostackallocvariablemake goexperiment

[qmuntal]

CL 653856 made small variable-sized make calls to allocate on the stack rather than on the heap. This is causing widespread failures among Windows code using syscall and x/sys/windows, given that it is easy to take wrong assumptions about object lifespans and memory layout. See for example #73199 and #73170.

We will fix the bugs found in the Go standard library and in x/sys/windows, but we don't have enough code coverage to be 100% sure all bugs related to CL 653856 are found and fixed, and we can't control third-party code making wrong assumptions.

We should add a goexperiment setting to disable stack allocated variable-sized make calls to facilitate upgrading to Go 1.25. This setting could be named nostackallocvariablemake..

@randall77 @golang/compiler

[mateusz834]

Godebugs are a runtime thing, right? That would have to be a goexperiment as this was a compiler change, right?

[qmuntal]

Godebugs are a runtime thing, right? That would have to be a goexperiment as this was a compiler change, right?

Good catch, will update the issue.

[thepudds]

FWIW, when I was looking at some of the related problems earlier, I had been thinking along similar lines, except I was thinking it might be better to do a compiler debug flag (rather than a GOEXPERIMENT or GODEBUG flag). It's maybe a little more convenient to be able to enable/disable per package and there are a couple of other reasons, though GOEXPERIMENT could be a reasonable choice as well.

In any event, I sent https://go.dev/cl/663795 for discussion. (Still WIP).

@qmuntal, let me know if that overlaps with something you already have in progress.

We can probably hash out best approach here (GOEXPERIMENT or GODEBUG or cmd/compile debug flag or ...).

I think would also be nice to hook up to the hash-based bisect, which I plan to try in a bit.

[randall77]

I think a GOEXPERIMENT makes the most sense.
We could conceivably make this a GODEBUG, at the cost of a global flag check in the generated code. I will look into how feasible that would be.

[mateusz834]

We could conceivably make this a GODEBUG, at the cost of a global flag check in the generated code. I will look into how feasible that would be.

What about a Default GOEXPERIMENT value based on go.mod version? Is that possible?

[randall77]

That would certainly be possible. We did it for the loop variable change, I believe.
That said, that was for a change to Go program semantics. The changes we're discussing here are not in that category, so it may be overkill.

[gopherbot]

Change https://go.dev/cl/663795 mentions this issue: cmd/compile/internal/escape: add hash for bisecting stack allocation of variable-sized makeslice