cmd/compile: write barrier code is sometimes preemptible when compiled with -N [1.21 backport]

[randall77]

package main

var g *int

func h() {
	g = nil
}

When compiled with -N (optimizations off), the empty block on the write-barrier-disabled branch is marked as preemptible when it shouldn't be.

	0x0018 00024 (/Users/khr/gowork/tmp7.go:6)	MOVWU	runtime.writeBarrier(SB), R0
	0x0020 00032 (/Users/khr/gowork/tmp7.go:6)	CBZW	R0, 40
	0x0024 00036 (/Users/khr/gowork/tmp7.go:6)	JMP	44
	0x0028 00040 (/Users/khr/gowork/tmp7.go:6)	PCDATA	$0, $-1
	0x0028 00040 (/Users/khr/gowork/tmp7.go:6)	JMP	64                     <= this one
	0x002c 00044 (/Users/khr/gowork/tmp7.go:6)	PCDATA	$0, $-2
	0x002c 00044 (/Users/khr/gowork/tmp7.go:6)	MOVD	main.g(SB), R0
	0x0034 00052 (/Users/khr/gowork/tmp7.go:6)	CALL	runtime.gcWriteBarrier1(SB)
	0x0038 00056 (/Users/khr/gowork/tmp7.go:6)	MOVD	R0, (R25)
	0x003c 00060 (/Users/khr/gowork/tmp7.go:6)	JMP	64
	0x0040 00064 (/Users/khr/gowork/tmp7.go:6)	MOVD	ZR, main.g(SB)

(PCDATA of -1 is preemptible, -2 is unpreemptible).

This could lead to very rare but nasty heap corruption when compiling with optimizations off (e.g. when running with Delve).

[gopherbot]

Change https://go.dev/cl/518755 mentions this issue: [release-branch.go1.21] cmd/compile: ensure empty blocks in write barriers are marked unpreemptible

[dmitshur]

@randall77 Is there a corresponding upstream issue (i.e., for Go1.22 milestone) for this backport request? Does this apply to Go 1.20 too, or only Go 1.21?

Per https://go.dev/wiki/MinorReleases:

As soon as an interested party thinks an issue should be considered for backport, they open one or two “child” issues titled like package: title [1.17 backport]. The issue should include a link to the original issue and a short rationale about why the backport might be needed.

[randall77]

This applies only to 1.21, 1.20 does not have the bug in question.
There's no upstream (1.22) issue, but there are CLs (518055 and 518539). They aren't easily backportable as is because the code has changed in the meantime.
CL 518755 is a simpler, safer backport for just the version of the bug that manifests in 1.21.

[gopherbot]

Closed by merging 25ec110 to release-branch.go1.21.