cmd/compile: writebarrier code not happy with single(ish)-block-loops that contain writebarriers

[dr2chase]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

1.9 devel:
go version devel +e2948f7efe Mon Feb 13 20:30:31 2017 +0000 darwin/amd64

What operating system and processor architecture are you using (go env)?

amd64/darwin

What did you do?

https://play.golang.org/p/72MnkjS4P4

What did you expect to see?

This works in the playground.

*pa[5]= 6

What did you see instead?

# command-line-arguments
./bar.go:9: internal compiler error: value v42 depends on WB store v44 in the same block b8

goroutine 1 [running]:
runtime/debug.Stack(0x0, 0x0, 0x0)
	/.../go/src/runtime/debug/stack.go:24 +0x79
cmd/compile/internal/gc.Fatalf(0x16c3354, 0x34, 0xc420349890, 0x3, 0x3)
	/.../go/src/cmd/compile/internal/gc/subr.go:175 +0x230
cmd/compile/internal/gc.(*ssaExport).Fatalf(0x19bbd55, 0x90900000001, 0x16c3354, 0x34, 0xc420349890, 0x3, 0x3)
	/.../go/src/cmd/compile/internal/gc/ssa.go:4939 +0x67
cmd/compile/internal/ssa.(*Config).Fatalf(0xc42038c000, 0x90900000001, 0x16c3354, 0x34, 0xc420349890, 0x3, 0x3)
	/.../go/src/cmd/compile/internal/ssa/config.go:345 +0x76
cmd/compile/internal/ssa.(*Func).Fatalf(0xc42036fd40, 0x16c3354, 0x34, 0xc420349890, 0x3, 0x3)
	/.../go/src/cmd/compile/internal/ssa/func.go:416 +0x72
cmd/compile/internal/ssa.writebarrier(0xc42036fd40)
	/.../go/src/cmd/compile/internal/ssa/writebarrier.go:106 +0xb3f
cmd/compile/internal/ssa.Compile(0xc42036fd40)
	/.../go/src/cmd/compile/internal/ssa/compile.go:70 +0x2c4
cmd/compile/internal/gc.buildssa(0xc4200c3180, 0x0)
	/.../go/src/cmd/compile/internal/gc/ssa.go:181 +0x10d7
cmd/compile/internal/gc.compile(0xc4200c3180)
	/.../go/src/cmd/compile/internal/gc/pgen.go:366 +0x2d3
cmd/compile/internal/gc.funccompile(0xc4200c3180)
	/.../go/src/cmd/compile/internal/gc/dcl.go:1226 +0xdc
cmd/compile/internal/gc.Main()
	/.../go/src/cmd/compile/internal/gc/main.go:473 +0x203c
main.main()
	/.../go/src/cmd/compile/main.go:50 +0xfe

There's a check in the writebarrier code that might not be necessary, or might need to exclude phi functions:

// make sure that no value in this block depends on WB stores
for _, w := range b.Values {
	if w.Op == OpStoreWB || w.Op == OpMoveWB || w.Op == OpMoveWBVolatile || w.Op == OpZeroWB {
		continue
	}
	for _, a := range w.Args {
		if wbs.contains(a.ID) {
			f.Fatalf("value %v depends on WB store %v in the same block %v", w, a, b)
		}
	}
}

The offending input looks like this:

b8: ← b6 b10
v42 = Phi <mem> v17 v44
v54 = Phi <int> v8 v46
v43 = NilCheck <void> v26 v42
v44 = StoreWB <mem> [8] v26 v36 v42
Plain → b9

b9: ← b8
v35 = Const64 <int> [1]
v46 = Add64 <int> v35 v54
v48 = Less64 <bool> v46 v53
If v48 → b10 b11

b10: ← b9
Plain → b8

[cherrymui]

I have mailed a sequence of CLs for the new writebarrier implementation this morning. The new code doesn't have this problem.

[cherrymui]

In particular, CL https://go-review.googlesource.com/c/36834/ is the one most relevant to this issue.

[cherrymui]

And if we were to fix the old implementation, yes, Phi function needs to be special-cased in that check.

[gopherbot]

CL https://golang.org/cl/36940 mentions this issue.

[gopherbot]

CL https://golang.org/cl/36834 mentions this issue.