cmd/compile: regression in writebarrier pass

[dsnet]

c4ef597 caused a regression where the following coder no longer compiles:

type Foo struct{ A, B time.Duration }

func Bar(fs []Foo) string {
	ss := make([]string, 2*len(fs))
	for i, h := range fs {
		ss[2*i] = fmt.Sprintf("%v %v", h.A, h.B.Seconds())
		ss[2*i+1] = fmt.Sprintf("%v %v", h.A, h.B.Seconds())
	}
	return strings.Join(ss, ",")
}

It now panics with:

./main.go:16: internal compiler error: attempt to load unspilled value v53 = MOVQload <time.Duration> {h} [8] v2 v117

goroutine 1 [running]:
runtime/debug.Stack(0x0, 0x0, 0x0)
	/home/rawr/Projects/go/src/runtime/debug/stack.go:24 +0x79
cmd/compile/internal/gc.Fatalf(0xaccaeb, 0x22, 0xc4204a4600, 0x1, 0x1)
	/home/rawr/Projects/go/src/cmd/compile/internal/gc/subr.go:175 +0x230
cmd/compile/internal/gc.(*ssaExport).Fatalf(0xdd0af5, 0x100d00000001, 0xaccaeb, 0x22, 0xc4204a4600, 0x1, 0x1)
	/home/rawr/Projects/go/src/cmd/compile/internal/gc/ssa.go:4929 +0x67
cmd/compile/internal/ssa.(*Config).Fatalf(0xc420452000, 0x100d00000001, 0xaccaeb, 0x22, 0xc4204a4600, 0x1, 0x1)
	/home/rawr/Projects/go/src/cmd/compile/internal/ssa/config.go:345 +0x76
cmd/compile/internal/ssa.(*Func).Fatalf(0xc4204b4000, 0xaccaeb, 0x22, 0xc4204a4600, 0x1, 0x1)
	/home/rawr/Projects/go/src/cmd/compile/internal/ssa/func.go:416 +0x72
cmd/compile/internal/ssa.(*regAllocState).allocValToReg(0xc4204d83c0, 0xc420453988, 0xffce, 0x100000001, 0xc400001233, 0xc4204551e8)
	/home/rawr/Projects/go/src/cmd/compile/internal/ssa/regalloc.go:454 +0x58e
cmd/compile/internal/ssa.(*regAllocState).regalloc(0xc4204d83c0, 0xc4204b4000)
	/home/rawr/Projects/go/src/cmd/compile/internal/ssa/regalloc.go:1188 +0x1731
cmd/compile/internal/ssa.regalloc(0xc4204b4000)
	/home/rawr/Projects/go/src/cmd/compile/internal/ssa/regalloc.go:135 +0x62
cmd/compile/internal/ssa.Compile(0xc4204b4000)
	/home/rawr/Projects/go/src/cmd/compile/internal/ssa/compile.go:70 +0x2c4
cmd/compile/internal/gc.buildssa(0xc4200b9180, 0x0)
	/home/rawr/Projects/go/src/cmd/compile/internal/gc/ssa.go:173 +0x1060
cmd/compile/internal/gc.compile(0xc4200b9180)
	/home/rawr/Projects/go/src/cmd/compile/internal/gc/pgen.go:366 +0x2d0
cmd/compile/internal/gc.funccompile(0xc4200b9180)
	/home/rawr/Projects/go/src/cmd/compile/internal/gc/dcl.go:1226 +0xdc
cmd/compile/internal/gc.Main()
	/home/rawr/Projects/go/src/cmd/compile/internal/gc/main.go:473 +0x202d
main.main()
	/home/rawr/Projects/go/src/cmd/compile/main.go:50 +0x101

\cc @cherrymui @dr2chase @josharian

[gopherbot]

CL https://golang.org/cl/37250 mentions this issue.

[funny-falcon]

Catched with same bit in code with defer.
Code looks like:

    func (bld *DbBuilder) Finish() error {
        defer bld.Unlink() // <- here is an error
        // ... other code
    }

Unfortunately, can't build small reproduce case.

[funny-falcon]

Changeset 37250 doesn't fix for me at the moment.

go version devel +5bbe1ce Sat Feb 18 19:44:10 2017 -0500 linux/amd64
$ go build
./builder.go:239: internal compiler error: attempt to load unspilled value v1780 = Phi <int> v282 v449
....

[funny-falcon]

I reduced case to minimal. Looks like removing any line makes it compile

package dbfile

import (
	"encoding/binary"
)

type DbBuilder struct {
	arr        []int
}

func (bld *DbBuilder) Finish() error {
	defer bld.Finish()

	var hash []byte
	for _, ixw := range bld.arr {
		for {
			if ixw != 0 {
				return nil
			}
			ixw--
		insertOne:
			for {
				for i := 0; i < 1; i++ {
					if binary.LittleEndian.Uint16(hash[i:]) == 0 {
						break insertOne
					}
				}
			}
		}
	}

	return nil
}

[cherrymui]

@funny-falcon Thanks for the repro! However, the related values are not touched in the writebarrier pass, and they are in the correct order at that point. Later the tighten pass got something wrong. So it's a different bug. I'll take a look.

[funny-falcon]

@cherrymui panic("") could be changed to any non-empty line: ixw = 0, return nil or break - all still trigger error. But removing that line makes it compileable.

[cherrymui]

@funny-falcon, I filed issue #19217, since it is a different bug, and even apply to Go 1.8.

[funny-falcon]

@cherrymui, thank you