cmd/compile: "attempt to load unspilled value" compiling valid code

[josharian]

Attempt to compile:

package x

type T struct {
    i int
    e interface{}
}

func (t *T) F() bool {
    if t.i != 0 {
        return false
    }
    _, ok := t.e.(string)
    return ok
}

var x int

func g(t *T) {
    if t.F() || true {
        if t.F() {
            x = 0
        }
    }
}

Result:

x.go:19: internal compiler error: attempt to load unspilled value v69 = MOVQload <*uint8> [8] v5 v1

git bisect points at 40f2b57, CL 19728.

This is blocking other compiler changes I'm working on.

cc @brtzsnr @dr2chase @randall77

[brtzsnr]

Looks like an issue in the register allocator and that cl is just triggering it. I don't know enough about the register allocator, so I cannot be sure.

[josharian]

May be, although it is also possible that this case tickles deadcode into feeding the register allocator invalid SSA. I haven't looked.

In any case, when this is fixed, we should also remove the go:noinline work around for IsSlice (in type.go) introduced in CL 21611.

[gopherbot]

CL https://golang.org/cl/22330 mentions this issue.

[gopherbot]

CL https://golang.org/cl/22334 mentions this issue.

[randall77]

Combination of:

1. phielimValue leaving graph with values not dominating uses (https://go-review.googlesource.com/c/22335/)
2. bug in the dominator graph calculation ( cl/22334 )
3. bug in check() not detecting 1 or 2 ( cl/22330 )
   All in all, a very nice test case.

[dr2chase]

Should be fixed, see https://go-review.googlesource.com/#/c/22347/

[gopherbot]

CL https://golang.org/cl/22426 mentions this issue.