cmd/compile: unnecessary branching

[randall77]

type T struct {
    x, y, z int
}
func f(x interface{}) int {
    t, ok := x.(*T)
    if ok && t != nil {
        return 7
    }
    return 3
}

The SSA compiler for this function generates some extra branching logic that seems unnecessary.

    0x0000 00000 (bin.go:12)    LEAQ    type.*"".T(SB), AX
    0x0007 00007 (bin.go:8) MOVQ    "".x+8(FP), CX
    0x000c 00012 (bin.go:8) CMPQ    AX, CX
    0x000f 00015 (bin.go:8) JNE $0, 49
    0x0011 00017 (bin.go:7) MOVQ    "".x+16(FP), AX
    0x0016 00022 (bin.go:9) JNE 29
    0x0018 00024 (bin.go:9) TESTQ   AX, AX
    0x001b 00027 (bin.go:9) JNE $0, 39
    0x001d 00029 (bin.go:12)    MOVQ    $3, "".~r1+24(FP)
    0x0026 00038 (bin.go:12)    RET
    0x0027 00039 (bin.go:10)    MOVQ    $7, "".~r1+24(FP)
    0x0030 00048 (bin.go:10)    RET
    0x0031 00049 (bin.go:8) MOVQ    $0, AX
    0x0038 00056 (bin.go:9) JMP 22

The 4th line there could be JNE directly to line 29. Instead it goes to 49/56, rejoins the main flow at 22, and then goes to 29.
Something wrong with the shortcircuit pass, maybe?

[bradfitz]

Related: the ok value can be ignored entirely. If t is non-nil, that implies that ok is true anyway:

func f(x interface{}) int {
    t, _ := x.(*T)
    if t != nil {
        return 7
    }
    return 3
}

[josharian]

Related: #12628