cmd/compile: internal compiler error: Flag* ops should never make it to codegen

[ALTree]

$ gotip version
go version devel +e92be18fd8 Wed Jun 10 14:56:01 2020 +0000 linux/amd64

The following program:

package p

func f() {
	if len([]int{})-1 < len([]int{}) {
	}

	var st struct {
		i int
	}
	g := func() string {
		return ""
	}
	h := func(string) string {
		return g() + g()
	}
	s, i := "", 0

	st.i = len(s)
	i = len(h(s[i+0:i+1])) + len(s[len(s)+1:i+1])
	s = s[(len(s[i+1:len(s)+1])+1):len(h(""))+1] + (s[i+1 : len([]int{})+i])
	i = 1 + len([]int{len([]string{s[i+len([]int{}) : len(s)+i]})})

	var ch chan int
	ch <- len(h("")) - len(s)
}

Crashes the tip compiler when built for arm or arm64:

$ GOARCH=arm gotip tool compile crash.go 

crash.go:4:20: internal compiler error: 'f': Flag* ops should never make it to codegen v294 = FlagLT_ULT <flags>

goroutine 1 [running]:
runtime/debug.Stack(0xd4bb80, 0xc0000b8008, 0x0)
	/home/alberto/go/src/runtime/debug/stack.go:24 +0x9f
cmd/compile/internal/gc.Fatalf(0xc000018180, 0x32, 0xc00000c0c0, 0x2, 0x2)
	/home/alberto/go/src/cmd/compile/internal/gc/subr.go:199 +0x1b0
cmd/compile/internal/gc.(*ssafn).Fatalf(0xc0003ca510, 0x414000000002, 0xc61cd5, 0x2c, 0xc000044040, 0x1, 0x1)
	/home/alberto/go/src/cmd/compile/internal/gc/ssa.go:6844 +0x1a5
cmd/compile/internal/ssa.(*Value).Fatalf(...)
	/home/alberto/go/src/cmd/compile/internal/ssa/value.go:384
cmd/compile/internal/arm.ssaGenValue(0xc0003eb400, 0xc0004080a0)
	/home/alberto/go/src/cmd/compile/internal/arm/ssa.go:865 +0x2b3d
cmd/compile/internal/gc.genssa(0xc00010cc60, 0xc0003eb380)
	/home/alberto/go/src/cmd/compile/internal/gc/ssa.go:6071 +0x702
cmd/compile/internal/gc.compileSSA(0xc00010c2c0, 0x0)
	/home/alberto/go/src/cmd/compile/internal/gc/pgen.go:327 +0x3a5
cmd/compile/internal/gc.compile(0xc00010c2c0)
	/home/alberto/go/src/cmd/compile/internal/gc/pgen.go:275 +0x345
cmd/compile/internal/gc.funccompile(0xc00010c2c0)
	/home/alberto/go/src/cmd/compile/internal/gc/pgen.go:220 +0xc5
cmd/compile/internal/gc.Main(0xc6e018)
	/home/alberto/go/src/cmd/compile/internal/gc/main.go:747 +0x344f
main.main()
	/home/alberto/go/src/cmd/compile/main.go:52 +0xb1

It compiles fine on go1.14.4, so this is a tip regression in two First Class Ports. I'm tentatively labelling this as a release-blocker, but leaving to you to decide if this is fine.

cc @randall77 @cherrymui

[mariecurried]

This error seems to have been caused by e031318

[ALTree]

Thanks @marigonzes for bisecting.

cc @shawn-xdji and @zhangfannie too, then.

[cherrymui]

Thanks. It seems the new noov ops also need to handle constant flags. (I also leave a comment on the CL.)

[shawndx]

@marigonzes Thanks for locating the culprit commit, I'm working on a fix.

[shawndx]

Hi @ALTree I'm trying to figure out more testing scenarios, could you please shed a light what kind of code snippet would trigger the rewriting? I tried some expressions like "if const_function1() <OP> const_function2()" but no lucky. Thanks.

[ALTree]

@shawn-xdji Sorry, I'm afraid I can't help. In general it can be quite hard to come up with a Go program that will trigger a specific SSA rule (unless the rule is a very peculiar one). Maybe Cherry or Keith have a better insight.

[shawndx]

The simplest one I could figure out is

package main
func f1() {
        const c = 1
        var a = 0
        var v *int = &a
        if *v-c < len([]int{}) {
        } else {
                panic("bad")
        }
}

By adjusting 'c', '*v' and the operator I think all concerned rules could be tested, not sure if it could be simplified further. I will install them to test/fixedbugs along with the fix.

[gopherbot]

Change https://golang.org/cl/237860 mentions this issue: cmd/compile: handle constant flags for 'noov' Ops

[gopherbot]

Change https://golang.org/cl/238077 mentions this issue: cmd/compile: redo flag constant ops

[gopherbot]

Change https://golang.org/cl/237999 mentions this issue: cmd/compile: redo flag constant ops for arm64

[gopherbot]

Change https://golang.org/cl/238677 mentions this issue: cmd/compile: Install testcases for flag constant Ops