cmd/compile: Export of FOR (and FORUNTIL) assumes ExprOrNil for OFOR.Right, but it is StmtOrNil

[dr2chase]

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

1.11dev

What did you do?

I enabled inlining for OFOR nodes, which caused the export of an OFOR node, which failed.

<autogenerated>:1: internal compiler error: cannot export = (20) node
==> please file an issue and assign to gri@

goroutine 1 [running]:
runtime/debug.Stack(0x0, 0x0, 0x0)
	/usr/local/google/home/drchase/work/go-ssa/src/runtime/debug/stack.go:24 +0xa7
cmd/compile/internal/gc.Fatalf(0xbe0f4c, 0x47, 0xc0068091f8, 0x2, 0x2)
	/usr/local/google/home/drchase/work/go-ssa/src/cmd/compile/internal/gc/subr.go:182 +0x1f7
cmd/compile/internal/gc.(*exportWriter).expr(0xc0059fa900, 0xc00219e900)
	/usr/local/google/home/drchase/work/go-ssa/src/cmd/compile/internal/gc/iexport.go:1319 +0x2a0
cmd/compile/internal/gc.(*exportWriter).exprsOrNil(0xc0059fa900, 0xc00219e880, 0xc00219e900)
	/usr/local/google/home/drchase/work/go-ssa/src/cmd/compile/internal/gc/iexport.go:1341 +0x8d
cmd/compile/internal/gc.(*exportWriter).stmt(0xc0059fa900, 0xc00219e800)
	/usr/local/google/home/drchase/work/go-ssa/src/cmd/compile/internal/gc/iexport.go:1053 +0x544
cmd/compile/internal/gc.(*exportWriter).stmtList(0xc0059fa900, 0xc006809318)
	/usr/local/google/home/drchase/work/go-ssa/src/cmd/compile/internal/gc/iexport.go:974 +0xa6
cmd/compile/internal/gc.(*iexporter).doInline(0xc005f1c000, 0xc00046f1d0)
	/usr/local/google/home/drchase/work/go-ssa/src/cmd/compile/internal/gc/iexport.go:505 +0xb4
cmd/compile/internal/gc.(*exportWriter).funcExt(0xc0059fa870, 0xc00046f1d0)
	/usr/local/google/home/drchase/work/go-ssa/src/cmd/compile/internal/gc/iexport.go:953 +0x144
cmd/compile/internal/gc.(*iexporter).doDecl(0xc005f1c000, 0xc00046f1d0)
	/usr/local/google/home/drchase/work/go-ssa/src/cmd/compile/internal/gc/iexport.go:434 +0x136
cmd/compile/internal/gc.iexport(0xc0055a2000)
	/usr/local/google/home/drchase/work/go-ssa/src/cmd/compile/internal/gc/iexport.go:276 +0x38d
cmd/compile/internal/gc.dumpexport(0xc004606000)
	/usr/local/google/home/drchase/work/go-ssa/src/cmd/compile/internal/gc/export.go:79 +0x8f
cmd/compile/internal/gc.dumpCompilerObj(0xc004606000)
	/usr/local/google/home/drchase/work/go-ssa/src/cmd/compile/internal/gc/obj.go:119 +0x39
cmd/compile/internal/gc.dumpobj1(0x7fffbe08a3cf, 0x23, 0x3)
	/usr/local/google/home/drchase/work/go-ssa/src/cmd/compile/internal/gc/obj.go:70 +0x15b
cmd/compile/internal/gc.dumpobj()
	/usr/local/google/home/drchase/work/go-ssa/src/cmd/compile/internal/gc/obj.go:51 +0x60
cmd/compile/internal/gc.Main(0xbe3d58)
	/usr/local/google/home/drchase/work/go-ssa/src/cmd/compile/internal/gc/main.go:684 +0x277c
main.main()
	/usr/local/google/home/drchase/work/go-ssa/src/cmd/compile/main.go:49 +0x96

Root cause is

	case OFOR:
...
		w.exprsOrNil(n.Left, n.Right) 
...

because n.Right is the increment of the for loop:

	OFOR      // for Ninit; Left; Right { Nbody }

i.e., n.Right is a statement, not an expression, and may be nil.

[dr2chase]

@gri, @mdempsky could you give this a look, if it is easy and low risk it would help with inlining for loops in 1.11, requested in #14768.

[griesemer]

This shouldn't be too hard; the current code simply makes a wrong assumption for OFOR.

@dr2chase Do you have a complete reproducible example for me so I can test this?

[dr2chase]

Requires a slightly tweaked compiler so it will attempt to inline OFOR, will put together that CL. I'll just reconnect all the inlining knobs, and add one for OFOR, we'll need this anyway to work on heuristics.

[gopherbot]

Change https://golang.org/cl/118996 mentions this issue: cmd/compile: add some inliner knobs for parameter search

[dr2chase]

There you go, a new CL for inlining knobs, plus a reproducer (except this one complains about a different node, 26):

cat > main.go <<EOF
package main

func Foo(x int) int {
	a := 0
	for i := 0; i < x; i++ {
		a += a*a + i
	}
	return a
}

func main() {
	for i := 0; i < 7; i++ {
		println("Foo(", i, ")=", Foo(i))
	}
}
EOF
go build -gcflags -d=env=INLLOG=1:INLFOREXTRA=1 main.go

With this set of flags, it fails with the original error (node 20) somewhere compiling runtime

go build -gcflags all=-d=env=INLLOG=1:INLFOREXTRA=1 main.go

[gopherbot]

Change https://golang.org/cl/119595 mentions this issue: cmd/compile: fix exporting of 'for' loops