gccgo: dereference of nil pointer to zero-width type does not panic

[mdempsky]

The Go spec says "For an operand x of pointer type *T, the pointer indirection *x denotes the variable of type T pointed to by x. If x is nil, an attempt to evaluate *x will cause a run-time panic."

This program correctly panics with cmd/compile, but does not with gccgo 8.0.0:

package main

func main() {
	var p *struct{}
	*p = *p
}

/cc @ianlancetaylor

[gopherbot]

Change https://golang.org/cl/170013 mentions this issue: compiler: require nil check when dereferecing a pointer to zero-sized type

[cherrymui]

The problem seems not specific to zero-sized type.

package main

func main() {
	var p *struct{ x int }
	*p = *p
}

This also does not panic. I think as long as the type is not big (>4096 bytes), if the backend somehow optimizes the dereference away, it will not panic.

[ianlancetaylor]

I think you're right. But I don't particularly want to force a nil check for all pointer dereferences. In principle we could do it only if we think the backend might eliminate the dereference.

[gopherbot]

Change https://golang.org/cl/176459 mentions this issue: compiler: improve escape analysis on interface conversions

[gopherbot]

Change https://golang.org/cl/176579 mentions this issue: test: use a real use function in nilptr2.go