Normalization of method selectors
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Normalization of method selectors
Go allows simplified forms of some selectors.
For example, in the following program, t1.M1
is a simplified form of (*t1).M1
,
and t2.M2
is a simplified form of (&t2).M2
. At compile time, the compiler will normalize the simplified forms to their original respective full forms.
The following program prints 0
and 9
, because the modification to t1.X
has no effects on the evaluation result of *t1
during evaluating (*t1).M1
.
package main
type T struct {
X int
}
func (t T) M1() int {
return t.X
}
func (t *T) M2() int {
return t.X
}
func main() {
var t1 = new(T)
var f1 = t1.M1 // <=> (*t1).M1
t1.X = 9
println(f1()) // 0
var t2 T
var f2 = t2.M2 // <=> (&t2).M2
t2.X = 9
println(f2()) // 9
}
In the following code, the function foo
runs okay, but the function bar
will produce a panic. The reason is s.M
is a simplified form of (*s.T).M
. At compile time, the compiler will normalize the simplified form to it original full form. At runtime, if s.T
is nil, then the evaluation of *s.T
will cause a panic. The two modifications to s.T
have no effects on the evaluation result of *s.T
.
package main
type T struct {
X int
}
func (t T) M() int {
return t.X
}
type S struct {
*T
}
func foo() {
var s = S{T: new(T)}
var f = s.M // <=> (*s.T).M
s.T = nil
f()
}
func bar() {
var s S
var f = s.M // panic
s.T = new(T)
f()
}
func main() {
foo()
bar()
}
Please note that, interface method values and method values got through reflection
will be expanded to the promoted method values with a delay.
For example, in the following program, the modification to s.T.X
has effects on the return results of the method values got through reflection and interface ways.
package main
import "reflect"
type T struct {
X int
}
func (t T) M() int {
return t.X
}
type S struct {
*T
}
func main() {
var s = S{T: new(T)}
var f = s.M // <=> (*s.T).M
var g = reflect.ValueOf(&s).Elem().
MethodByName("M").
Interface().(func() int)
var h = interface{M() int}(s).M
s.T.X = 3
println( f() ) // 0
println( g() ) // 3
println( h() ) // 3
}
Source: https://github.com/golang/go/issues/47863
However, as of Go toolchain 1.18, there is a bug in the official standard
Go compiler implementation. The compiler will de-virtualize some interface
methods at compile time but the de-virtualizations are made too far
to be correct. For example, the following program should print 2 2
,
but it prints 1 2
.
package main
type I interface{ M() }
type T struct{
x int
}
func (t T) M() {
println(t.x)
}
func main() {
var t = &T{x: 1}
var i I = t
var f = i.M
defer f() // 2 (correct)
// i.M is de-virtualized as (*t).M at compile time (bug).
defer i.M() // 1 (wrong)
t.x = 2
}
It is unclear when this bug will be fixed now.
Source: https://github.com/golang/go/issues/52072
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