Fix Fetch and OpDeref

[a-peyrard]

First commit is fixing some issue with runtime.Deref which is called by OpDeref, pointer of interface were not de-referenced.
Second commit is creating a fix for #466. This is de-referencing everything in Fetch before fetching the attribute.
As discussed in the issue, one other approach could be to ensure we have a OpDeref operation added during the compilation phase.

[a-peyrard]

I will have a look tomorrow to see if I can understand where I would need to modify the code to add the OpDeref operation.
I would also like to check if the modifications are not impacting the overall performance as the two modifications are for functions used during runtime.

[antonmedv]

Results looks promising.

[antonmedv]

Looks like we going to loose reflect.Indirect here.

[antonmedv]

Here is ooriginal change: 6f0c855

[a-peyrard]

We are still doing the "indirect" in the deref function as we are calling Elem() of the value is a pointer.
Which should be doing the same as the Indirect function, which is also doing Elem() behind the scene:

func Indirect(v Value) Value {
	if v.Kind() != Pointer {
		return v
	}
	return v.Elem()
}

[a-peyrard]

I just ran the benchrmak, it seems we are in the same order of magnitude:
before:

Benchmark_largeStructAccess-12           1000000               134.4 ns/op

after:

Benchmark_largeStructAccess-12           1000000               132.2 ns/op

[antonmedv]

Cool! let me check it a bit more as runtime changes should be done very carefully.

[a-peyrard]

I was trying to figure out where we would need to add an OpDeref operation. I was thinking to add one there:
https://github.com/antonmedv/expr/blob/709c5dd55aa7cfc1b47fc67ee32ac15af6db967a/compiler/compiler.go#L590
like that:

if op == OpFetch {
		c.derefInNeeded(base)
		c.compile(node.Property)
		c.emit(OpFetch)
	}

This is fixing the problem when we try to access a field, but introducing a bug when fetch is used to get a method. This test for example is failing: TestDeref_method_on_int_pointer as the element has been dereferenced and then doesn't contain any method named Bar.

Any idea where I should add a OpDeref, as Fetch is used to fetch fields and methods, it might be a bit tricky w/o splitting the memberNode into two separate structures, one to retrieve methods, and one to retrieve fields.

The other solution would be to not add any OpDeref and just do the deref if needed during the Fetch (as implemented in this PR)

[antonmedv]

The other solution would be to not add any OpDeref and just do the deref if needed during the Fetch (as implemented in this PR)

Let's use this for now.

[antonmedv]

The other solution would be to not add any OpDeref and just do the deref if needed during the Fetch (as implemented in this PR)

Lets use this for now.