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pointers to slice elements in a []byte("string") are != #1569

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thehowl opened this issue Jan 23, 2024 · 0 comments · Fixed by #1601
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pointers to slice elements in a []byte("string") are != #1569

thehowl opened this issue Jan 23, 2024 · 0 comments · Fixed by #1601
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🐞 bug Something isn't working 📦 🤖 gnovm Issues or PRs gnovm related

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@thehowl
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thehowl commented Jan 23, 2024

Before you ask: yes, this is specifically byte slices which are created as a result of string conversion.

package main

func main() {
	{
		b := []byte("ABCDEFGHIJKL")
		a := b
		println(&b[0] == &a[0], b[0], a[0])

		// modifying the underlying array modifies both a[0] and b[0],
		// as it should
		a[0] = 11
		println(a[0], b[0])
	}

	{
		b := []byte{1, 2, 3}
		a := b
		println(&b[0] == &a[0], b[0], a[0])

		// modifying the underlying array modifies both a[0] and b[0],
		// as it should
		a[0] = 11
		println(a[0], b[0])
	}
}

// Output:
// false 65 65
// 11 11
// true 1 1
// 11 11
@thehowl thehowl added 🐞 bug Something isn't working 📦 🤖 gnovm Issues or PRs gnovm related labels Jan 23, 2024
@thehowl thehowl added this to the 🚀 main.gno.land (required) milestone Jan 23, 2024
@thehowl thehowl assigned thehowl and unassigned thehowl Jan 23, 2024
@thehowl thehowl changed the title pointers to slice elements in a byte slice are != pointers to slice elements in a []byte("string") are != Jan 23, 2024
@ltzmaxwell ltzmaxwell self-assigned this Jan 29, 2024
@deelawn deelawn linked a pull request Feb 14, 2024 that will close this issue
petar-dambovaliev pushed a commit that referenced this issue Apr 9, 2024
This is a fix to #1569 . Thanks to @thehowl for identifying this issue
and providing a thorough and insightful analysis.

Here is an further analysis based on #1569 :

```go
package main

func main() {
	{
		b := []byte("ABCDEFGHIJKL")
		a := b
		println(&a[0] == &a[0])
	}
}
```
this comparison would be false too.


The root cause for this is the way pointer values is obtained and
compared, e.g.
```go
package main

func main() {
	c := []byte{'A'}
	a := c
	println(&c[0])
	println(&c[0] == &a[0])
}
```

in this code snippet, the address of the c[0], (&c[0]) is obtained from
this:
```go
    ev := fillValueTV(store, &av.List[ii]) // by reference
```
that is a reference to the element of the underlying list of arrayValue,

in case like this:
```go
package main

func main() {
	{
		b := []byte("ABCDEFGHIJKL")
		a := b
		println(&a[0] == &a[0])
	}
}
```
the address is obtained by
```go
	bv := &TypedValue{ // heap alloc, so need to compare value rather than pointer in isEql(), line 482
		T: DataByteType,
		V: DataByteValue{
			Base:     av,
			Index:    ii,
			ElemType: et,
		},
	}
```
that is a new allocated *TV, which implies the value of it would not be
same within the first and second & operation.
So we should actually compare the concrete value rather than the
pointers for this case.
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