Re: [go-nuts] testing whether two maps are the same object

['Axel Wagner' via golang-nuts]

On Tue, Jul 18, 2023 at 5:22 PM Jochen Voss  wrote:

> Dear Jason,
>
> Thanks for the idea of using reflect.  Do you know whether
> `reflect.ValueOf(a).UnsafePointer()` is somehow "safer" than
> `*(*uintptr)(unsafe.Pointer())`?
>

For what it's worth: I'd argue comparing
`reflect.ValueOf(a).UnsafePointer()` provides stronger guarantees of
portability and correctness than comparing
`*(*uintptr)(unsafe.Pointer())`. Also, you should probably use
`*(*unsafe.Pointer)(unsafe.Pointer())` if anything. It does the same
thing, when it works, but I'm not sure that the conversion to `uintptr`
that is happening is guaranteed to work under a moving GC. A comparison
isn't necessarily arithmetic  - and
those rules are just what `gc` is specifying anyways, an implementation is
free to choose others, in theory.

That's because the latter relies on maps being "pointer-shaped". In theory,
an implementation might not represent a map as a plain pointer. Though I
admittedly having trouble coming up with an alternative design.

Plus, the documentation  of
`reflect.Value.UnsafePointer` *also* doesn't really say *what* the returned
pointer is for a map and what its semantics are. But it seems somewhat
safer to assume that an implementation that would *not* represent a map as
a plain pointer, would implement `reflect.Value.UnsafePointer` in a way
that still allowed you to compare them for identity purposes.

But, as you might be able to tell, all of this is a bit of a nitpick of how
willing you are to make assumptions about reasonable ways an implementation
might deviate from what they *have* to do. I'm pretty sure there is no way
to compare map values that is keeping *strictly* in the bounds of
well-defined behavior. I would, personally, feel comfortable using either
and to just assume that I never have to run my code in a sufficiently wild
implementation. And I'd probably try to benchmark/test if the `reflect`
based version changes performance characteristics (I would assume it makes
it easier for the map to escape, but maybe maps *always* escape anyways)
and if not, use `reflect`.


> reflect.DeepEqual will not work, since it just compares contents of the
> maps: https://go.dev/play/p/tE_qZI2cKEd
>
> All the best,
> Jochen
> On Tuesday, 18 July 2023 at 15:52:24 UTC+1 Jason Phillips wrote:
>
>> You can also use the reflect package rather than (directly) reaching for
>> unsafe. The reflect.DeepEqual function does something along the lines of:
>> https://go.dev/play/p/IVt0Z-mxugh
>>
>> On Tuesday, July 18, 2023 at 10:45:18 AM UTC-4 Jan Mercl wrote:
>>
>>> On Tue, Jul 18, 2023 at 4:35 PM Jochen Voss  wrote:
>>>
>>> > Is there a better way?
>>>
>>> I have never been here and please don't do this:
>>> https://go.dev/play/p/x4QYJubXMnQ
>>>
>> --
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> 
> .
>

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Re: [go-nuts] testing whether two maps are the same object

[Jochen Voss]

Dear Jason,

Thanks for the idea of using reflect.  Do you know whether 
`reflect.ValueOf(a).UnsafePointer()` is somehow "safer" than 
`*(*uintptr)(unsafe.Pointer())`?

reflect.DeepEqual will not work, since it just compares contents of the 
maps: https://go.dev/play/p/tE_qZI2cKEd

All the best,
Jochen
On Tuesday, 18 July 2023 at 15:52:24 UTC+1 Jason Phillips wrote:

> You can also use the reflect package rather than (directly) reaching for 
> unsafe. The reflect.DeepEqual function does something along the lines of: 
> https://go.dev/play/p/IVt0Z-mxugh
>
> On Tuesday, July 18, 2023 at 10:45:18 AM UTC-4 Jan Mercl wrote:
>
>> On Tue, Jul 18, 2023 at 4:35 PM Jochen Voss  wrote: 
>>
>> > Is there a better way? 
>>
>> I have never been here and please don't do this: 
>> https://go.dev/play/p/x4QYJubXMnQ 
>>
>

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Re: [go-nuts] testing whether two maps are the same object

[Jason Phillips]

You can also use the reflect package rather than (directly) reaching for 
unsafe. The reflect.DeepEqual function does something along the lines 
of: https://go.dev/play/p/IVt0Z-mxugh

On Tuesday, July 18, 2023 at 10:45:18 AM UTC-4 Jan Mercl wrote:

> On Tue, Jul 18, 2023 at 4:35 PM Jochen Voss  wrote:
>
> > Is there a better way?
>
> I have never been here and please don't do this:
> https://go.dev/play/p/x4QYJubXMnQ
>

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Re: [go-nuts] testing whether two maps are the same object

[Jan Mercl]

On Tue, Jul 18, 2023 at 4:35 PM Jochen Voss  wrote:

> Is there a better way?

I have never been here and please don't do this:
https://go.dev/play/p/x4QYJubXMnQ

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[go-nuts] testing whether two maps are the same object

[Jochen Voss]

Dear all,

To implement the "eq" operator in a simple PostScript interpreter, I need 
to determine whether two maps are the same object.

This can be done by adding a new element to one map, and checking whether 
the new entry also appears in the second map, but as you can imagine, the 
resulting code is quite ugly.  See https://go.dev/play/p/AfembYDt3en for an 
implementation of this idea.

Is there a better way?

Many thanks,
Jochen

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