http://d.puremagic.com/issues/show_bug.cgi?id=9975

           Summary: pointsTo asserts because of false pointer in union
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: code@dawg.eu


--- Comment #0 from Martin Nowak <code@dawg.eu> 2013-04-21 05:42:44 PDT ---
import std.exception;

struct S
{
    union
    {
        size_t sz;
        string s;
    }
}

void main()
{
    S a, b;
    a.sz = -1;
    assert(!pointsTo(a, b));
}

----

The problem is that pointsTo checks every .tupleof member thus the example asserts because a.s pointsTo b.

http://www.digitalmars.com/d/archives/digitalmars/D/learn/overzealous_pointsTo_23203.html

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monarchdodra@gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |monarchdodra@gmail.com


--- Comment #1 from monarchdodra@gmail.com 2013-04-24 14:20:12 PDT ---
But isn't this the expected behavior though?

By setting sz to -1, s becomes a string (dynamic array), that starts at null, and spans size_t.max. So in that sense, b is inside a.sz, so a points to b.

Looking at the provided link, where it shows that non anonymous unions don't show this "problem", I'm really tempted to say it actually reveils that pointsTo *doesn't* support unions correctly, as *both* asserts should have failed...

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--- Comment #2 from monarchdodra@gmail.com 2013-04-26 08:44:36 PDT ---
https://github.com/D-Programming-Language/phobos/pull/1277

Fix Issue 9975 - pointsTo asserts because of false pointer in union

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--- Comment #3 from github-bugzilla@puremagic.com 2013-05-25 22:00:52 PDT ---
Commit pushed to master at https://github.com/D-Programming-Language/dmd

https://github.com/D-Programming-Language/dmd/commit/7518f83d0082e5cd339ef5d7c215f13a0287ad4b Do not include test case for issue 9975

It's not behavior that we guarantee.

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--- Comment #4 from Kenji Hara <k.hara.pg@gmail.com> 2013-05-25 22:12:10 PDT ---
(In reply to comment #3)
> Commit pushed to master at https://github.com/D-Programming-Language/dmd
> 
> https://github.com/D-Programming-Language/dmd/commit/7518f83d0082e5cd339ef5d7c215f13a0287ad4b Do not include test case for issue 9975
> 
> It's not behavior that we guarantee.

The commit is not for this issue. Please ignore it.

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--- Comment #5 from github-bugzilla@puremagic.com 2013-06-16 13:51:37 PDT ---
Commits pushed to master at https://github.com/D-Programming-Language/phobos

https://github.com/D-Programming-Language/phobos/commit/e44adcd9edfd025fa7074501e1743e54ecbbda33 Fix Issue 9975 - pointsTo asserts because of false pointer in union

https://github.com/D-Programming-Language/phobos/commit/af14a7f0aad058587fa498bbcdfd0ad84c204deb Merge pull request #1277 from monarchdodra/9975

Fix Issue 9975 - pointsTo asserts because of false pointer in union

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--- Comment #6 from Martin Nowak <code@dawg.eu> 2013-06-17 16:08:14 PDT ---
This function is used throughout std.algorithm so marking this as documented behavior is not a good solution because it mean basic algorithms like sort can't be used with such structs.

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--- Comment #7 from monarchdodra@gmail.com 2013-06-17 23:44:57 PDT ---
(In reply to comment #6)
> This function is used throughout std.algorithm so marking this as documented behavior is not a good solution because it mean basic algorithms like sort can't be used with such structs.

Which behavior exactly are you mentioning? And do you mean *any* structs, or just structs containing unions?

As far as documentation goes, all I added was that when doing pointsTo on a union, you "could" get a false positive. I don't think there is any way around it, since pointsTo would have no way to know *which* members to check.

That, and a false positive is always safer than a false negative.

In case of the original example in this bug, it is not tested nor documented.

Could you explain in more detail what you think is wrong and/or what should be changed?

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--- Comment #8 from Martin Nowak <code@dawg.eu> 2013-06-18 01:53:05 PDT ---
(In reply to comment #7)
> (In reply to comment #6)
> > This function is used throughout std.algorithm so marking this as documented behavior is not a good solution because it mean basic algorithms like sort can't be used with such structs.
> 
> Which behavior exactly are you mentioning? And do you mean *any* structs, or just structs containing unions?
> 
Returning false positives.
Any struct that might trigger false positives on pointsTo.

It's really a serious issue, I stumbled over this while sorting JSON values.
I don't think we can afford to say that sort or swap can't be used with
algebraic types because they trigger false positive assertions.
Of course this is mainly an issue with swap so we need to look at both sides.

> Could you explain in more detail what you think is wrong and/or what should be
changed?

Maybe it's possible to mitigate the problem by reliably identifying that
something can't be a valid pointer.
But to really fix the swap issue we could make this configurable, i.e. either
add an `ignoreFalsePointer` parameter or return a tri-state result. Then swap
could at least work for most cases.

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--- Comment #9 from monarchdodra@gmail.com 2013-06-18 02:48:36 PDT ---
(In reply to comment #8)
> (In reply to comment #7)
> > (In reply to comment #6)
> > > This function is used throughout std.algorithm so marking this as documented behavior is not a good solution because it mean basic algorithms like sort can't be used with such structs.
> > 
> > Which behavior exactly are you mentioning? And do you mean *any* structs, or just structs containing unions?
> > 
> Returning false positives.
> Any struct that might trigger false positives on pointsTo.

It'd be even worst to have a false negative, IMO. Of course, the best would be to not have any "false".

> It's really a serious issue, I stumbled over this while sorting JSON values.
> I don't think we can afford to say that sort or swap can't be used with
> algebraic types because they trigger false positive assertions.
> Of course this is mainly an issue with swap so we need to look at both sides.

Just to be clear, algebraic types don't trigger false positives.

The only things that answer true to pointsTo are true pointer types (including slices and classes). Everything other basic type is statically known not to be a pointer type. Basically, points to knows what is pointer type, and what isn't. There are (AFAIK) no "false pointers"

The problem really only appears once unions get into the way.

If a union contains an int, and a pointer (which is basically the example provided), then there is literally no way to know which of the union members should be taken into account. The union legitimately contains both the states pointsTo and !pointsTo :/ That's where the problem is. Given that pointsTo anser true as soon as at least a member pointsTo something, the reasonable answer to give in case of dual state is true...

It *is* a problem, but I don't think it is as big a problem as you think it is (or at least, not in your wording of "sort can't be used with structs").

A tristate answer might be the answer. Given that pointsTo is mostly only ever used in assertive conditions, we could assert only if pointsTo is true. If pointsTo is "maybe", the swap can be more lenient and go ahead anyways...?

We'd need to have a tribool type in phobos first though, if we want to do this in a (mostly) non breaking way.

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