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digitalmars.D.bugs - [Issue 11287] New: NRVO should remove dtor call completely

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=11287

           Summary: NRVO should remove dtor call completely
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Keywords: rejects-valid
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: k.hara.pg gmail.com



This is similar to bug 11286, but slightly different.

Test case:

struct A
{
    ~this() {}
}

A getA() pure
{
    A a;        // line 7
    return a;   // NRVO
}

void main()
{
    A a = getA();
}

Output:

test.d(7): Error: pure function 'test.getA' cannot call impure function
'test.A.~this'

In the function getA(), the variable 'a' will be moved out to the caller by
NRVO, and its dtor won't be called. So compiler should accept the case
properly.

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next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=11287


monarchdodra gmail.com changed:

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



Question:

Doesn't this mean that the compiler *must* implement NRVO? I thought NRVO was
an optimization *opportunity*?

If
//----
A getA() pure
{
    A a;        // line 7
    return a;   // NRVO
}
//----

Is legal, then it means that only compilers hat implement NRVO will handle
this.

Sorry if it's a stupid question.

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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 Question:
 
 Doesn't this mean that the compiler *must* implement NRVO? I thought NRVO was
 an optimization *opportunity*?
 
 If
 //----
 A getA() pure
 {
     A a;        // line 7
     return a;   // NRVO
 }
 //----
 
 Is legal, then it means that only compilers hat implement NRVO will handle
 this.
 
 Sorry if it's a stupid question.


Unfortunately current D language spec does not mention about NRVO, so It's
still one of the optimizations. But, at least, current dmd always apply NRVO
for the specific test case.

And, in D we can check the semantic analysis result by using is(typeof()) and
__traits(compiles). I believe that "if these compile-time checker primitives
return true, it should mean that the specific D code will generate correct
runtime code, and vice versa". For example, since "cannot access frame pointer"
error had not been able to detect in compile-time, but now it can.

Back to the case, dmd could generate correct binary for the code by using NRVO.
So dmd should not cause "cannot call impure function" error.

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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Max Samukha <samukha voliacable.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |samukha voliacable.com



PDT ---



 Unfortunately current D language spec does not mention about NRVO, so It's
 still one of the optimizations. But, at least, current dmd always apply NRVO
 for the specific test case.


How are the standard library's constructs such as "scoped" supposed to work if
NRVO is optional?

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prev sibling parent d-bugmail puremagic.com writes:
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 Unfortunately current D language spec does not mention about NRVO, so It's
 still one of the optimizations. But, at least, current dmd always apply NRVO
 for the specific test case.


 
 How are the standard library's constructs such as "scoped" supposed to work if
 NRVO is optional?


Yes. std.typecons.scoped requires NRVO in order to implement its own semantic.
That's why NRVO should be mentioned properly in language spec.

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