http://d.puremagic.com/issues/show_bug.cgi?id=5540

           Summary: Probable bug-hiding redundancies
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: bearophile_hugs@eml.cc


--- Comment #0 from bearophile_hugs@eml.cc 2011-02-07 12:33:29 PST ---
This is a real bug in good C++ code:

if ((p_newHeader.frame_num != m_lastSlice.frame_num) ||
    (p_newHeader.pic_parameter_set_id !=
     p_newHeader.pic_parameter_set_id) ||
    (p_newHeader.field_pic_flag != p_newHeader.field_pic_flag) ||
    (p_newHeader.bottom_field_flag != m_lastSlice.bottom_field_flag)
    ){
    return false;
}


The bug is of the kind:
if (x != x) {}

Two other similar bugs:
if (x == x) {}

auto y = x - x;


For the compiler it's easy to spot such three situations. They are correct code, yet experience shows that often such redundancies hide bugs. So I suggest to add to DMD warnings for such tree situations.

See also bug 3878

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--- Comment #1 from bearophile_hugs@eml.cc 2011-02-07 15:05:52 PST ---
Another case, in good C++ code (probably caused by not complete copy & paste &
modify):

return Contains(Sphere(lss.mP0, lss.mRadius)) &&
       Contains(Sphere(lss.mP0, lss.mRadius));

That is of this kind:
x && x

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--- Comment #2 from bearophile_hugs@eml.cc 2011-04-07 15:08:11 PDT ---
Reference:
http://www.viva64.com/en/a/0068/

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--- Comment #3 from bearophile_hugs@eml.cc 2011-06-19 16:38:35 PDT ---
See a very nice example of bug of this kind, that's easy to find for a compiler or lint:

http://www.digitalmars.com/webnews/newsgroups.php?art_group=digitalmars.D&article_id=139021

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--- Comment #4 from bearophile_hugs@eml.cc 2011-06-19 16:44:30 PDT ---
(In reply to comment #3)
> See a very nice example of bug of this kind, that's easy to find for a compiler or lint:
> 
> http://www.digitalmars.com/webnews/newsgroups.php?art_group=digitalmars.D&article_id=139021

The bug in Andrej Mitrovic code was:

if (mmioRead(hmmio, cast(LPSTR)&drum, DRUM.sizeof != DRUM.sizeof))
{
    mmioClose(hmmio, 0);
    return szErrorCannotRead;
}

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--- Comment #5 from bearophile_hugs@eml.cc 2011-06-29 13:24:48 PDT ---
See also:
http://www.viva64.com/en/b/0103/
http://www.viva64.com/en/d/0090/

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--- Comment #6 from bearophile_hugs@eml.cc 2011-06-29 16:56:41 PDT ---
A class of bug-hiding redundancy:

if (x == 10)
    foo1();
else if (x == 20)
    foo2();
else if (x == 10) // ***
    foo3();


Another class, two assignments to the same variable in a row:

x = foo();
x = bar();


While this is OK:

x = 1;
x = x + 1;
x = foo(x);


Another class (the two branches are equal):

if (x)
    foo();
else
    foo();

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