http://d.puremagic.com/issues/show_bug.cgi?id=3632

           Summary: modify float is float to do a bitwise compare
           Product: D
           Version: unspecified
          Platform: Other
        OS/Version: Linux
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: schveiguy@yahoo.com


--- Comment #0 from Steven Schveighoffer <schveiguy@yahoo.com> 2009-12-18 12:04:05 PST ---
I was trying to test that my array append code is working properly.  In order to do this, I want to verify that when I set the length to a larger number that the additional space is filled with T.init

This test works great for most all types except floating point:

assert(a[$-1] is T.init)

It was pointed out that Phobos has isIdentical, but that doesn't work with literals for some reason:

float a;
assert(isIdentical(a, float.init)) // fails!

But it shouldn't be this hard.  Why does 'is' do a bitwise compare for everything *except* floating point numbers?  I understand that the spec states that is converts to == for builtin types, but it seems inconsistent because == for all builtin types except floating point *is* a bitwise compare.  It seems there is no easy way to get a bitwise compare for floating points, and the method provided doesn't work properly.  It should be braindead simple to get the compiler to do a bitwise compare.

I propose that for floating point types, x is y be equivalent to a bitwise compare.  It is easy to say in the spec "x is y does a bitwise compare, which for all builtin types except for floating point types is equivalent to equality"

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Don <clugdbug@yahoo.com.au> changed:

           What    |Removed                     |Added
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                 CC|                            |clugdbug@yahoo.com.au


--- Comment #1 from Don <clugdbug@yahoo.com.au> 2009-12-18 22:49:41 PST ---
(In reply to comment #0)
> It was pointed out that Phobos has isIdentical, but that doesn't work with literals for some reason:
> 
> float a;
> assert(isIdentical(a, float.init)) // fails!

That's because of the recently discovered NaN bug; the code generated for uninitialized floats and doubles needs to change. It's not a problem with isIdentical.

> But it shouldn't be this hard.  Why does 'is' do a bitwise compare for everything *except* floating point numbers?

> I propose that for floating point types, x is y be equivalent to a bitwise compare.  It is easy to say in the spec "x is y does a bitwise compare, which for all builtin types except for floating point types is equivalent to equality"

I think this would be a good idea. I've thought that several times myself. I created the isIdentical() function because I think there's a need for it.

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yebblies <yebblies@gmail.com> changed:

           What    |Removed                     |Added
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           Keywords|                            |patch
                 CC|                            |yebblies@gmail.com


--- Comment #2 from yebblies <yebblies@gmail.com> 2011-06-14 23:50:06 PDT ---
Possible implementation. https://github.com/D-Programming-Language/dmd/pull/126

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Walter Bright <bugzilla@digitalmars.com> changed:

           What    |Removed                     |Added
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             Status|NEW                         |RESOLVED
                 CC|                            |bugzilla@digitalmars.com
         Resolution|                            |FIXED


--- Comment #3 from Walter Bright <bugzilla@digitalmars.com> 2011-06-25 23:20:33 PDT ---
https://github.com/D-Programming-Language/dmd/commit/0d93cf4333df6e167f9027eb1a4b0aa9a940ff19

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bearophile_hugs@eml.cc changed:

           What    |Removed                     |Added
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                 CC|                            |bearophile_hugs@eml.cc


--- Comment #4 from bearophile_hugs@eml.cc 2011-06-26 00:49:51 PDT ---
Do you know what's missing in the list of bug 3981 ?

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kennytm@gmail.com changed:

           What    |Removed                     |Added
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             Status|RESOLVED                    |REOPENED
                 CC|                            |kennytm@gmail.com
         Resolution|FIXED                       |


--- Comment #5 from kennytm@gmail.com 2011-06-26 01:37:29 PDT ---
This is *not yet fixed*. The current implementation in DMD


           real_t v1 = e1->toReal();
           real_t v2 = e2->toReal();
           cmp = !memcmp(&v1, &v2, sizeof(real_t));


will not work, at least on OS X, because while 'real_t' ('long double') is only
a 80-bit floating point number (occupying 10 bytes), with alignment
'sizeof(real_t)' will take 16 bytes. The extra 6 bytes of paddings are often
filled with garbage. This makes even

       4.0 is 4.0

to return 'false'.

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kennytm@gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|REOPENED                    |RESOLVED
         Resolution|                            |FIXED


--- Comment #6 from kennytm@gmail.com 2011-06-27 07:21:00 PDT ---
https://github.com/D-Programming-Language/dmd/commit/32740

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yebblies <yebblies@gmail.com> changed:

           What    |Removed                     |Added
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             Status|RESOLVED                    |REOPENED
         Resolution|FIXED                       |


--- Comment #7 from yebblies <yebblies@gmail.com> 2011-06-27 08:19:40 PDT ---
Reopening as the commit above will cause the following assert to fail: static assert(real.init !is real.nan);

And the padding issue has only been fixed for the compile time evaluation, not runtime.

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--- Comment #8 from Walter Bright <bugzilla@digitalmars.com> 2011-06-27 11:32:17 PDT ---
(In reply to comment #7)
> Reopening as the commit above will cause the following assert to fail: static assert(real.init !is real.nan);

This is intended. All nans are regarded as the same (even signalling and
non-signalling).

> And the padding issue has only been fixed for the compile time evaluation, not runtime.

You're right.

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--- Comment #9 from bearophile_hugs@eml.cc 2011-06-27 13:33:12 PDT ---
(In reply to comment #8)

> This is intended. All nans are regarded as the same (even signalling and
> non-signalling).

So we have to use std.math.isIdentical() to tell apart floating point values on the base of the their bit patterns.

Related: I think almost no one uses the NaN payloads because (beside being a niche need) almost no language gives easy and explicit support to manage those payloads (while in std.math there are functions like getNaNPayload).

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