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digitalmars.D.bugs - [Issue 4397] New: D1 const does not work

reply d-bugmail puremagic.com writes:
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           Summary: D1 const does not work
           Product: D
           Version: D1 & D2
          Platform: Other
        OS/Version: All
            Status: NEW
          Keywords: wrong-code
          Severity: regression
          Priority: P1
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: nfxjfg gmail.com


--- Comment #0 from nfxjfg gmail.com 2010-06-27 10:30:32 PDT ---
In the following example, foo() will create an array literal on each call,
instead of returning a statically shared / compile time value. On D2, the same
may happen when using enum instead of const (I didn't test it).

In many cases, this codegen bug may go unnoticed, which makes it just worse.

$ cat rtz.d
struct X {
    int[] a;
}

const cX = X([1,2]);

X foo() {
    return cX;
}

$ dmd -c rtz.d
$ objdump -d -Mintel rtz.o -r
...
00000000 <_D3rtz3fooFZS3rtz1X>:
   0:   55                      push   ebp
   1:   8b ec                   mov    ebp,esp
   3:   83 ec 08                sub    esp,0x8
   6:   53                      push   ebx
   7:   6a 02                   push   0x2
   9:   6a 01                   push   0x1
   b:   6a 02                   push   0x2
   d:   b8 00 00 00 00          mov    eax,0x0
                        e: R_386_32     _D11TypeInfo_Ai6__initZ
  12:   50                      push   eax
  13:   e8 fc ff ff ff          call   14 <_D3rtz3fooFZS3rtz1X+0x14>
                        14: R_386_PC32  _d_arrayliteralT
  18:   89 c1                   mov    ecx,eax
  1a:   bb 02 00 00 00          mov    ebx,0x2
  1f:   89 5d f8                mov    DWORD PTR [ebp-0x8],ebx
  22:   89 4d fc                mov    DWORD PTR [ebp-0x4],ecx
  25:   8b 55 fc                mov    edx,DWORD PTR [ebp-0x4]
  28:   8b 45 f8                mov    eax,DWORD PTR [ebp-0x8]
  2b:   83 c4 10                add    esp,0x10
  2e:   5b                      pop    ebx
  2f:   c9                      leave  
  30:   c3                      ret

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nfxjfg gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|D1 const does not work      |D1 const/CTFE does not work


--- Comment #1 from nfxjfg gmail.com 2010-07-11 22:02:35 PDT ---
Does anyone know a workaround for this?
It's starting to get annoying, and Walter probably died, so it will take a
while until this is fixed.

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nfxjfg gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|D1 const/CTFE does not work |const/CTFE does not work


--- Comment #2 from nfxjfg gmail.com 2010-07-11 22:04:16 PDT ---
(Wrong code on v2.046 too, when switching const with enum.)

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Don <clugdbug yahoo.com.au> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |clugdbug yahoo.com.au
           Severity|regression                  |major


--- Comment #3 from Don <clugdbug yahoo.com.au> 2010-07-26 01:25:23 PDT ---
This generated code is identical on 1.020: this is not a regression.
Downgrading to major.
I don't see any evidence that it's wrong-code, either: it seems to be a
performance issue. Finally, it looks like a duplicate of bug 2356.

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--- Comment #4 from Sobirari Muhomori <dfj1esp02 sneakemail.com> 2010-07-28
12:04:59 PDT ---
Shouldn't literals be immutable and the code - invalid?

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--- Comment #5 from Don <clugdbug yahoo.com.au> 2010-07-28 12:29:57 PDT ---
(In reply to comment #4)
 Shouldn't literals be immutable and the code - invalid?
I think they *should*. I argued strongly for immutable array literals. But I lost. So the code is valid, but likely to be very slow for a very long time. -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
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--- Comment #6 from nfxjfg gmail.com 2010-07-28 13:05:25 PDT ---
It's not valid. This is a systems programming language, and the compiler can't
just randomly insert memory allocations. What if you wrote a kernel in D? I
insist on the wrong-code keyword.

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--- Comment #7 from nfxjfg gmail.com 2010-07-28 13:06:29 PDT ---
and the compiler can't just randomly insert memory allocations.
Add "that are not supposed to be there". -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
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--- Comment #8 from Sobirari Muhomori <dfj1esp02 sneakemail.com> 2010-07-28
19:57:40 PDT ---
Well... there can be a problem with immutable literals because immutability is
transitive... hmm...
Even if literals aren't immutable, compiler can still catch assignment of
literal to mutable array and report error.

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Don <clugdbug yahoo.com.au> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|wrong-code                  |performance


--- Comment #9 from Don <clugdbug yahoo.com.au> 2010-07-28 23:01:36 PDT ---
(In reply to comment #8)
 Well... there can be a problem with immutable literals because immutability is
 transitive... hmm...
 Even if literals aren't immutable, compiler can still catch assignment of
 literal to mutable array and report error.
But it is EXPLICITLY LEGAL to assign a literal to a mutable array. There is an invisible dup by design. I don't like this, I argued strongly against it, but it's in there. This isn't wrong code. (Similarly, you can write const C x = new C; The C will be allocated on the heap, even though it will never change afterwards). -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
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Christian Kamm <kamm-removethis incasoftware.de> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |kamm-removethis incasoftwar
                   |                            |e.de


--- Comment #10 from Christian Kamm <kamm-removethis incasoftware.de>
2010-07-29 07:40:11 PDT ---
The main problem from my point of view is that this fails:

struct X {
    int[] a;
}

const cX = X([1,2]);

void main()
{
  assert(cX.a.ptr is cX.a.ptr);
}

Which is an issue very similar to bug 2526 .

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--- Comment #11 from nfxjfg gmail.com 2010-09-01 12:34:43 PDT ---
Btw. this is a CTFE problem, and the first example is in D1. I don't know what
the hell is with D2 and immutable and implicit dups, but to get the same
behavior on D2, replace const with enum.

The array should be on the *datsegment*, not somehow constructed on the fly.

Not sure about the exact testcase in bug 2356, but it's definitely different
from writing "int[3] x; x[] = [1,2,3];".

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nfxjfg gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code


--- Comment #12 from nfxjfg gmail.com 2010-10-23 06:58:06 PDT ---
I just noticed that the wrong-code keyword is gone.
Sorry that's just bullshit.

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nfxjfg gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |WONTFIX


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Brad Roberts <braddr puremagic.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
                 CC|                            |braddr puremagic.com
         Resolution|WONTFIX                     |


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yebblies <yebblies gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|REOPENED                    |RESOLVED
                 CC|                            |yebblies gmail.com
         Resolution|                            |DUPLICATE


--- Comment #13 from yebblies <yebblies gmail.com> 2012-02-02 15:32:18 EST ---
*** This issue has been marked as a duplicate of issue 2526 ***

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Don <clugdbug yahoo.com.au> changed:

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


--- Comment #14 from Don <clugdbug yahoo.com.au> 2012-02-02 06:16:18 PST ---
This isn't the same as bug 2526, which is an accepts-invalid bug. This has
nothing to do with templates.

The glue layer needs to detect when a constant is being initialized with an
array literal, and when that happens, change it into a value on the static data
segment (ie, it needs to create an XXXinit initializer). Doing this would also
help with bug 2356, but it isn't the same. (Bug 2356 will often need to create
values at runtime, so it will need to copy data from the initializer).

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--- Comment #15 from Don <clugdbug yahoo.com.au> 2013-03-19 04:51:15 PDT ---
It passes if you specify the type in the const/enum.

The reason is, that in declaration.c, VarDeclaration::semantic(), we see this
code:

else if (storage_class & (STCconst | STCimmutable | STCmanifest) ||
                 type->isConst() || type->isImmutable())
        {
            /* Because we may need the results of a const declaration in a
             * subsequent type, such as an array dimension, before semantic2()
             * gets ordinarily run, try to run semantic2() now.
             * Ignore failure.
             */
            if (!global.errors && !inferred)
                 ....

If we're inferring type inference, then semantic2 doesn't get run on the
variable declaration (cX in the original bug report).

So when it runs semantic on "return cx;", DsymbolExp::semantic() for cx,
returns cx->init, which hasn't had semantic run on it yet. So it behaves like a
copy-and-paste of the initializer.

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Don <clugdbug yahoo.com.au> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Depends on|                            |9953


--- Comment #16 from Don <clugdbug yahoo.com.au> 2013-04-18 01:54:50 PDT ---
There is no compiler bug here, it is working as designed, at least for D2.
However, the design doesn't make sense. I have opened bug 9953 for the design
change.

It would be possible to give the desired behaviour in D1, though it's hardly
worthwhile. In D2 I think it is a logical impossibility. IMHO this should just
fail to compile. You should write "static const" instead of "enum" when you
want to declare a constant of a reference type.

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Apr 18 2013