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digitalmars.D.bugs - [Issue 3183] New: Spec of align attribute needs work

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=3183

           Summary: Spec of align attribute needs work
           Product: D
           Version: unspecified
          Platform: All
               URL: http://www.digitalmars.com/d/archives/digitalmars/D/Sp
                    ec_of_align_attribute_is_a_mess_88129.html
        OS/Version: All
            Status: NEW
          Keywords: spec
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: smjg iname.com
            Blocks: 677,3118


There are ambiguities and inconsistencies in how the align attribute is
described in the D specification.  What follows is a cut-down version of the
unanswered post at the given URL, with a few bits thrown in from other posts.

1. Is the whole purpose of it for portability of data or for C ABI
compatibility?

ctod.html:
"Clearly, since much of the point to setting alignment is for 
portability of data, a portable means of expressing it is necessary."

attribute.html:
"AlignAttribute is meant for C ABI compatiblity [sic], which is not the 
same thing as binary compatibility across diverse platforms. For that, 
use packed structs:"

2. attribute.html describes align as matching the behaviour of the companion C
compiler.  What if there's no companion C compiler, or the companion C compiler
doesn't support alignment control at the level it has been used in the D code?

3. I had for ages made out align to be purely about the relative alignment of
members within a struct.  But a discussion at
http://tinyurl.com/nqs2n3#N87948
(that message onwards) got me confused until I realised that people were
interpreting it as absolute alignment.  The spec doesn't clarify one way or the
other.  Which is it meant to be?

4. Following on from point 3, should it apply only to structs as is currently
documented?  Or should it work on unions as well?  Even arbitrary variable
declarations?

5. And how should the compiler handle it if used where inapplicable?  There are
the odd bits in the spec on this, but it wants rethinking.  This is just one
case of issue 3118.

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next sibling parent d-bugmail puremagic.com writes:
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Walter Bright <bugzilla digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bugzilla digitalmars.com





02:44:07 PDT ---
1. C ABI compatibility.

2. There's always a companion C compiler, as D relies on the existence of the C
runtime library.

3. It's the way the companion C compiler aligns.

4. Unions members are all at offset 0. Unions are aligned according to the
alignment of their largest field.

5. It is ignored for non-struct fields.

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 2. There's always a companion C compiler, as D relies on the existence of the C
 runtime library.


So a prerequisite for writing a D compiler from scratch is to write a C
compiler first?  Still, that doesn't answer my question beyond the first 7
words.


 3. It's the way the companion C compiler aligns.


Still, that this may be absolute or relative depending on the companion C
compiler ought to be mentioned in the D spec at least to crack down on the user
confusion of which I have been a victim.


 4. Unions members are all at offset 0. Unions are aligned according to the
 alignment of their largest field.


You mean aligned in a containing struct, or aligned absolutely _if_ the
implementation interprets alignment as absolute?  Neither seems to be happening
in my test (1.046):

----------
align(1) struct Qwert {
    byte b1;
    align(1) struct {
        byte b2;
        union {
            align (4) struct { int i; }
        }
    }
}

pragma(msg, "Qwert.alignof == " ~ Qwert.alignof.stringof);
pragma(msg, "Qwert.b1.alignof == " ~ Qwert.b1.alignof.stringof);
pragma(msg, "Qwert.b2.alignof == " ~ Qwert.b2.alignof.stringof);
pragma(msg, "Qwert.i.alignof == " ~ Qwert.i.alignof.stringof);
pragma(msg, "Qwert.b1.offsetof == " ~ Qwert.b1.offsetof.stringof);
pragma(msg, "Qwert.b2.offsetof == " ~ Qwert.b2.offsetof.stringof);
pragma(msg, "Qwert.i.offsetof == " ~ Qwert.i.offsetof.stringof);
----------
Qwert.alignof == 1u
Qwert.b1.alignof == 1u
Qwert.b2.alignof == 1u
Qwert.i.alignof == 4u
Qwert.b1.offsetof == 0u
Qwert.b2.offsetof == 1u
Qwert.i.offsetof == 2u
----------

But to both 4 and 5, I said "should", i.e. I was pondering over what's
desirable and whether the spec ought to be changed, not what the current spec
or behaviour is.

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prev sibling parent d-bugmail puremagic.com writes:
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Walter Bright <bugzilla digitalmars.com> changed:

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





13:26:26 PDT ---
Fixed dmd 1.047 and 2.032

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