Re: [boost] optional/type_with_alignment.hpp vs. metrowerks 8.3 PPC CFM

From: Mat Marcus (mmarcus-boost_at_[hidden])
Date: 2003-09-02 16:23:17

--On Tuesday, September 02, 2003 3:32 PM -0400 Douglas Paul Gregor 
<gregod_at_[hidden]> wrote:

> On Tue, 2 Sep 2003, Mat Marcus wrote:
>> --On Tuesday, September 02, 2003 2:00 PM -0400 Douglas Gregor
>> <gregod_at_[hidden]> wrote:
>> > I suspect they are both '4', but that leaves me even more confused
>> > as to why  the alignment of std::pair<double, double> would be 8
>> > (and how to get a POD  type with alignment 8 for ourselves!).
>>
>> Yes, they are both 4. I suspect that the problem may have to do with
>> some unusual alignment rules on this platform. I believe that the
>> rule is something like: a double at the beginning of a struct
>> forces 8 byte alignment, but a double in the middle does not. That
>> is, given
>>
>> struct A {
>> 	double d;
>> 	int i;
>> };
>>
>> struct B {
>> 	int i;
>> 	double d;
>> };
>>
>> then:
>> sizeof(A) == 16;
>> sizeof(B) == 12;
>> alignment_of<A>::value == 8;
>> alignment_of<B>::value == 4;
>>
>> Fun, eh? I can double check the compiler release notes for the exact
>> rule if you like.
>>
>>  - Mat
>
> Are there any other crazy rules like this that you know of? We could
> just add "struct A" from above to the list of types, or if there is a
> CodeWarrior-specific extension (e.g., something like "#pragma pack"
> or "atttribute(aligned...)") we could start using compiler-specific
> extensions to do this, which wouldn't be a bad thing to do anyway.
>
> 	Doug

There is a #pragma pack. This is the craziest rule that I know of. 
Perhaps Howard or Andreas could give a more definitive answer. 
Following is an excerpt from some release notes for this 
platform/target:

 - Mat

 =====================================================================
===
General Notes for 3.0
======================================================================
==

- extended alignment modes, Metrowerks conformed to the "Macintosh 
C/C++ ABI
  Standard Specification Revision 1.3" dated Dec. 5, 1996, and "Inside 
Mac OS X:
  Mac Mach-O Runtime Architecture", version Preliminary dated March 
2002. Apple
  has now decided that both documents are incorect, and that the 
defintion of
  align=power is what ever MrC and GCC are doing.

  The rub here is that acording to this change in behavior doubles in 
a structure
  are not 8 byte aligned unless the user aligns them manual by 
inserting padding,
  or if they happen to be the first element in the structure.

  Rather then not support the existing behavior a number of new 
alignment modes now
  exist:

        #pragma options align=power_mw

        always implements the rule:     When in power_mw alignment 
mode, a special exception
        is made when the first embedding element of a struct or any 
element of a union has
        the data type of double -- in this case, the embedding 
alignment for all (directly
        included/top level) double members in the aggregate is 8. By 
the rules of
        computing the embedding alignment for the struct being 
declared, this will also cause
        the embedding alignment of the entire struct or union to be at 
least 8 as well. The
        aggregate table entry for power is 8 for such structs.

        #pragma options align=power_gcc

        always implements the rules used by gcc/MrC:
        - the embedding alignment of the first element in a data 
structure is equal to
          the elements natural alignment
        - for subsequent elements with a natural alignment less than 
4, the embedding
          alignment of each element is equal to its natural alignment
    - for subsequent elements that have a natural alignment greater 
than 4
      bytes, the embedding alignment is 4, unless the element is a 
vector data type

    #pragma options align=natural

    always implement simple alignment with no special rules based on 
the type
    of the first member of a structure. doubles and long longs are 8 
byte aligned.

    #pragma options align=power

    maps to either power_mw or power_gcc depending on the setting of 
the option:

    #pragma alignment_metrowerks on|off|reset

    If struct alignment preferences were set to "PowerPC" 
alignment_metrowerks defaults on.
    If struct alignment preferences were set to "PowerPC MrC/GCC" 
alignment_metrowerks defaults off.
    If struct alignment preferences were set to "PowerPC Natural" 
alignment_metrowerks defaults off.

    When on power==power_mw
    when off power==power_gcc.