[m-rev.] for review: fix float field structure padding/packing problems

[Julien Fischer]

On Tue, 6 Dec 2011, Paul Bone wrote:

> On Mon, Dec 05, 2011 at 05:50:36PM +1100, Peter Wang wrote:
>> Can someone apply this and test with MSVC and MinGW?
>> I've only tested the single precision floats on 64-bit Linux part.
>>
>> Would it be worth avoiding the C extensions when
>> sizeof(MR_Float) == sizeof(MR_Word)?  That can't be evaluated by the C
>> preprocessor, though.
>>
>
> If this is possible then there's an improvement below (see my in-line reply)
> that can be made.
>
>> diff --git a/runtime/mercury_float.h b/runtime/mercury_float.h
>> index 049477f..28d5728 100644
>> --- a/runtime/mercury_float.h
>> +++ b/runtime/mercury_float.h
>> @@ -29,6 +29,21 @@
>>  #define MR_FLOAT_WORDS		((sizeof(MR_Float) + sizeof(MR_Word) - 1) \
>>  					/ sizeof(MR_Word))
>>
>> +  /*
>> +  ** MR_Float_Aligned is a word-aligned version of MR_Float.  This is necessary
>> +  ** when MR_Float is single precision on a 64-bit platform to avoid packing
>> +  ** consecutive float fields, or when MR_Float is double precision on some
>> +  ** 32-bit platforms (e.g. Win32) to avoid introducing padding.
>> +  */
>> +#if defined(MR_GNUC) || defined(MR_CLANG)
>> +  typedef MR_Float MR_Float_Aligned __attribute__((aligned(sizeof(MR_Word))));
>> +#elif defined(MR_MSVC)
>> +  typedef _declspec(align(sizeof(MR_Word))) MR_Float MR_Float_Aligned;
>> +#else
>> +  /* Hope for the best. */
>> +  typedef MR_Float MR_Float_Aligned;
>> +#endif
>> +
>
> This last alternative could raise an error to tell the user that we don't
> support his/her compiler on this combination of word size and float size.

Raising an error at this point would prevent Mercury from working with
any but the three C compilers listed.  That seems a bit rough given that
this problem only affects certain problems or grades.

Julien.
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