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

[Julien Fischer]

Hi,

On Mon, 5 Dec 2011, Peter Wang wrote:

> :- pred mlds_output_scalar_cell_group_type_and_name(mlds_to_c_opts::in,
> 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;

You didn't really think it would be that easy, did you? ;-)

s/_declspec/__declspec/

and apparently the argument of the align attribute must be an integer
constant, so we can't use an expression such as sizeof(MR_Word).
(This is with MSVC 2008, I haven't yet tried a later version.)

I think the appropriate thing to do here is add MR_BYTES_PER_WORD back
to the runtime and use that in the above.  (I say add back, because the
only place it appears is in mercury_conf_bootstrap.h.)

In the meantime I've just hardcoded a value in there - I'll let you know
how that went once my workspace finishes building.

Julien.
--------------------------------------------------------------------------
mercury-reviews mailing list
Post messages to:       mercury-reviews at csse.unimelb.edu.au
Administrative Queries: owner-mercury-reviews at csse.unimelb.edu.au
Subscriptions:          mercury-reviews-request at csse.unimelb.edu.au
--------------------------------------------------------------------------