Re: [lto] PATCH: preserve precision of odd-sized integral types

[Mark Mitchell <mark at codesourcery dot com>]

Sandra Loosemore wrote:
> This patch addresses one of the weirdnesses Kenny noted that showed up 
> when reading and writing bit_size_type (68 bit precision) through the 
> DWARF encoding.  OK to commit?

> 2006-10-10  Sandra Loosemore  <sandra@codesourcery.com>
>
> 	* gcc/dwarf2out.c (base_type_die): Add DW_AT_bit_size and
> 	DW_AT_bit_offset attributes for integral types with a precision
> 	smaller than the bit size of their containing object.
> 	* gcc/lto/lto.c (lto_read_base_type_DIE): Do the inverse when
> 	constructing integral types.

This is cool because it's the first example of one of the positive 
side-effects I've been expecting from LTO: we'll get better debug 
information!

> !   byte_size = int_size_in_bytes (type);
> !   add_AT_unsigned (base_type_result, DW_AT_byte_size, byte_size);
>     add_AT_unsigned (base_type_result, DW_AT_encoding, encoding);
>   
> +   /* Emit information to allow the LTO front end to reconstruct integral
> +      types whose precision is less than the bit width of their containing
> +      object, e.g., bitsizetype.  */

I don't think the LTO comment here is appropriate.  This is just 
generating more correct debugging information.

> +   if (TREE_CODE (type) == INTEGER_TYPE
> +       && TYPE_PRECISION (type) < byte_size * BITS_PER_UNIT)
> +     {
> +       add_AT_unsigned (base_type_result, DW_AT_bit_size,
> + 		       TYPE_PRECISION (type));
> +       add_AT_unsigned (base_type_result, DW_AT_bit_offset,
> + 		       byte_size * BITS_PER_UNIT - TYPE_PRECISION (type));
> +     }

Does DW_AT_bit_offset probably depend on the {bit,byte}-endianness of 
the target?  The DWARF spec says:

"The bit offset attribute describes the offset in bits of the high order 
bit of a value of the given type from the high order bit of the storage 
unit used to contain that value."

So, for a big-endian system, I'd sort-of expect the value to be zero. 
Daniel, can you help here?  I see that in the reader, you check that 
that the values are as you expect in this respect, but I'm not sure that 
the data you're emitting is correct for big-endian systems.

--
Mark Mitchell
CodeSourcery
mark@codesourcery.com
(650) 331-3385 x713