Re: [PATCH] default_type_align: Use type_length_units

[Simon Marchi <simon.marchi@polymtl.ca>]

On 2018-06-14 12:59, Tom Tromey wrote:
> This certainly is correct according to the comment in gdbtypes.h:
> 
>   /* * Return the alignment of the type in target addressable memory
>      units.
> 
> However, I was curious to know what DWARF specifies, to see whether
> dwarf2read was doing this as well.  But DWARF seems pretty silent on
> this topic:
> 
>   A debugging information entry may have a DW_AT_alignment attribute
>   whose value of class constant is a positive, non-zero, integer
>   describing the alignment of the entity.
> 
> So maybe this is depending on what your compiler does?  I think I did
> not really consider this issue too deeply when writing the alignment
> support :(

 From what I remember, nothing is well defined in DWARF regarding 
non-8-bit-bytes.  In the past, I have contributed fixes for such 
architectures based on how things work with our (Ericsson) internal 
compiler, and nobody complained so far.  There is a BoF at Cauldron 
though to talk about how to make the non-8-bit-bytes support more 
"official" and testable [1], if you're interested!

Our compiler doesn't emit DW_AT_alignment AFAIK, so I don't know what it 
would emit.  But it would make sense if it was in addressable memory 
units, just like type sizes.

> Another possible issue is that gdbtypes.c:type_align uses TYPE_LENGTH 
> in
> one spot but I suppose it ought to use type_length_units if this patch
> goes in.

This use seems related to C++, and our compiler is C only, so I can't 
test that.  But it would make sense, I can include that change if you 
prefer.

Simon

[1] 
https://gcc.gnu.org/wiki/cauldron2018#Non-8-bit-byte_architecture_support_BoF