Re: [RFC] decimal float point patch based on libdecnumber: gdb patch

[Wu Zhou <woodzltc@cn.ibm.com>]

Daniel Jacobowitz wrote:
>> But variables/constants of _Decimal32, _Decimal64 and _Decimal128 (which 
>> are the DFP extension to c language types) in the memory are stored in 
>> little-endian on x86, and big-endian on ppc64.  So the byte swapping is 
>> needed on x86.
> 
> OK, that makes sense: note that this is needed precisely when
> converting from a target decimal float to a host decimal128.  That
> is a better time to do the conversion.

Yes.  It should be a better time.

>> So one option is for us to keep the byte swapping code in gdb, and when the 
>> byte order in libdecnumber is changed to host byte order, we can easily 
>> delete them.
> 
> This, however, is not correct.  Libdecnumber will presumably change to
> use host endianness.  GDB will fetch numbers in target endianness.
> If you're using a native i386 debugger, then you won't need to swap;
> but if you're using an i386 <-> powerpc debugger, then you will.  The
> swap will need to be in the same place, just with a different
> condition.

Yes. Swapping is still needed in cross debugging. I will give some more thought to this, to see if I 
can work out a solution in the near future.

> I would recommend that you always store the bytes in struct value in
> target endianness.  Then, have two functions which convert between a
> "struct value" and a "decimal128".  Then it should be clear which one
> has which representation.
> 
> Then, for instance, you can use decimal128 in typed_val_decfloat, and
> in the argument of value_from_decfloat.  And that function can be
> responsible for the exchange.  Similarly, in print_decimal_floating,
> you have bytes in target endianness; you can convert them to a "struct
> value", which will have the same bytes, and convert the value to a
> decimal128 which you can print.
> 
> The only part of that which is tricky is converting the bytes back to
> a struct value.  You could write a new function, value_from_bytes,
> to do that; just like value_from_longest.
> 
Thanks for these valuable suggestion.

Regards
- Wu Zhou