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

[Daniel Jacobowitz <drow@false.org>]

On Sun, Sep 03, 2006 at 04:53:14PM +0800, Wu Zhou wrote:
> It is like this.  The structures decimal32, decimal64 and decimal128 are 
> big-endian in current libdecnumber implementation:
> 
> typedef struct
> {
>   uint8_t bytes[DECIMAL128_Bytes];      /* decimal128: 1, 5, 12, 110 bits */
> } decimal128;
> 
> 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.

> Ben Elliston is planning to change the memory layout of 
> decimal32/decimal64/decimal128 to host byte order in later 
> libdecnumber/gcc.  Then the byte swapping will not be needed in gdb.  But 
> that is when GCC gets to stage 1 again, which might be around the end of 
> this year.
> 
> 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.

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.

-- 
Daniel Jacobowitz
CodeSourcery