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

Daniel Jacobowitz
2006-09-03T16:44:21Z

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