Re: Debugger support for __float128 type?

[Mark Kettenis <mark dot kettenis at xs4all dot nl>]

> Date: Wed, 30 Sep 2015 19:33:44 +0200 (CEST)
> From: "Ulrich Weigand" <uweigand@de.ibm.com>
> 
> Hello,
> 
> I've been looking into supporting __float128 in the debugger, since we're
> now introducing this type on PowerPC.  Initially, I simply wanted to do
> whatever GDB does on Intel, but it turns out debugging __float128 doesn't
> work on Intel either ...
> 
> The most obvious question is, how should the type be represented in
> DWARF debug info in the first place?  Currently, GCC generates on i386:
> 
>         .uleb128 0x3    # (DIE (0x2d) DW_TAG_base_type)
>         .byte   0xc     # DW_AT_byte_size
>         .byte   0x4     # DW_AT_encoding
>         .long   .LASF0  # DW_AT_name: "long double"
> 
> and
> 
>         .uleb128 0x3    # (DIE (0x4c) DW_TAG_base_type)
>         .byte   0x10    # DW_AT_byte_size
>         .byte   0x4     # DW_AT_encoding
>         .long   .LASF1  # DW_AT_name: "__float128"
> 
> On x86_64, __float128 is encoded the same way, but long double is:
> 
>         .uleb128 0x3    # (DIE (0x31) DW_TAG_base_type)
>         .byte   0x10    # DW_AT_byte_size
>         .byte   0x4     # DW_AT_encoding
>         .long   .LASF0  # DW_AT_name: "long double"
> 
> Now, GDB doesn't recognize __float128 on either platform, but on i386
> it could at least in theory distinguish the two via DW_AT_byte_size.
> 
> But on x86_64 (and also on powerpc), long double and __float128 have
> the identical DWARF encoding, except for the name.
> 
> Looking at the current DWARF standard, it's not really clear how to
> make a distinction, either.  The standard has no way to specifiy any
> particular floating-point format; the only attributes for a base type
> of DW_ATE_float encoding are related to the size.
> 
> (For the Intel case, one option might be to represent the fact that
> for long double, there only 80 data bits and the rest is padding, via
> some combination of the DW_AT_bit_size and DW_AT_bit_offset or
> DW_AT_data_bit_offset attributes.  But that wouldn't help for PowerPC
> since both long double and __float128 really use 128 data bits,
> just different encodings.)
> 
> Some options might be:
> 
> - Extend the official DWARF standard in some way
> 
> - Use a private extension (e.g. from the platform-reserved
>   DW_AT_encoding value range)
> 
> - Have the debugger just hard-code a special case based
>   on the __float128 name 
> 
> Am I missing something here?  Any suggestions welcome ...
> 
> B.t.w. is there interest in fixing this problem for Intel?  I notice
> there is a GDB bug open on the issue, but nothing seems to have happened
> so far: https://sourceware.org/bugzilla/show_bug.cgi?id=14857

Perhaps you should start with explaining what __float128 actually is
on your specific platform?  And what long double actually is.

I'm guessing long double is a what we sometimes call an IBM long
double, which is essentially two IEEE double-precision floating point
numbers packed together and that __float128 is an attempt to fix
history and have a proper IEEE quad-precision floating point type ;).
And that __float128 isn't actually implemented in hardware.

I fear that the idea that it is possible to determine the floating
point type purely from the size is fairly deeply engrained into the
GDB code base.  Fixing this won't be easy.  The easiest thing to do
would probably be to define a separate ABI where long double is IEEE
quad-precision.  But the horse is probably already out of the barn on
that one...

Making the decision based on the name is probably the easiest thing to
do.  Butq keep in mind that other OSes that currently don't support
IBM long doubles and where long double is the same as double, may want
to define long double to be IEEE quad-precision floating point on
powerpc.

The reason people haven't bothered to fix this, is probably because
nobody actually implements quad-precision floating point in hardware.
And software implementations are so slow that people don't really use
them unless they need to.  Like I did to nomerically calculate some
asymptotic expansions for my Thesis work...