Re: incorrect signed data

[Mark Wielaard <mjw at redhat dot com>]

On Thu, 2014-02-06 at 13:56 +0100, Mark Wielaard wrote:
> On Wed, 2014-02-05 at 14:59 -0800, Josh Stone wrote:
> > Here's gdb precedent for the status quo, not sign-extending, in the
> > comment right before dwarf2_const_value_data in gdb/dwarf2read.c:
> > 
> > /* Given an attr with a DW_FORM_dataN value in host byte order,
> >    zero-extend it as appropriate for the symbol's type.  The DWARF
> >    standard (v4) is not entirely clear about the meaning of using
> >    DW_FORM_dataN for a constant with a signed type, where the type is
> >    wider than the data.  The conclusion of a discussion on the DWARF
> >    list was that this is unspecified.  We choose to always zero-extend
> >    because that is the interpretation long in use by GCC.  */
> 
> I also found the following comment in the GCC sources:
> 
>       /* ??? It would be slightly more efficient to use a scheme like is
>          used for unsigned constants below, but gdb 4.x does not sign
>          extend.  Gdb 5.x does sign extend.  */
> 
> That is just before trying to output a possible signed constant value.
> It will always use DW_FORM_sdata as sleb128 for that.
> 
> The GCC comment is incomplete because you showed that recent GDBs (at
> least since 2010) reverted to the 4.x behavior again.

And I now found the discussion on the gdb patches list:
https://sourceware.org/ml/gdb-patches/2010-07/msg00437.html

Which funnily brings it back to us again:

"Both GCC and elfutils agreed that DW_FORM_data* are "just bits" -- no
sign extension is to be applied."

Grin,

Mark