RFC gdbserver and tdesc and powerpc (stuck on a gdbserver assert)

[will schmidt]

Hi, 
   I've been working on a target-description rework for powerpc, this is
a continuation of work that Rogerio has posted rfc patches for sometime last year.

I've run into a stumbling block with the init_target_desc code in gdbserver,
and am not sure how to best proceed.

  gdbserver/tdesc.cc:  init_target_desc(...)  iterates through the provided
features and populates the reg_defs structure.  The code currently has an assert
with a comment:

        /* Register number will increase (possibly with gaps) or be zero.  */
        gdb_assert (regnum == 0 || regnum >= tdesc->reg_defs.size ());

This trips on powerpc (with the WIP tdesc patch set), potentially in several
locations, since our features contain registers that are intermixed across the
ranges, so we end up with regnos that numerically belong earlier in the
tdesc->reg_defs structure, but they belong in the features where they are.
In particular; 
The Powerpc "core" features includes regnums 0-31 (gprs r0..r31),
a gap, then 64-69 (PC,MSR,CR,LR,CTR,XER).
The subsequent "fpu" feature fills in that gap as it includes regnums
32-63 (f0..f31), and 70 (fpscr).

There may or may not be an issue with the subsequent altivec and vsx register sets,
since we have some overlapping ranges there.   

I could split apart the features into smaller bits, but this would scramble the
documented powerpc features descriptions (as seen in gdb.texinfo).   
I've tried just disabling the assert, but I'm not certain that is sufficient, I currently
also see some partial transfer errors between gdb and gdbserver that i've not sorted out.

Appreciate any thoughts on how I should proceed.
Thanks
-Will