[RFC] Passing MIPS debug hints between gcc and gdb

[Richard Sandiford]

Fred Fish <fnf@specifix.com> writes:
> Currently gcc supports generating two zero content sections, the names
> of which are supposed to be used by gdb.
>
> One specifies the ABI, is of the form ".mdebug.abiXX", and gdb does
> currently look for it.
>
> Another has a name of the form ".gcc_compiled_longXX" and gdb does not
> currently look for it.

Ah.  Presumably the patch to do that was never contributed then.
It does exist somewhere!

> This is supposed to handle the case where gcc has been given the
> command line option -mlong32 or -mlong64.  Since these can be used
> independently of the ABI, the current gcc code should be changed to
> always generate this section.

Right.

> However, the size of pointers can also be affected by -mlongXX.  In
> theory we can figure out how the pointer size is affected by knowing
> the ABI and other stuff, but it's much simplier to just let gcc
> specify it the same way as the size of longs.

You're right: the size of everything is determined by the combination of
the ABI "name" and long size.  The gdb patch I mentioned did do that IIRC.
(I didn't write that patch myself, and my memory's hazy.)  It was certainly
the intention that gdb work this out for itself.

> I have found, from chasing other MIPS gdb testsuite failures, that gdb
> also needs to know when code has been compiled with soft float, on
> hardware that supports hard float, so that it can call functions by
> hand with float arguments in the right location.  So we need another
> section for that.
>
> Just to complicate things, the latest development binutils linker
> removes sections with no contents, so all of these disappear in the
> output file.

Using empty sections was always a hack, to be honest (modelled on the
old .gcc_compiled_v3 thing, whatever it was called).  I suppose we
should be using note sections really.

You didn't mention it, but your patch removes the EABI conditional above
the .gcc_compiled_long* code.  I'm not comfortable with adding the
sections unconditionally though.  It may confuse IRIX.

As far as I'm concerned, EABI is the only ABI where we support both
-mlong32 and -mlong64.  ISTR someone deciding to use -mabi=o64 -mlong64
as well.  Since both EABI and o64 are basically GNU inventions, I'd be
OK with relaxing the condition to EABI || o64 if it's o64 you need this
for.  I suppose we should make the incorrect -mlong* option an error for
other ABIs.  (Another one for the TODO list.)

Richard