Re: new register allocator and HARD_REGNO_CALL_PART_CLOBBERED

[Daniel Berlin <dberlin at dberlin dot org>]

On Tuesday, May 13, 2003, at 03:40  PM, Joern Rennecke wrote:

> Daniel Berlin wrote:
> > On Tue, 13 May 2003, Joern Rennecke wrote:
> >
> > > Daniel Berlin wrote:
> > No, they are invalid. It frequently gives us regs we can't use *at 
> > all*
> > (IE regardless of call status, they just aren't even okay for the
> > mode at all).
>
> That is a different problem then, which indeed would be a bug in 
> regclass.

> > > You can do this ahead of time, by looking at the modes of the pseudos
> > > you want to allocate in the register.
> >
> > Which is what i suggested, rather than putting it in get_free_reg.
> > The portion of code we have now that looks at the modes is regclass.
> > So i suggested it goes there.
>
> That would require to create new register classes to cover all
> call-used/call-saved boundaries, both for full and partial saves, plus 
> the
> resultant unions and intersections.
No it wouldn't require new regclasses.
It simply requires giving a HARD_REG_SET of preferred registers, rather 
than a register class.
This is what the new register allocator converts the regclass into 
anyway.
The whole notion of explicit register classes hardly ever meshes well 
with reality.
What we really need is for each port to have a function, that when 
passed a struct df (or something) with dataflow info already generated, 
gives us two arrays.  First, for each pseudo, a valid list of preferred 
usable registers (in terms of constraints like call clobbered or not, 
not in terms of non-conflicting with any other pseudo's set of 
registers)for that pseudo.  Second, a valid list of *all* usable 
registers for the pseudo.
This is basically what web_class ends up doing.
We really shouldn't be trying to shoehorn pseudos into a fixed set of 
register classes for no good reason.

> So it seems that choose_spill_colors should take 
> HARD_REGNO_CALL_PART_CLOBBERED
> into account, except that we are short of a mode there.
No, this is also wrong.
Note that choose_spill_colors uses the usable_regs. Usable_regs should 
*not* have any unusable regs in it.
The only reason we have to test HARD_REGNO_MODE_OK *now* is because 
some ports (or regclass itself) are broken, so we end up with unusable 
regs in usable_regs (since it's initialized based on register class).
Get it?
It was to work around broken ports, but it shouldn't be necessary.

> Going back a bit in this thread, I see:
> Herman ten Brugge:
> > I do not think the problem is in df.c. The code in df.c walks through 
> > the
> > rtl and sets clobbered/used register info. We do not yet know what 
> > mode
> > the hard registers will get (reg_raw_mode can not be used here). The 
> > mode
> > is assigned in the new-ra pass.
>
> I must say that I don't understand why reg_raw_mode can not be used 
> there.
> reg_raw_mode should be HFmode for the c4x registers in question, and
> Dale Johannesen's patch to df_insn_refs_record should find the 
> calls/register
> combinations that could potentially cause a problem.
It's actually my patch, sent to Dale privately.