Re: Unrevied patches^3

[Geoff Keating <geoffk at geoffk dot org>]

Richard Sandiford <rsandifo@redhat.com> writes:

> Geoff Keating <geoffk@geoffk.org> writes:
> > This patch isn't right.  What is the MIPS port doing that it has a
> > LO_SUM without a corresponding HIGH?  Why can it not represent this
> > using PLUS, or some other arithmetic operation, instead?
> 
> I don't understand.  The LO_SUM does have a high part, but it isn't
> always represented as a HIGH:
> 
>    - If the symbol refers to small data, the high part is $gp
>      (this is like what the alpha port does, except the alpha
>      port only uses it after reload).
> 
>    - When generating PIC, the high part is a load from the GOT.
>      It's represented as such even before reload (which this makes
>      various things easier).
> 
> Are you saying that the first operand to a LO_SUM must be provided
> specifically by a HIGH, rather than some other "more specific"
> rtx that evaluates to the same thing?

You should probably look up the documentation for LO_SUM and HIGH.

It doesn't have to be represented as a HIGH, it is usually a register.
You have RTL like:

(set (reg $gp) (high (symbol_ref "foo")))
(set ... (lo_sum (reg $gp) (symbol_ref "foo")))

but the middle-end has no problem with this.

I'm not completely sure how MIPS small data works, but I thought it
didn't do this.  I thought that the point of using small data was that
the offset to it did not have a high part (it fits in 16 signed bits,
thus "small"), and that the register used as a base was fixed.  In
that case, I would recommend simply writing

(set ... (symbol_ref "foo"))

and leaving $gp out of it.  (This is what rs6000 does, and HIGH/LO_SUM
was originally invented for rs6000.)

I don't understand what you mean by "the high part is a load from the
GOT".  If you can load the high part from the GOT, why don't you just
load the whole address from there?

-- 
- Geoffrey Keating <geoffk@geoffk.org>