Re: PATCH [11/n]: Prepare x32: PR rtl-optimization/48155: Reload doesn't handle subreg properly

["Ulrich Weigand" <uweigand at de dot ibm dot com>]

H.J. Lu wrote:
> On Mon, Jun 27, 2011 at 7:52 AM, Ulrich Weigand <uweigand@de.ibm.com> wrote:
> > H.J. Lu wrote:
> >
> >> Given input:
> >>
> >> (plus:SI (subreg:SI (plus:DI (reg/f:DI 7 sp)
> >> =A0 =A0 =A0 =A0 =A0 =A0 (const_int 16 [0x10])) 0)
> >> =A0 =A0 (const_int -1 [0xffffffffffffffff]))
> >
> > Once again, this seems weird as legitimate address ... =A0If this really
> > can occur validly, there'll probably need to be an insn+splitter and/or
> > a secondard reload provided by the back-end to handle it.
> 
> This is the valid memory address for any instructions which
> take a memory operand under x32.  How will insn+splitter and/or
> a secondard reload help x32 here? Do I implement such a thing for
> all instructions which take a memory operand?

Well, if this *is* already accepted as valid, then I don't understand
why it is getting reloaded in the first place.

> > With your change below, it seems you're just falling through to
> > the generic gen_rtx_SET case, right? =A0 How does this help?
> >
> 
> I added ix86_simplify_base_disp to i386.c to handle such cases.

I see.  It appears that this routine is used within ix86_decompose_address,
which means that:
- addresses containing constructs as above will be accepted as valid
- the simplification will *not* be done in place in the RTL stream,
  but on the fly every time the address is looked at

This explains why just emitting a plain SET is accepted.  But if this
is the case, then the PLUS case in gen_reload should also have worked,
since the first thing it tries is just a plain SET as well:

[snip]
         The simplest approach is to try to generate such an insn and see if it
         is recognized and matches its constraints.  If so, it can be used.
[snip]
      insn = emit_insn_if_valid_for_reload (gen_rtx_SET (VOIDmode, out, in));
      if (insn)
        return insn;

Can you check in your test case why this isn't accepted here?


In general, I'm wondering if it really a good idea to accept
complex non-simplified expressions like the above as valid addresses,
instead of simplifying them directly where they are generated, and
then just accepting the simplified versions.  That simplification
could occur e.g. in a secondary reload for PLUS (in those cases
where we actually require a reload), or in a legitimize_reload_address
handler (in those cases where the address can remain in place).

Bye,
Ulrich

-- 
  Dr. Ulrich Weigand
  GNU Toolchain for Linux on System z and Cell BE
  Ulrich.Weigand@de.ibm.com