input address reload issue

[Aurelien Buhrig <aurelien.buhrig.gcc@gmail.com>]

Hi all, and best wishes for the happy new year!

I'm porting a private 4.6 backend to GCC 6 and facing a reload issue and
I would appreciate a little help to cope with it.

The issue happens when reloading:

(set (reg:QI 47 [ _9 ])
  (mem:QI (plus:SI (reg/v/f:SI 68 [orig:51 in ] [51])
                (const_int 1 [0x1])

My understanding is that IRA allocates hardregs to allocno which are
replaced by the reload pass which generates new reloads or spills regs
when needed, right?

Here the IRA chooses a reg (named r2)which makes the mem address not
legitimate. Is it valid to allocate a reg which makes non legitimate
address?

Assuming it is, my understanding is that the reload chooses a legitimate
reg (named a0 here) and shall emit insns (in emit_reload_insns) to set
a0 correctly (RELOAD_FOR_INPUT_ADDRESS). Right?

So the insn:
(set (reg:QI 0 r0) (mem:QI (plus:SI (reg:SI 2 r2)(const_int 1))

is transformed into:
(set (reg:SI 8 a0) (reg:SI 2 r2))
(set (reg:SI 8 a0) (const_int 1))
(set (reg:SI 8 a0) (plus:SI (reg:SI 8 a0) (reg:SI 8 a0)))
(set (reg:QI 0 r0) (mem:QI (reg:SI 8 a0))

This "basic" transformation requires two reload regs, but only one is
given/used/possible from the rl structure (in emit_reload_insns).

So where does the issue comes from? The need for 2 reload regs, the
transformation which is too "basic" and could be optimized to use only
one reload reg, or any wrong/missing reload target hook?

Thanks in advance.
Aurélien