lra: Avoid cycling on certain subreg reloads [PR96796]

lra: Avoid cycling on certain subreg reloads [PR96796]

This PR is about LRA cycling for a reload of the form:

----------------------------------------------------------------------------
Changing pseudo 196 in operand 1 of insn 103 on equiv [r105:DI*0x8+r140:DI]
      Creating newreg=287, assigning class ALL_REGS to slow/invalid mem r287
      Creating newreg=288, assigning class ALL_REGS to slow/invalid mem r288
  103: r203:SI=r288:SI<<0x1+r196:DI#0
      REG_DEAD r196:DI
    Inserting slow/invalid mem reload before:
  316: r287:DI=[r105:DI*0x8+r140:DI]
  317: r288:SI=r287:DI#0
----------------------------------------------------------------------------

The problem is with r287.  We rightly give it a broad starting class of
POINTER_AND_FP_REGS (reduced from ALL_REGS by preferred_reload_class).
However, we never make forward progress towards narrowing it down to
a specific choice of class (POINTER_REGS or FP_REGS).

I think in practice we rely on two things to narrow a reload pseudo's
class down to a specific choice:

(1) a restricted class is specified when the pseudo is created

    This happens for input address reloads, where the class is taken
    from the target's chosen base register class.  It also happens
    for simple REG reloads, where the class is taken from the chosen
    alternative's constraints.

(2) uses of the reload pseudo as a direct input operand

    In this case get_reload_reg tries to reuse the existing register
    and narrow its class, instead of creating a new reload pseudo.

However, neither occurs here.  As described above, r287 rightly
starts out with a wide choice of class, ultimately derived from
ALL_REGS, so we don't get (1).  And as the comments in the PR
explain, r287 is never used as an input reload, only the subreg is,
so we don't get (2):

----------------------------------------------------------------------------
         Choosing alt 13 in insn 317:  (0) r  (1) w {*movsi_aarch64}
      Creating newreg=291, assigning class FP_REGS to r291
  317: r288:SI=r291:SI
    Inserting insn reload before:
  320: r291:SI=r287:DI#0
----------------------------------------------------------------------------

IMO, in this case we should rely on the reload of r316 to narrow
down the class of r278.  Currently we do:

----------------------------------------------------------------------------
         Choosing alt 7 in insn 316:  (0) r  (1) m {*movdi_aarch64}
      Creating newreg=289 from oldreg=287, assigning class GENERAL_REGS to r289
  316: r289:DI=[r105:DI*0x8+r140:DI]
    Inserting insn reload after:
  318: r287:DI=r289:DI
---------------------------------------------------

i.e. we create a new pseudo register r289 and give *that* pseudo
GENERAL_REGS instead.  This is because get_reload_reg only narrows
down the existing class for OP_IN and OP_INOUT, not OP_OUT.

But if we have a reload pseudo in a reload instruction and have chosen
a specific class for the reload pseudo, I think we should simply install
it for OP_OUT reloads too, if the class is a subset of the existing class.
We will need to pick such a register whatever happens (for r289 in the
example above).  And as explained in the PR, doing this actually avoids
an unnecessary move via the FP registers too.

The patch is quite aggressive in that it does this for all reload
pseudos in all reload instructions.  I wondered about reusing the
condition for a reload move in in_class_p:

          INSN_UID (curr_insn) >= new_insn_uid_start
          && curr_insn_set != NULL
          && ((OBJECT_P (SET_SRC (curr_insn_set))
               && ! CONSTANT_P (SET_SRC (curr_insn_set)))
              || (GET_CODE (SET_SRC (curr_insn_set)) == SUBREG
                  && OBJECT_P (SUBREG_REG (SET_SRC (curr_insn_set)))
                  && ! CONSTANT_P (SUBREG_REG (SET_SRC (curr_insn_set)))))))

but I can't really justify that on first principles.  I think we
should apply the rule consistently until we have a specific reason
for doing otherwise.

gcc/
	PR rtl-optimization/96796
	* lra-constraints.c (in_class_p): Add a default-false
	allow_all_reload_class_changes_p parameter.  Do not treat
	reload moves specially when the parameter is true.
	(get_reload_reg): Try to narrow the class of an existing OP_OUT
	reload if we're reloading a reload pseudo in a reload instruction.

gcc/testsuite/
	PR rtl-optimization/96796
	* gcc.c-torture/compile/pr96796.c: New test.