Tighten LRA test for reloading the inner reg of a paradoxical subreg

Tighten LRA test for reloading the inner reg of a paradoxical subreg

This patch fixes an LRA cycling problem on the attached testcase.
The original insn was:

(insn 74 72 76 8 (set (reg:V2DI 287 [ _166 ])
        (subreg:V2DI (reg/v/f:DI 112 [ d ]) 0)) 1060 {*aarch64_simd_movv2di}
     (nil))

which IRA converted to:

(insn 74 72 580 8 (set (reg:V2DI 287 [ _166 ])
        (subreg:V2DI (reg/v/f:DI 517 [orig:112 d ] [112]) 0)) 1060 {*aarch64_simd_movv2di}
     (nil))

after creating loop allocnos.  It happens that the ALLOCNO_WMODEs for
both 112 and 517 were not set to V2DI due to another bug that I'll post
a separate patch for, but we nevertheless got a valid allocation of
register 1.

LRA's first try at constraining the instruction gave:

         Choosing alt 5 in insn 74:  (0) ?w  (1) r {*aarch64_simd_movv2di}

at which point all was good.  But LRA later decided it needed
to spill r517:

    Spill r517 after risky transformations

so the next constraint attempt gave:

         Choosing alt 0 in insn 74:  (0) =w  (1) m {*aarch64_simd_movv2di}

which was still good.  Then during inheritance we had:

      Creating newreg=672 from oldreg=517, assigning class GENERAL_REGS to inheritance r672
    Original reg change 517->672 (bb8):
   74: r287:V2DI=r672:DI#0
    Add inheritance<-original before:
  939: r672:DI=r517:DI

    Inheritance reuse change 517->672 (bb8):
  620: r572:DI=r672:DI
      REG_DEAD r672:DI

    Use smallest class of POINTER_REGS and GENERAL_REGS
      Creating newreg=673 from oldreg=517, assigning class POINTER_REGS to inheritance r673
    Original reg change 517->673 (bb8):
  936: r669:DI=r673:DI
    Add inheritance<-original before:
  940: r673:DI=r517:DI

("Use smallest class of POINTER_REGS and GENERAL_REGS" ought to
give GENERAL_REGS.  That might be a missed optimisation, and probably
due to both classes having the same number of allocatable registers.
I'll look at that as a follow-on.)

Thus LRA created two inheritance registers for r517, one (r673)
that included the unallocatable x31 and another (r672) that didn't.
The r672 references included the paradoxical subreg in insn 74 but the
r673 ones didn't.  LRA then allocated x30 to r673, which was a valid
choice.

Later LRA decided to "undo" the inheritance for insn 620, but because
of the double inheritance, it got confused as to what the original
situation was, and made insn 74 use the other inheritance register
instead of r517:

********** Undoing inheritance #2: **********

Inherit 11 out of 12 (91.67%)
   Insn after restoring regs:
  620: r572:DI=r517:DI
      REG_DEAD r517:DI
    Change reload insn:
   74: r287:V2DI=r673:DI#0       <-------------------
   Insn after restoring regs:
  939: r517:DI=r673:DI
      REG_DEAD r673:DI

This might be a bug in itself: we should probably look through sets
of other inheritance pseudos to find the "real" origin.

Either way, at this point we had a situation in which r673 was used in an
insn whose subreg was larger than the biggest_mode that r673 had when it
was allocated.  While x30 was valid for the original biggest_mode, it
wasn't valid for this subreg use.

The next attempt to constrain insn 74 was:

        Choosing alt 5 in insn 74:  (0) ?w  (1) r {*aarch64_simd_movv2di}
      Creating newreg=684, assigning class GENERAL_REGS to r684
   74: r287:V2DI=r684:V2DI
    Inserting insn reload before:
  951: r684:V2DI=r673:DI#0

where LRA reloaded the SUBREG rather than the SUBREG_REG.  And it
then cycled trying the same thing when reloading the reload (and the
reload of the reload, etc.).

What it should be doing here is reloading the SUBREG_REG instead.
There's already code to cope with this case when the paradoxical
subreg falls outside the class (which isn't true here, since r673
is POINTER_REGS and POINTER_REGS includes x31).  But I think we
should also test whether LRA is entitled to allocate the spanned
registers.  Not doing that seems like a bug regardless of the above
missed optimisation and the mix-up undoing inheritance.

2018-05-30  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* lra-constraints.c (simplify_operand_subreg): In the paradoxical
	case, check whether the outer register overlaps an unallocatable
	register, not just whether it fits the required class.

gcc/testsuite/
	* g++.dg/torture/aarch64-vect-init-1.C: New test.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@261531 138bc75d-0d04-0410-961f-82ee72b054a4