simplify_operand_subreg tries to detect whether the allocation for
a pseudo in a paradoxical subreg is also valid for the outer mode.
The condition it used to check for an invalid combination was:
else if (REG_P (reg)
&& REGNO (reg) >= FIRST_PSEUDO_REGISTER
&& (hard_regno = lra_get_regno_hard_regno (REGNO (reg))) >= 0
&& (hard_regno_nregs (hard_regno, innermode)
< hard_regno_nregs (hard_regno, mode))
&& (regclass = lra_get_allocno_class (REGNO (reg)))
&& (type != OP_IN
|| !in_hard_reg_set_p (reg_class_contents[regclass],
mode, hard_regno)
|| overlaps_hard_reg_set_p (lra_no_alloc_regs,
mode, hard_regno)))
I think there are two problems with this:
(1) It never actually checks whether the hard register is valid for the
outer mode (in the hard_regno_mode_ok sense). If it isn't, any attempt
to reload in the outer mode is likely to cycle, because the implied
regno/mode combination will be just as invalid next time
curr_insn_transform sees the subreg.
(2) The check is valid for little-endian only. For big-endian we need
to move hard_regno backwards.
Using simplify_subreg_regno should avoid both problems.
As the existing comment says, IRA should always take subreg references
into account when allocating hard registers, so this fix-up should only
really be needed for pseudos allocated by LRA itself.
gcc/
2020-03-21 Richard Sandiford <richard.sandiford@arm.com>
PR rtl-optimization/94052
* lra-constraints.c (simplify_operand_subreg): Reload the inner
register of a paradoxical subreg if simplify_subreg_regno fails
to give a valid hard register for the outer mode.
gcc/testsuite/
2020-03-21 Tamar Christina <tamar.christina@arm.com>
PR target/94052
* gcc.target/aarch64/pr94052.C: New test.
