Improve spilling for variable-width slots

Improve spilling for variable-width slots

Once SVE is enabled, a general AArch64 spill slot offset will be

  A + B * VL

where A is a constant and B is a multiple of the SVE vector length.
The offsets in SVE load and store instructions are a multiple of VL
(and so can encode some values of B), while offsets for standard AArch64
load and store instructions aren't (and encode some values of A).

We therefore get better spill code if variable-sized slots are grouped
together separately from constant-sized slots, and if variable-sized
slots are not reused for constant-sized data.  Then, spills to the
constant-sized slots can add B * VL to the offset first, creating a
common anchor point for spills with the same B component but different
A components.  Similarly, spills to variable-sized slots can add A to
the offset first, creating a common anchor point for spills with the same
A component but different B components.

This patch implements the sorting and grouping side of the optimisation.
A later patch creates the anchor points.

The patch is a no-op on other targets.

2018-01-03  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* lra-spills.c (pseudo_reg_slot_compare): Sort slots by whether
	they are variable or constant sized.
	(assign_stack_slot_num_and_sort_pseudos): Don't reuse variable-sized
	slots for constant-sized data.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r256208