Allow single-element interleaving for non-power-of-2 strides

[Jeff Law]

On 11/17/2017 08:33 AM, Richard Sandiford wrote:
> This allows LD3 to be used for isolated a[i * 3] accesses, in a similar
> way to the current a[i * 2] and a[i * 4] for LD2 and LD4 respectively.
> Given the problems with the cost model underestimating the cost of
> elementwise accesses, the patch continues to reject the VMAT_ELEMENTWISE
> cases that are currently rejected.
> 
> Tested on aarch64-linux-gnu (with and without SVE), x86_64-linux-gnu
> and powerpc64le-linux-gnu.  OK to install?
> 
> Richard
> 
> 
> 2017-11-17  Richard Sandiford  <richard.sandiford@linaro.org>
> 	    Alan Hayward  <alan.hayward@arm.com>
> 	    David Sherwood  <david.sherwood@arm.com>
> 
> gcc/
> 	* tree-vect-data-refs.c (vect_analyze_group_access_1): Allow
> 	single-element interleaving even if the size is not a power of 2.
> 	* tree-vect-stmts.c (get_load_store_type): Disallow elementwise
> 	accesses for single-element interleaving if the group size is
> 	not a power of 2.
> 
> gcc/testsuite/
> 	* gcc.target/aarch64/sve_struct_vect_18.c: New test.
> 	* gcc.target/aarch64/sve_struct_vect_18_run.c: Likewise.
> 	* gcc.target/aarch64/sve_struct_vect_19.c: Likewise.
> 	* gcc.target/aarch64/sve_struct_vect_19_run.c: Likewise.
OK.
jeff