Hi all,

Consider the code:
typedef long long v2di __attribute__ ((vector_size (16)));
  
void
store_laned (v2di x, long long *y)
{
   y[0] = x[1];
   y[3] = x[0];
}

AArch64 GCC will generate:
store_laned:
         umov    x1, v0.d[0]
         st1     {v0.d}[1], [x0]
         str     x1, [x0, 24]
         ret

It moves the zero lane into a core register and does a scalar store when instead it could have used a scalar FP store
that supports the required addressing mode:
store_laned:
         st1     {v0.d}[1], [x0]
         str     d0, [x0, 24]
         ret

Combine already tries to match this pattern:

Trying 10 -> 11:
Failed to match this instruction:
(set (mem:DI (plus:DI (reg/v/f:DI 76 [ y ])
             (const_int 24 [0x18])) [1 MEM[(long long int *)y_4(D) + 24B]+0 S8 A64])
     (vec_select:DI (reg/v:V2DI 75 [ x ])
         (parallel [
                 (const_int 0 [0])
             ])))

but we don't match it in the backend. It's not hard to add it, so this patch does that for all the relevant vector modes.
With this patch we generate the second sequence above and in SPEC2006 eliminate some address computation instructions
because we use the more expressive STR instead of ST1 or we eliminate such moves to the integer registers because we
can just do the store of the D-reg.

Bootstrapped and tested on aarch64-none-linux-gnu.

Ok for trunk?

Thanks,
Kyrill

2017-04-21  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

	* config/aarch64/aarch64-simd.md (aarch64_store_lane0<mode>):
	New pattern.

2017-04-21  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

	* gcc.target/aarch64/store_lane0_str_1.c: New test.