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Manually unroll the inner loop of Neon sad16x_4d()

Manually unrolling the inner loop is sufficient to stop the compiler
getting confused and emitting inefficient code.

Co-authored by: James Greenhalgh <james.greenhalgh@arm.com>

Bug: b/181236880
Change-Id: I860768ce0e6c0e0b6286d3fc1b94f0eae95d0a1a

--- a/vpx_dsp/arm/sad4d_neon.c

+++ b/vpx_dsp/arm/sad4d_neon.c

@@ -243,7 +243,7 @@

 static INLINE void sad16x_4d(const uint8_t *src_ptr, int src_stride,

                              const uint8_t *const ref_array[4], int ref_stride,

                              uint32_t *res, const int height) {

-  int i, j;

+  int i;

   const uint8_t *ref_loop[4] = { ref_array[0], ref_array[1], ref_array[2],

                                  ref_array[3] };

   uint16x8_t sum[4] = { vdupq_n_u16(0), vdupq_n_u16(0), vdupq_n_u16(0),

@@ -252,10 +252,15 @@

   for (i = 0; i < height; ++i) {

     const uint8x16_t s = vld1q_u8(src_ptr);

     src_ptr += src_stride;

-    for (j = 0; j < 4; ++j) {

-      sad16_neon(ref_loop[j], s, &sum[j]);

-      ref_loop[j] += ref_stride;

-    }

+    /* Manual unrolling here stops the compiler from getting confused. */

+    sad16_neon(ref_loop[0], s, &sum[0]);

+    ref_loop[0] += ref_stride;

+    sad16_neon(ref_loop[1], s, &sum[1]);

+    ref_loop[1] += ref_stride;

+    sad16_neon(ref_loop[2], s, &sum[2]);

+    ref_loop[2] += ref_stride;

+    sad16_neon(ref_loop[3], s, &sum[3]);

+    ref_loop[3] += ref_stride;

   }

   sad_512_pel_final_neon(sum, res);