Add comments in vp9_diamond_search_sad_avx()

Added comments related to re-arranging the
elements of the SAD vector to find the
minimum.

Change-Id: I58b702d304a6cdd32f04775fba603e39c19a8947

diff --git a/vp9/encoder/x86/vp9_diamond_search_sad_avx.c b/vp9/encoder/x86/vp9_diamond_search_sad_avx.c
index c00579e..63c35df 100644 (file)
--- a/vp9/encoder/x86/vp9_diamond_search_sad_avx.c
+++ b/vp9/encoder/x86/vp9_diamond_search_sad_avx.c
@@ -233,12 +233,19 @@ int vp9_diamond_search_sad_avx(const MACROBLOCK *x,
         if (UNLIKELY(local_best_sad == 0xffff)) {
           __m128i v_loval_d, v_hival_d, v_loidx_d, v_hiidx_d, v_sel_d;
 
+          // Re-arrange the values in v_sad_d as follows:
+          // v_loval_d[0] = v_sad_d[0], v_loval_d[1] = v_sad_d[2]
+          // v_loval_d[2] = v_sad_d[1], v_loval_d[3] = v_sad_d[3]
+          // v_loidx_d stores the corresponding indices 0, 2, 1, 3
+          // This re-arrangement is required to ensure that when there exists
+          // more than one minimum, the one with the least index is selected
           v_loval_d = _mm_shuffle_epi32(v_sad_d, 0xd8);
           v_loidx_d = _mm_set_epi32(3, 1, 2, 0);
 
           v_hival_d = _mm_srli_si128(v_loval_d, 8);
           v_hiidx_d = _mm_srli_si128(v_loidx_d, 8);
 
+          // Compare if v_sad_d[1] < v_sad_d[0], v_sad_d[3] < v_sad_d[2]
           v_sel_d = _mm_cmplt_epi32(v_hival_d, v_loval_d);
 
           v_loval_d = _mm_blendv_epi8(v_loval_d, v_hival_d, v_sel_d);
@@ -246,6 +253,7 @@ int vp9_diamond_search_sad_avx(const MACROBLOCK *x,
           v_hival_d = _mm_srli_si128(v_loval_d, 4);
           v_hiidx_d = _mm_srli_si128(v_loidx_d, 4);
 
+          // min(v_sad_d[2], v_sad_d[3]) < min(v_sad_d[0], v_sad_d[1])
           v_sel_d = _mm_cmplt_epi32(v_hival_d, v_loval_d);
 
           v_loval_d = _mm_blendv_epi8(v_loval_d, v_hival_d, v_sel_d);