shithub: libvpx

Download patch

ref: ab0e7a237a6e9796c15a8858caac04dea3593d62
parent: ada850786c7e0a5d6024b7716db9896124fd483e
author: Johann <johannkoenig@google.com>
date: Tue Aug 2 11:59:35 EDT 2016 

Use shifted value for sinpi8sqrt2

The value 35468 changes sign when stored in int16_t:
implicit conversion from 'int' to 'int16_t' (aka 'short')
changes value from 35468 to -30068

This negation requires adding back the original value to compensate.
Shifting the value keeps the value positive and saves a post-vqdmulh
shift.

This technique is used in webp and idct_dequant_full_2x_neon

BUG=b/28027557

Change-Id: I0c5ce09bea170fe08061856c2af6f841a557e0c3


--- a/test/idct_test.cc
+++ b/test/idct_test.cc
@@ -115,6 +115,10 @@
 }
 
 INSTANTIATE_TEST_CASE_P(C, IDCTTest, ::testing::Values(vp8_short_idct4x4llm_c));
+#if HAVE_NEON
+INSTANTIATE_TEST_CASE_P(NEON, IDCTTest,
+                        ::testing::Values(vp8_short_idct4x4llm_neon));
+#endif
 #if HAVE_MMX
 INSTANTIATE_TEST_CASE_P(MMX, IDCTTest,
                         ::testing::Values(vp8_short_idct4x4llm_mmx));
--- a/vp8/common/arm/neon/dequant_idct_neon.c
+++ b/vp8/common/arm/neon/dequant_idct_neon.c
@@ -11,7 +11,11 @@
 #include <arm_neon.h>
 
 static const int16_t cospi8sqrt2minus1 = 20091;
-static const int16_t sinpi8sqrt2 = 35468;
+// 35468 exceeds INT16_MAX and gets converted to a negative number. Because of
+// the way it is used in vqdmulh, where the result is doubled, it can be divided
+// by 2 beforehand. This saves compensating for the negative value as well as
+// shifting the result.
+static const int16_t sinpi8sqrt2 = 35468 >> 1;
 
 void vp8_dequant_idct_add_neon(int16_t *input, int16_t *dq, unsigned char *dst,
                                int stride) {
@@ -60,10 +64,8 @@
   q3 = vqdmulhq_n_s16(q2, sinpi8sqrt2);
   q4 = vqdmulhq_n_s16(q2, cospi8sqrt2minus1);
 
-  q3 = vshrq_n_s16(q3, 1);
   q4 = vshrq_n_s16(q4, 1);
 
-  q3 = vqaddq_s16(q3, q2);
   q4 = vqaddq_s16(q4, q2);
 
   d10 = vqsub_s16(vget_low_s16(q3), vget_high_s16(q4));
@@ -90,10 +92,8 @@
   d12 = vqadd_s16(d2tmp2.val[0], d2tmp3.val[0]);
   d13 = vqsub_s16(d2tmp2.val[0], d2tmp3.val[0]);
 
-  q3 = vshrq_n_s16(q3, 1);
   q4 = vshrq_n_s16(q4, 1);
 
-  q3 = vqaddq_s16(q3, q2);
   q4 = vqaddq_s16(q4, q2);
 
   d10 = vqsub_s16(vget_low_s16(q3), vget_high_s16(q4));
--- a/vp8/common/arm/neon/shortidct4x4llm_neon.c
+++ b/vp8/common/arm/neon/shortidct4x4llm_neon.c
@@ -11,7 +11,11 @@
 #include <arm_neon.h>
 
 static const int16_t cospi8sqrt2minus1 = 20091;
-static const int16_t sinpi8sqrt2 = 35468;
+// 35468 exceeds INT16_MAX and gets converted to a negative number. Because of
+// the way it is used in vqdmulh, where the result is doubled, it can be divided
+// by 2 beforehand. This saves compensating for the negative value as well as
+// shifting the result.
+static const int16_t sinpi8sqrt2 = 35468 >> 1;
 
 void vp8_short_idct4x4llm_neon(int16_t *input, unsigned char *pred_ptr,
                                int pred_stride, unsigned char *dst_ptr,
@@ -40,10 +44,8 @@
   d12 = vqadd_s16(vget_low_s16(q1s16), vget_high_s16(q1s16));  // a1
   d13 = vqsub_s16(vget_low_s16(q1s16), vget_high_s16(q1s16));  // b1
 
-  q3s16 = vshrq_n_s16(q3s16, 1);
   q4s16 = vshrq_n_s16(q4s16, 1);
 
-  q3s16 = vqaddq_s16(q3s16, q2s16);
   q4s16 = vqaddq_s16(q4s16, q2s16);
 
   d10 = vqsub_s16(vget_low_s16(q3s16), vget_high_s16(q4s16));  // c1
@@ -71,10 +73,8 @@
   d12 = vqadd_s16(vget_low_s16(q1s16), vget_high_s16(q1s16));  // a1
   d13 = vqsub_s16(vget_low_s16(q1s16), vget_high_s16(q1s16));  // b1
 
-  q3s16 = vshrq_n_s16(q3s16, 1);
   q4s16 = vshrq_n_s16(q4s16, 1);
 
-  q3s16 = vqaddq_s16(q3s16, q2s16);
   q4s16 = vqaddq_s16(q4s16, q2s16);
 
   d10 = vqsub_s16(vget_low_s16(q3s16), vget_high_s16(q4s16));  // c1