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Use shifted value for sinpi8sqrt2

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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
This commit is contained in:
Johann 2016-08-02 15:59:35 -07:00
parent ada850786c
commit ab0e7a237a
3 changed files with 14 additions and 10 deletions
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4
test/idct_test.cc
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@ -115,6 +115,10 @@ TEST_P(IDCTTest, TestWithData) {
}
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));

10
vp8/common/arm/neon/dequant_idct_neon.c
View File

@ -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 @@ void vp8_dequant_idct_add_neon(int16_t *input, int16_t *dq, unsigned char *dst,
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 @@ void vp8_dequant_idct_add_neon(int16_t *input, int16_t *dq, unsigned char *dst,
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));

10
vp8/common/arm/neon/shortidct4x4llm_neon.c
View File

@ -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 @@ void vp8_short_idct4x4llm_neon(int16_t *input, unsigned char *pred_ptr,
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 @@ void vp8_short_idct4x4llm_neon(int16_t *input, unsigned char *pred_ptr,
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