ref: 67ac68e399f892e50455281215f28c042422f38e
parent: d217c87139a3218d9dc4154782de53b9d0cc1119
author: Johann <johannkoenig@google.com>
date: Thu May 4 05:07:28 EDT 2017
variance neon: assert overflow conditions Change-Id: I12faca82d062eb33dc48dfeb39739b25112316cd
--- a/vpx_dsp/arm/variance_neon.c
+++ b/vpx_dsp/arm/variance_neon.c
@@ -9,6 +9,7 @@
*/
#include <arm_neon.h>
+#include <assert.h>
#include "./vpx_dsp_rtcd.h"
#include "./vpx_config.h"
@@ -32,7 +33,13 @@
return vget_lane_s32(c, 0);
}
-// w * h must be less than 2048 or sum_s16 may overflow.
+// The variance helper functions use int16_t for sum. 8 values are accumulated
+// and then added (at which point they expand up to int32_t). To avoid overflow,
+// there can be no more than 32767 / 255 ~= 128 values accumulated in each
+// column. For a 32x32 buffer, this results in 32 / 8 = 4 values per row * 32
+// rows = 128. Asserts have been added to each function to warn against reaching
+// this limit.
+
// Process a block of width 4 four rows at a time.
static void variance_neon_w4x4(const uint8_t *a, int a_stride, const uint8_t *b,
int b_stride, int h, uint32_t *sse, int *sum) {
@@ -41,6 +48,9 @@
int32x4_t sse_lo_s32 = vdupq_n_s32(0);
int32x4_t sse_hi_s32 = vdupq_n_s32(0);
+ // Since width is only 4, sum_s16 only loads a half row per loop.
+ assert(h <= 256);
+
for (i = 0; i < h; i += 4) {
const uint8x16_t a_u8 = load_unaligned_u8q(a, a_stride);
const uint8x16_t b_u8 = load_unaligned_u8q(b, b_stride);
@@ -73,7 +83,6 @@
*sse = (uint32_t)horizontal_add_s32x4(vaddq_s32(sse_lo_s32, sse_hi_s32));
}
-// w * h must be less than 2048 or sum_s16 may overflow.
// Process a block of any size where the width is divisible by 16.
static void variance_neon_w16(const uint8_t *a, int a_stride, const uint8_t *b,
int b_stride, int w, int h, uint32_t *sse,
@@ -83,6 +92,10 @@
int32x4_t sse_lo_s32 = vdupq_n_s32(0);
int32x4_t sse_hi_s32 = vdupq_n_s32(0);
+ // The loop loads 16 values at a time but doubles them up when accumulating
+ // into sum_s16.
+ assert(w / 8 * h <= 128);
+
for (i = 0; i < h; ++i) {
for (j = 0; j < w; j += 16) {
const uint8x16_t a_u8 = vld1q_u8(a + j);
@@ -117,7 +130,6 @@
*sse = (unsigned int)horizontal_add_s32x4(vaddq_s32(sse_lo_s32, sse_hi_s32));
}
-// w * h must be less than 2048 or sum_s16 may overflow.
// Process a block of width 8 two rows at a time.
static void variance_neon_w8x2(const uint8_t *a, int a_stride, const uint8_t *b,
int b_stride, int h, uint32_t *sse, int *sum) {
@@ -125,6 +137,9 @@
int16x8_t sum_s16 = vdupq_n_s16(0);
int32x4_t sse_lo_s32 = vdupq_n_s32(0);
int32x4_t sse_hi_s32 = vdupq_n_s32(0);
+
+ // Each column has it's own accumulator entry in sum_s16.
+ assert(h <= 128);
do {
const uint8x8_t a_0_u8 = vld1_u8(a);