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Pad 'Left' when building under ASan

The neon intrinsics are not able to load just the 4 values that are
used. In vpx_dsp/arm/intrapred_neon.c:dc_4x4 it loads 8 values for both
the 'above' and 'left' computations, but only uses the sum of the first
4 values.

BUG=webm:1268

Change-Id: I937113d7e3a21e25bebde3593de0446bf6b0115a

--- a/vp8/common/reconintra4x4.c

+++ b/vp8/common/reconintra4x4.c

@@ -15,6 +15,8 @@

 #include "vp8_rtcd.h"

 #include "blockd.h"

 #include "reconintra4x4.h"

+#include "vp8/common/common.h"

+#include "vpx_ports/mem.h"

 typedef void (*intra_pred_fn)(uint8_t *dst, ptrdiff_t stride,

                               const uint8_t *above, const uint8_t *left);

@@ -38,8 +40,19 @@

                           int left_stride, B_PREDICTION_MODE b_mode,

                           unsigned char *dst, int dst_stride,

                           unsigned char top_left) {

-  unsigned char Left[4];

   unsigned char Aboveb[12], *Above = Aboveb + 4;

+#if HAVE_NEON

+  // Neon intrinsics are unable to load 32 bits, or 4 8 bit values. Instead, it

+  // over reads but does not use the extra 4 values.

+  unsigned char Left[8];

+#if VPX_WITH_ASAN

+  // Silence an 'uninitialized read' warning. Although uninitialized values are

+  // indeed read, they are not used.

+  vp8_zero_array(Left, 8);

+#endif  // VPX_WITH_ASAN

+#else

+  unsigned char Left[4];

+#endif  // HAVE_NEON

   Left[0] = yleft[0];

   Left[1] = yleft[left_stride];

--- a/vpx_ports/mem.h

+++ b/vpx_ports/mem.h

@@ -48,4 +48,14 @@

 #define CONVERT_TO_BYTEPTR(x) ((uint8_t *)(((uintptr_t)(x)) >> 1))

 #endif  // CONFIG_VP9_HIGHBITDEPTH

+#if !defined(__has_feature)

+#define __has_feature(x) 0

+#endif  // !defined(__has_feature)

+

+#if __has_feature(address_sanitizer) || __SANITIZE_ADDRESS__

+#define VPX_WITH_ASAN 1

+#else

+#define VPX_WITH_ASAN 0

+#endif  // __has_feature(address_sanitizer) || __SANITIZE_ADDRESS

+

 #endif  // VPX_PORTS_MEM_H_