shithub: dav1d

Info  •  Files  •  Log  •  Branches

Avoid masking the lsb in high bit-depth stride calculations

We specify most strides in bytes, but since C defines offsets
in multiples of sizeof(type) we use the PXSTRIDE() macro to
downshift the strides by one in high-bit depth templated files.

This however means that the compiler is required to mask away
the least significant bit, because it could in theory be non-zero.

Avoid that by telling the compiler (when compiled in release mode)
that the lsb is in fact guaranteed to always be zero.

--- a/include/common/bitdepth.h

+++ b/include/common/bitdepth.h

@@ -31,6 +31,8 @@

 #include <stdint.h>

 #include <string.h>

+#include "common/attributes.h"

+

 #if !defined(BITDEPTH)

 typedef void pixel;

 typedef void coef;

@@ -47,7 +49,7 @@

 #define iclip_pixel iclip_u8

 #define PIX_HEX_FMT "%02x"

 #define bitfn(x) x##_8bpc

-#define PXSTRIDE(x) x

+#define PXSTRIDE(x) (x)

 #define highbd_only(x)

 #define HIGHBD_DECL_SUFFIX /* nothing */

 #define HIGHBD_CALL_SUFFIX /* nothing */

@@ -70,7 +72,10 @@

 #define HIGHBD_TAIL_SUFFIX , bitdepth_max

 #define bitdepth_from_max(bitdepth_max) (32 - clz(bitdepth_max))

 #define bitfn(x) x##_16bpc

-#define PXSTRIDE(x) (x >> 1)

+static inline ptrdiff_t PXSTRIDE(const ptrdiff_t x) {

+    assert(!(x & 1));

+    return x >> 1;

+}

 #define highbd_only(x) x

 #else

 #error invalid value for bitdepth