AV_WN64 is meant for unaligned data, but the existing av_alias* unions
(without a definition for the av_alias attribute - we don't have one
for MSVC) indicate to the compiler that they would have sufficient
alignment for normal access, i.e. the compiler is free to assume
8 byte alignment.
On ARM, this makes sure that AV_WN64 (or two consecutive AV_WN32) is
done with two str instructions instead of one strd.
---
Or should we define av_alias to __unaligned in attributes.h instead?
---
libavutil/intreadwrite.h | 5 +++++
1 file changed, 5 insertions(+)
diff --git a/libavutil/intreadwrite.h b/libavutil/intreadwrite.h
index f77fd60..0ba9254 100644
--- a/libavutil/intreadwrite.h
+++ b/libavutil/intreadwrite.h
@@ -197,6 +197,11 @@ union unaligned_16 { uint16_t l; } __attribute__((packed))
av_alias;
# define AV_RN(s, p) (*((const __unaligned uint##s##_t*)(p)))
# define AV_WN(s, p, v) (*((__unaligned uint##s##_t*)(p)) = (v))
+#elif defined(_MSC_VER) && (defined(_M_ARM) || defined(_M_X64))
+
+# define AV_RN(s, p) (*((const __unaligned uint##s##_t*)(p)))
+# define AV_WN(s, p, v) (*((__unaligned uint##s##_t*)(p)) = (v))
+
#elif AV_HAVE_FAST_UNALIGNED
# define AV_RN(s, p) (((const av_alias##s*)(p))->u##s)
--
2.7.4
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