libvpx: Pull from upstream

source/libvpx/vp9/common/vp9_systemdependent.h

Index: source/libvpx/vp9/common/vp9_systemdependent.h
===================================================================
--- source/libvpx/vp9/common/vp9_systemdependent.h	(revision 247498)
+++ source/libvpx/vp9/common/vp9_systemdependent.h	(working copy)
@@ -11,6 +11,10 @@
 #ifndef VP9_COMMON_VP9_SYSTEMDEPENDENT_H_
 #define VP9_COMMON_VP9_SYSTEMDEPENDENT_H_
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #ifdef _MSC_VER
 #include <math.h>
 #define snprintf _snprintf
@@ -34,7 +38,46 @@
 }
 #endif
 
+// use GNU builtins where available.
+#if defined(__GNUC__) && \
+    ((__GNUC__ == 3 && __GNUC_MINOR__ >= 4) || __GNUC__ >= 4)
+static INLINE int get_msb(unsigned int n) {
+  return 31 ^ __builtin_clz(n);
+}
+#elif defined(_MSC_VER) && _MSC_VER > 1310 && \
+      (defined(_M_X64) || defined(_M_IX86))
+#include <intrin.h>
+#pragma intrinsic(_BitScanReverse)
+
+static INLINE int get_msb(unsigned int n) {
+  unsigned long first_set_bit;
+  _BitScanReverse(&first_set_bit, n);
+  return first_set_bit;
+}
+#else
+// Returns (int)floor(log2(n)). n must be > 0.
+static INLINE int get_msb(unsigned int n) {
+  int log = 0;
+  unsigned int value = n;
+  int i;
+
+  for (i = 4; i >= 0; --i) {
+    const int shift = (1 << i);
+    const unsigned int x = value >> shift;
+    if (x != 0) {
+      value = x;
+      log += shift;
+    }
+  }
+  return log;
+}
+#endif
+
 struct VP9Common;
 void vp9_machine_specific_config(struct VP9Common *cm);
 
+#ifdef __cplusplus
+}  // extern "C"
+#endif
+
 #endif  // VP9_COMMON_VP9_SYSTEMDEPENDENT_H_