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Restrict get_msb inputs

Add a warning and assert that inputs for get_msb must not be zero.

Change-Id: I8c6f289ff13248f6e3a8bc24aab3712ed33022a6

--- a/vpx_ports/bitops.h

+++ b/vpx_ports/bitops.h

@@ -11,6 +11,8 @@

 #ifndef VPX_PORTS_BITOPS_H_

 #define VPX_PORTS_BITOPS_H_

+#include <assert.h>

+

 #include "vpx_ports/msvc.h"

 #ifdef _MSC_VER

@@ -25,10 +27,15 @@

 extern "C" {

 #endif

+// These versions of get_msb() are only valid when n != 0 because all

+// of the optimized versions are undefined when n == 0:

+// https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html

+

 // use GNU builtins where available.

 #if defined(__GNUC__) && \

     ((__GNUC__ == 3 && __GNUC_MINOR__ >= 4) || __GNUC__ >= 4)

 static INLINE int get_msb(unsigned int n) {

+  assert(n != 0);

   return 31 ^ __builtin_clz(n);

 }

 #elif defined(USE_MSC_INTRINSICS)

@@ -36,6 +43,7 @@

 static INLINE int get_msb(unsigned int n) {

   unsigned long first_set_bit;

+  assert(n != 0);

   _BitScanReverse(&first_set_bit, n);

   return first_set_bit;

 }

@@ -46,6 +54,8 @@

   int log = 0;

   unsigned int value = n;

   int i;

+

+  assert(n != 0);

   for (i = 4; i >= 0; --i) {

     const int shift = (1 << i);