Add MOD to match mod with powers of 2

include/global.h

@@ -85,6 +85,12 @@
 #define SAFE_DIV(a, b) ((a) / (b))
 #endif
 
+// The below macro does a%n, but (to match) will switch to a&(n-1) if n is a power of 2.
+// There are cases where GF does a&(n-1) where we would really like to have a%n, because
+// if n is changed to a value that isn't a power of 2 then a&(n-1) is unlikely to work as
+// intended, and a%n for powers of 2 isn't always optimized to use &.
+#define MOD(a, n)(((n) & ((n)-1)) ? ((a) % (n)) : ((a) & ((n)-1)))
+
 // Extracts the upper 16 bits of a 32-bit number
 #define HIHALF(n) (((n) & 0xFFFF0000) >> 16)