+/* Shift A right by B bits portably, by dividing A by 2**B and
+ truncating towards minus infinity. A and B should be free of side
+ effects, and B should be in the range 0 <= B <= INT_BITS - 2, where
+ INT_BITS is the number of useful bits in an int. GNU code can
+ assume that INT_BITS is at least 32.
+
+ ISO C99 says that A >> B is implementation-defined if A < 0. Some
+ implementations (e.g., UNICOS 9.0 on a Cray Y-MP EL) don't shift
+ right in the usual way when A < 0, so SHR falls back on division if
+ ordinary A >> B doesn't seem to be the usual signed shift. */
+#define SHR(a, b) \
+ (-1 >> 1 == -1 \
+ ? (a) >> (b) \
+ : (a) / (1 << (b)) - ((a) % (1 << (b)) < 0))
+