Question

My question is, within C++, is the following code defined? Some of it? And if it is, what’s it supposed to do in these four scenarios?

word <<  100;
word >>  100;
word << -100;
word >> -100;

word is a uint32_t

(This is for a bottleneck in a 3d lighting renderer. One of the more minor improvements in the inner most loop I wanna make is eliminating needless conditional forks. One of those forks is checking to see if a left shift should be done on several 32 bit words as part of a hamming weight count. If the left shift accepts absurd values, the checks don’t need done at all)

Answer 1

In the C++0X draft N3290, §5.8:

The behavior is undefined if the right operand is negative,
or greater than or equal to the length in bits of the promoted left
operand.

Note: the above paragraph is identical in the C++03 standard.

So the last two are undefined. The others, I believe depend on whether word is signed or not, if word is at least 101bits long. If word is “smaller” than 101bits, the above applies and the behavior is undefined.

Here are the next two sections of that paragraph in C++0X (these do differ in C++03):

The value of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits are zero-filled. If E1 has an unsigned
type, the value of the result is E1 × 2E2 , reduced modulo one more than the maximum value representable
in the result type. Otherwise, if E1 has a signed type and non-negative value, and E1 × 2E2 is representable
in the result type, then that is the resulting value; otherwise, the behavior is undefined.

The value of E1 >> E2 is E1 right-shifted E2 bit positions. If E1 has an unsigned type or if E1 has a signed
type and a non-negative value, the value of the result is the integral part of the quotient of E1/2E2 . If E1
has a signed type and a negative value, the resulting value is implementation-defined.