Question

I think I’m going insane with this.

I have a a piece of code that needs to create an (unsigned) integer with N consequent bits set to 1. To be exact I have a bitmask, and in some situations I’d like to set it to a solid rnage.

I have the following function:

void MaskAddRange(UINT& mask, UINT first, UINT count)
{
    mask |= ((1 << count) - 1) << first;
}

In simple words: 1 << count in binary representation is 100...000 (number of zeroes is count), subtracting 1 from such a number gives 011...111, and then we just left-shift it by first.

The above should yield correct result, when the following obvious limitation is met:

first + count <= sizeof(UINT)*8 = 32

Note that it should also work correctly for “extreme” cases.

• if count = 0 we have (1 << count) = 1, and hence ((1 << count) - 1) = 0.
• if count = 32 we have (1 << count) = 0, since the leading bit overflows, and according to C/C++ rules bitwise shift operators are not cyclic. Then ((1 << count) - 1) = -1 (all bits set).

However, as turned out, for count = 32 the formula doesn’t work as expected. As discovered:

UINT n = 32;
UINT x = 1 << n;
// the value of x is 1

Moreover, I’m using MSVC2005 IDE. When I evaluate the above expression in the debugger, the result is 0. However when I step over the above line, x gets value of 1. Lokking via the disassembler we see the following:

mov eax,1 
mov ecx,dword ptr [ebp-0Ch] // ecx = n
shl eax,cl                  // eax <<= LOBYTE(ecx)
mov dword ptr [ebp-18h],eax // n = ecx

There’s no magic indeed, compiler just used shl instruction. Then it seems that shl doesn’t do what I expected it should do. Either CPU decides to ignore this instruction, or the shift is treated modulo 32, or donno what.

My questions are:

• What is the correct behavior of shl/shr instructions?
• Is there a CPU flag controlling the bitshift instructions?
• Is this according to C/C++ standard?

Thanks in advance

Edit:

Thanks for answers. I’ve realized that (1) shl/shr indeed treat operand modulo 32 (or & 0x1F) and (2) C/C++ standard treats shift by more than 31 bits as undefined behavior.

Then I have one more question. How can I rewrite my “masking” expression to cover this extreme case too. It should be without branching (if, ?). What’d be the simplest expression?

Answer 1

1U << 32 is undefined behavior in C and in C++ when type unsigned int is 32-bit wide.

(C11, 6.5.7p3) "If the value of the right operand is negative or is greater than or equal to the width of the promoted left operand, the behavior is undefined"

(C++11, 5.8p1) "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."