INT14-A. Distinguish bitmaps from numeric types

Avoid performing bit manipulation and arithmetic operations on the same variable. Though such operations are valid and will compile, they can reduce code readability. Declaring a variable as containing a numeric value or a bitmap makes the programmer's intentions clearer and can lead to better code maintainability.

Non-Compliant Code Example 1

In this non-compliant code example, both bit manipulation and arithmetic manipulation is performed on the integer type x. The result is a (prematurely) optimized statement that assigns 5x + 1 to x for implementations where integers are represented as two's complement values.

int x = 50;
x += (x << 2) + 1;

Although this is a legal manipulation, the result of the shift depends on the underlying representation of the integer type, and is consequently implementation-defined. Additionally, the readability of the code is impaired.

Compliant Solution 1

In this compliant solution, the assignment statement is modified to reflect the arithmetic nature of x resulting in a clearer indication of the programmer's intentions.

int x = 50;
x = 5 * x + 1;

A reviewer may now recognize that the operation should be checked for integer overflow. This might not have been apparent in the original, non-compliant code example.

Non-Compliant Code Example 2

In this non-compliant code example, the programmer attempts to (prematurely) optimize dividing{{x}} by 4.

int x = -50;
x >>= 2;

Although this code is likely to perform a division by 4, it is not guaranteed to. If x has a signed type and a negative value, the operation is implementation-defined and could be implemented as either an arithmetic shift or a logical shift.

For example, if the internal representation of x is 0xFFFF FFCE (two's-complement), an arithmetic shift results in 0xFFFF FFF3 (-13 in two's-complement), while a logical shift results in 0x3FFF FFF3 (1 073 741 811 in two's-complement).

The result of the shift also depends on the underlying representation of the integer type.

Compliant Solution 2

In this compliant solution, the shift is replaced by a division operation so that the intention is clear.

int x = -50;
x /= 4;

The resulting value is now more likely to be consistent with the programmers expectations.

Compliant Practice

To further separate bit collections and numeric types, it might be prudent to define a bitmapped type. A programmer can then run automated tools over the code in question to verify that only bit manipulations are performed on variables of this type.

typedef int bitmap;
bitmap x = 0x7f3;
x = (x << 2) | 3; /* shifts in two 1-bits from the right */

Risk Assessment

By complicating information regarding how a variable is used in code, it is difficult to determine which checks must be performed to ensure data validity. Explicitly stating how a variable is used determines which checks to perform.

References

[[ISO/IEC 9899-1999]] Section 6.2.6.2, "Integer types"

Steele, G. L. 1977. Arithmetic shifting considered harmful. SIGPLAN Not. 12, 11 (Nov. 1977), 61-69.