Avoid performing bitwise and arithmetic operations on the same data. In particular, it is frequently the case that bitwise operations are performed on arithmetic values as a form of premature optimization. Bitwise operators include the unary operator ~ , and the binary operators <<, >>, &, ^, and |. Although 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 the code more maintainable.
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The typedef name uintN_t designates an unsigned integer type with width N. Consequently, uint32_t denotes an unsigned integer type with a width of exactly 32 bits. Bitmaps are normally declared as unsigned.
Non-Compliant Code Example (
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Left Shift)
In this non-compliant code example, both bit manipulation and arithmetic manipulation are 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.
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Although this is a valid 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 reduced.
Compliant Solution (
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Left Shift)
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.
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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 (
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Right Shift)
In this non-compliant code example, the programmer prematurely optimizes code by replacing a division with a right shift.
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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).
Compliant Solution (
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Right Shift)
In this compliant solution, the right shift is replaced by division.
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Performing bit manipulation and arithmetic operations on the same variable obscures the programmers programmer's intentions and reduces readability. This in turn makes it more difficult for a security auditor or maintainer to determine which checks must be performed to eliminate security flaws and ensure data integrity.
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