Signed integer overflow is undefined behavior 36. Consequently, implementations have considerable latitude in how they deal with signed integer overflow (see MSC15-C. Do not depend on undefined behavior). An implementation that defines signed integer types as being modulo, for example, need not detect integer overflow. Implementations may also trap on signed arithmetic overflows, or simply assume that overflows will never happen and generate object code accordingly. It is also possible for the same conforming implementation to emit code that exhibits different behavior in different contexts. For example, an implementation may determine that a signed integer loop control variable declared in a local scope cannot overflow and may emit efficient code on the basis of that determination, while the same implementation may determine that a global variable used in a similar context will wrap.
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The C Standard, 6.5.7, paragraph 4 [ISO/IEC 9899:2011], states:
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In almost every case, an attempt to shift by a negative number of bits or by more bits than exist in the operand indicates a logic error. These issues are covered by INT34-C. Do not shift an expression by a negative number of bits or by greater than or equal to the number of bits that exist in the operand.
Noncompliant Code Example
This noncompliant code example performs a left shift, after verifying that the number being shifted is not negative, and the number of bits to shift is valid. The PRECISION() macro and popcount() function provide the correct precision for any integer type (see INT35-C. Use correct integer precisions). However, because this code does no overflow check, it can result in an unrepresentable value.
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Compliant Solution
This compliant solution eliminates the possibility of overflow resulting from a left-shift operation:
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