GeneralPurpose-UndefinedBitwiseShift

Avoid computations or casts resulting in integer overflow

Required inputs: IR, StaticSemanticAnalysis

Finds undefined behavior caused by the bitwise left- and right-shift operator operating on integer types. For the following examples, assume CHAR_BIT == 8 && sizeof(int) == 4.

Bad code:

void f(int x, int y) {
    unsigned a = 1u << 32; // ERROR: Arithmetic computation may cause overflow
}

Good code:

void f(int x, int y) {
    unsigned a = 1u << 31; // OK: Does not cause overflow
}

Possible Messages

Key	Text	Severity	Disabled
certain_shift_amount_negative	Shift by a negative bit count (undefined behavior)	None	False
certain_shift_amount_too_large	Shift by the integer width or more (undefined behavior)	None	False
certain_shift_right_negative	Right shift with negative left-hand-side	None	False
static_overflow	Arithmetic computation may cause overflow	None	False

Options

• This rule shares the following common options: exclude_in_macros, exclude_messages_in_system_headers, excludes, extend_exclude_to_macro_invocations, includes, justification_checker, languages, post_processing, provider, report_at, severity

• The following places define options that affect this rule: Stylechecks, Analysis-GlobalOptions

abstract_interpretation_maximal_tracked_array_index

abstract_interpretation_maximal_tracked_array_index : int = 10

The number of explicit indices in array expressions per routine tracked by the "symbolic expression analysis". For example, consider the following program.

extern signed char a[6];
int main()
{
    if (a[2] < 0)
    {
        a[2]++;
    }
    if (a[3] < 0)
    {
        a[3]++;
    }
    if (a[4] < 0)
    {
        a[4]++;
    }
    return 0;
}

If the value of this option is set to 2, the first two array index expressions encountered in the routine are tracked. Hence, the analysis can use the facts a[2] < 0 and a[3] < 0 to infer that a[2]++ and a[3]++ do not overflow, but it will not track the third array access in this routine.

A higher value of the option can cause more consumption of memory and time for the analysis.

 

abstract_interpretation_overflow

abstract_interpretation_overflow : bool = False

Use abstract-interpretation-based "symbolic expression analysis" as additional postprocessing step.

 

abstract_interpretation_overflow_unrolling_level

abstract_interpretation_overflow_unrolling_level : int = 0

How many levels of conditions are traversed to compute additional constraints for the "symbolic expression analysis".

 

check_signed

check_signed : bool = True

Whether issues for signed integer operations should be reported. For casts including implicit conversions, the target type of the cast is used.

 

check_unsigned

check_unsigned : bool = True

Whether wrap-around for unsigned integer operations should be reported. For casts including implicit conversions, the target type of the cast is used.

 

relevant_expressions

relevant_expressions

Type: RelevantExpressions

Default: 'const_and_compile_time_constant'

Which (const / constant) expressions should be considered.

Note: this is only relevant for the purely static parts of the analysis. The StaticSemanticAnalysis-based checks for runtime errors will be performed independently.

 

suppress_well_defined_findings

suppress_well_defined_findings

Type: SuppressionMode

Default: 'DERIVE_FROM_IR'

Some overflows have well-defined semantics in all C/C++ standard versions. The typical example is UINT_MAX+1 which is well-defined as 0 via wraparound. This differs from INT_MAX+1 which is either undefined or implementation-defined depending on the considered standard version. Most CPUs will compute INT_MIN but this wraparound is not guaranteed by any C/C++ standard.

Both cases are overflows and are reported by this rule. However, one might want to suppress messages for the well-defined cases. To suppress these activate this option.

Different C and C++ standard versions differ in what is well-defined, implementation-defined, or undefined. Luckily, if we only consider well-defined and do not discern between implementation-defined and undefined, we end up with only two groups: pre-C++20 and since-C++20.

 

Option Types

These types are used by options listed above:

RelevantExpressions

An enumeration.

 

none

No (additional) checks for overflows in const-expressions or compile time constant expressions.

const_expressions_only

Whether the analysis should statically check const-expressions (i.e., const variables and literals) that might have been reduced to a literal during compilation.

const_and_compile_time_constant

Whether the analysis should statically check const-expressions (i.e., const variables and literals) as well as compile-time constant expressions (i.e., preprocessor defines, constexprs or literals) that might have been reduced to a literal during compilation.

SuppressionMode

An enumeration.

 

NONE

Suppress nothing.

PRE_CPP2020

Suppress findings that are well-defined before C++20. These are:

• Over- and underflows of unsigned integers during addition, subtraction, and multiplication
• Conversions from unsigned to unsigned integers
• Wrap-around caused by left-shifting of unsigned integer

CPP2020

Suppress findings that are well-defined since C++20. These are:

• Over- and underflows of unsigned integers during addition, subtraction, and multiplication
• Conversions between signed and unsigned integers
• Wrap-around caused by left-shifting
• Shifting negative integers

Surprising mechanics of C++20 signed narrow integers

Since C++20, casts between signed and unsigned are defined as two-complement wrap-around. Overflows of signed integers are still undefined behavior and are reported by this rule. But, due to integer promotion rules, certain expressions are computed using wider integer types, which can lead to the false impression that this is no longer the case, because no overflow findings are reported there.

Suppose, that the code is compiled on a platform where short is smaller than or equal to half the size of an int. Very commonly the sizes are 2 and 4. This assumption is thus true for many platforms.

In this case, narrow signed integer types such as short or signed char are first implicitly promoted to int before the arithmetic operation is executed. Because of this promotion, the actual operation does not overflow and is thus well-defined. After the operation, an implicit cast is performed to the narrower type. This cast is well-defined in C++20 as wrapping around.

Consider the following snippet:

    static_assert(sizeof(short) == 2);
    static_assert(sizeof(int) == 4);
    short a = 0x1000;
    short b = 0x1001;
    short c = a*b;

C++20 defines c as 0x1000. The reason is that a*b is implicitly promoted to static_cast<int> (a)*static_cast<int>(b). After the promotion, the multiplication does not overflow and yields a well-defined 0x1001000. This number is then implicitly cast to 0x1000 which is also a well-defined operation.

An analogous effect can be observed for signed short addition and multiplication. Another effect is that it is well-defined to shift by up to as many bits as int has even if the shifted integer has fewer bits.

DERIVE_FROM_IR

Derive the language version from the IR compilation flags and suppress findings accordingly.