MISRA C++ rule 5-0-3 false positive warning

Question

My static analyzer is throwing the following warning:

MCPP Rule 5-0-3: This complex expression is implicitly converted to a different essential type

for the following code:

void func(const uint32_t arg)
{
    //32U has underlying type uint8_t
    const uint32_t u32a = arg % 32U; //warning issued in this line
    const uint32_t u32b = (arg % static_cast<uint32_t>(32U)); //same warning issued in this line
    const uint32_t u32c = static_cast<uint32_t>(arg % 32U); //compliant
}

According to MISRA underlying type conversion rules:

Otherwise, if both operands have integral type, the underlying type of the expression can be found using the following:

– If the types of the operands are the same size, and either is unsigned, the result is unsigned.

– Otherwise, the type of the result is that of the larger type.

I think this warning may be a false positive because, despite the 32U being a uint8_t, the expression should take the underlying type of the larger type, in this case the uint32_t, thus making the need for the static_cast unnecessary.

Do you agree this is a false positive? Or am I looking at it all wrong?

EDIT: The MISRA standard states that:

The underlying type of an integer constant expression is therefore defined as follows:

1. If the actual type of the expression is signed integral, the underlying type is defined as the smallest signed integer type that is capable of representing its value.

2. If the actual type of the expression is unsigned integral, the underlying type is defined as the smallest unsigned integer type that is capable of representing its value.

3. In all other circumstances, the underlying type of the expression is defined as being the same as its actual type.

No. 2 is the reason why I've to assume that 32U has the underlying type of uint8_t.

Answer 1

I think this warning may be a false positive because, despite the 32U being a uint8_t

32U is not a uint8_t on any platform. With integer literals the smallest type you can express is a int/unsigned int. According to cppreference nnnnU can be a unsigned int, unsigned long int, or unsigned long long int. It does pick the first type that the literal can be stored in so 32U is a unsigned int.

So if you want to guarantee that 32U is the same type as uint32_t then you need the cast on the right hand side.

Answer 2

32U is of type unsigned, which is potentially a distinct type from uint32_t. Contrary to your statement, it is never of type uint8_t

A unsigned is only guaranteed to be able to represent the values in the range 0 to 65535, although it is permitted to support a larger range. What it actually can represent is implementation-defined. However, it cannot be a uint8_t, since a uint8_t cannot represent the range required of an unsigned.

This means, practically, three possibilities are that unsigned may be a 16-bit type, a 32-bit type, or even a 64-bit type - and these are, respectively, smaller, the same size, or larger than a uint32_t.

The result of the expression arg % 32U may therefore be of type uint32_t (if unsigned is 16-bit), uint32_t (if unsigned and uint32_t are both the same 32-bit type), or unsigned (if unsigned is a 64-bit type). In the last case, a conversion from unsigned to uint32_t is required to initialise u32a.

Your static analyser is warning you of this potential variation of behaviours between systems.

So, no, it is not a false positive.

Answer 3

You have found the relevant section. The type of the expression where the literal is located is unsigned, so the underlying type is the smallest one that can fit an unsigned value 32, meaning uint8_t. It would have the very same underlying type if the literal had been 32 without the U suffix (though that would violate other MISRA rules).

What MISRA is aiming for here is that in the specific expression uint32_t arg; ... arg % 32U there can never occur a dangerous implicit conversion. That being said, you can safely cast the literal to uint32_t and that should silence all warnings. Ensuring that there are no implicit type promotions in your code what-so-ever is good programming practice, no matter what MISRA says/doesn't say.

If the purpose of the static analyser is just to check for implicit promotions in general, then the warning is fine. If the purpose of your static analyser is to check for MISRA compliance, then it is a false positive.

The line arg % static_cast<uint32_t>(32U) should never yield any form of diagnostic, regardless of the purpose of the tool. That is certainly a false positive.