What is the strict aliasing rule?
The Strict aliasing rule allows us to avoid redundant machine code in cases in which it should be safe to assume that two pointers don’t point to the same memory block (see also the restrict keyword).
What is pointer aliasing?
Pointer aliasing is a hidden kind of data dependency that can occur in C, C++, or any other language that uses pointers for array addresses in arithmetic operations. Array data identified by pointers in C can overlap, because the C language puts very few restrictions on pointers.
What is strict aliasing and why do we care?
In C and C++ aliasing has to do with what expression types we are allowed to access stored values through. The compiler and optimizer are allowed to assume we follow the aliasing rules strictly, hence the term strict aliasing rule.
What is Type punning C++?
Type punning is the possibility of a programming language intentionally subvert the type system to treat a type as a different type. One typical way to do type punning in C++ is to read the member of a union with a different type from the one with which it was written.
What is aliasing give an example?
If the image data is processed in some way during sampling or reconstruction, the reconstructed image will differ from the original image, and an alias is seen. An example of spatial aliasing is the moiré pattern observed in a poorly pixelized image of a brick wall.
What does std :: launder do?
std::launder is used to “obtain a pointer to an object created in storage occupied by an existing object of the same type, even if it has const or reference members.”
Does C++ have aliasing?
In C, C++, and some other programming languages, the term aliasing refers to a situation where two different expressions or symbols refer to the same object. When references access that object in different ways—as both reads and stores—there are consequences for the order in which these mixed accesses can happen.
What is aliasing in DSP?
Aliasing is an unwanted case of sampling, where the minimum condition for accurate sampling is not met. Thus there is an overlap in the shifted replicas of the x(ω) signal. Consequently, the x(t) signal can neither be sampled accurately or recovered from its samples.
How do I stop punning type?
The only safe manner of using type punning is with unsigned char or well unsigned char arrays (because we know that members of array objects are strictly contiguous and there is not any padding bytes when their size is computed with sizeof() ).
What is restrict in C?
In the C programming language, restrict is a keyword that can be used in pointer declarations. restrict limits the effects of pointer aliasing, aiding optimizations. If the declaration of intent is not followed and the object is accessed by an independent pointer, this will result in undefined behavior.
What is aliasing error?
Aliasing errors occur when components of a signal are above the Nyquist frequency (Nyquist theory states that the sampling frequency must be at least two times the highest frequency component of the signal) or one half the sample rate. Aliasing errors are hard to detect and almost impossible to remove using software.
What is strict aliasing in C and C++?
In C and C++ aliasing has to do with what expression types we are allowed to access stored values through. In both C and C++ the standard specifies which expression types are allowed to alias which types. The compiler and optimizer are allowed to assume we follow the aliasing rules strictly, hence the term strict aliasing rule.
When asking about common undefined behavior in C, people sometimes refer to the strict aliasing rule. What are they talking about? A typical situation where you encounter strict aliasing problems is when overlaying a struct (like a device/network msg) onto a buffer of the word size of your system (like a pointer to uint32_t s or uint16_t s).
Does Char* alias other char* types?
The rules allow an exception for char* (including signed char and unsigned char ). It’s always assumed that char* aliases other types. However this won’t work the other way: there’s no assumption that your struct aliases a buffer of chars. This is only one potential minefield when overlaying two types onto each other.