Is Reinterpret_casting Between SIMD Vector Pointers and Corresponding Types Undefined Behavior in C ?

Is reinterpret_casting Between Hardware SIMD Vector Pointer and the Corresponding Type an Undefined Behavior?

In C , the usage of reinterpret_cast to convert a float pointer to a __m256 pointer and access float objects through a different pointer type raises concerns about potential undefined behaviors or violations of strict aliasing rules.

Undefined Behavior

According to the ISO C standard, the behavior of reinterpret_cast between different pointer types is implementation-defined and therefore not guaranteed to be portable across different platforms or compilers.

May-Alias Attribute

However, Intel's implementations of these intrinsics often define vector-pointer types like __m256* as having a "may-alias" attribute. This means that the compiler is allowed to assume that a __m256* could potentially alias anything else, including other vector types or their scalar components.

Safe to Dereference

As a result, it is generally considered safe to dereference a __m256* directly, without requiring the use of intrinsic functions like _mm256_load_ps. This is because the compiler is aware of the potential aliasing and will generate the appropriate instructions to perform the operation safely.

Strict Aliasing Violation

The usage of array types and direct access to vector elements raises concerns about violating the strict aliasing rules in C . The act of reinterpret_casting does not necessarily violate strict aliasing rules. However, accessing the resultant pointer-to-struct as the original type does. The strict aliasing rules define that direct access to a part of an object of the union type may violate the const-correctness, meaning a const union may not be accessed directly, and a non-const union may not be cast to a const-qualified type while modifying the value. Strict aliasing would only be violated, in this context, if you have accessed the reinterpret_cast pointer as a float pointer and modified the value. This is because the strict aliasing rules in C prohibit accessing an object through a pointer of a different type if it could violate the const-correctness of the object. That is not the case in the example.

Summary

In practice, using reinterpret_cast between hardware SIMD vector pointers and the corresponding float pointers is commonly used and accepted as safe on platforms that support Intel's intrinsics. However, it's important to note that this may not be the case on all platforms or implementations. Always refer to the documentation for the specific compiler and platform you are using for guidance on the proper usage of intrinsic functions and type conversions.

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