dereferenceable

Dereferenceable is a term used in computer science to describe a value, typically a pointer or reference, that can be safely dereferenced to access the object it designates. In formal terms, a pointer is dereferenceable for a given size if reading that many bytes starting from the pointer address yields defined behavior. The concept helps distinguish between mere pointer values and pointers guaranteed to provide valid memory access.

In LLVM and related compiler work, dereferenceable is an attribute attached to pointer values or parameters. A pointer marked dereferenceable(n) guarantees that at least n bytes of memory beginning at the pointed-to address are valid to read. This does not guarantee that the pointer remains valid after the current scope or that writes to that region are allowed, only that reads of that region are safe. If a dereference would access memory outside that region, the program may produce undefined behavior. The attribute is often used alongside other properties like nonnull and alignment to support optimizations and safety checks.

Dereferenceable is primarily a program-analysis and optimization aid rather than a language feature with runtime semantics in languages such as C or C++. It is used to improve alias analysis, enable vectorization, and provide formal proofs of safety in compiler IR and static-analysis tools. A typical use is a function parameter annotated as dereferenceable(4), which promises that reading the first four bytes of the pointer is defined, enabling more aggressive optimizations.

Limitations and caveats include that the guarantee is a contract the program must uphold; memory changes, deallocation, or lifetime issues can violate it. It is not a substitute for proper memory management but a tool to aid analysis and optimization. See also pointers, memory safety, static analysis, and LLVM IR.