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| -rw-r--r-- | llvm/docs/LangRef.rst | 29 |
1 files changed, 16 insertions, 13 deletions
diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst index 3b7d80ba376..56221c5154a 100644 --- a/llvm/docs/LangRef.rst +++ b/llvm/docs/LangRef.rst @@ -941,23 +941,26 @@ Currently, only the following parameter attributes are defined: .. _noalias: ``noalias`` - This indicates that pointer values :ref:`based <pointeraliasing>` on - the argument or return value do not alias pointer values that are - not *based* on it, ignoring certain "irrelevant" dependencies. For a - call to the parent function, dependencies between memory references - from before or after the call and from those during the call are - "irrelevant" to the ``noalias`` keyword for the arguments and return - value used in that call. The caller shares the responsibility with - the callee for ensuring that these requirements are met. For further - details, please see the discussion of the NoAlias response in :ref:`alias - analysis <Must, May, or No>`. + This indicates that objects accessed via pointer values + :ref:`based <pointeraliasing>` on the argument or return value are not also + accessed, during the execution of the function, via pointer values not + *based* on the argument or return value. The attribute on a return value + also has additional semantics described below. The caller shares the + responsibility with the callee for ensuring that these requirements are met. + For further details, please see the discussion of the NoAlias response in + :ref:`alias analysis <Must, May, or No>`. Note that this definition of ``noalias`` is intentionally similar - to the definition of ``restrict`` in C99 for function arguments, - though it is slightly weaker. + to the definition of ``restrict`` in C99 for function arguments. For function return values, C99's ``restrict`` is not meaningful, - while LLVM's ``noalias`` is. + while LLVM's ``noalias`` is. Furthermore, the semantics of the ``noalias`` + attribute on return values are stronger than the semantics of the attribute + when used on function arguments. On function return values, the ``noalias`` + attribute indicates that the function acts like a system memory allocation + function, returning a pointer to allocated storage disjoint from the + storage for any other object accessible to the caller. + ``nocapture`` This indicates that the callee does not make any copies of the pointer that outlive the callee itself. This is not a valid |
