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Clarify the description of the noalias attribute 

The previous description of the noalias attribute did not accurately specify
the implemented semantics, and the terminology used differed unnecessarily
from that used by the C specification to define the semantics of restrict. For
the argument attribute, the semantics can be precisely specified in terms of
objects accessed through pointers based on the arguments, and this is now what
is done.

Saying that the semantics are 'slightly weaker' than that provided by C99
restrict is not really useful without further elaboration, so that has been
removed from the sentence.

noalias on a return value is really used to mean that the function is
malloc-like (and, in fact, we use this attribute to represent
__attribute__((malloc)) in Clang), and this is a stronger guarantee than that
provided by restrict (because it is a property of the pointed-to memory region,
not just a guarantee on object access). Clarifying this is relevant to fixing
(and was motivated by the discussion on) PR21556.

llvm-svn: 222497
This commit is contained in:
Hal Finkel 2014-11-21 02:22:46 +00:00
parent 5c24658328
commit 12d36309c9
1 changed files with 16 additions and 13 deletions
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29
llvm/docs/LangRef.rst
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@ -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