Document __builtin_nontemporal_load and __builtin_nontemporal_store.

[mzolotukhin]

Summary

In r247104 I added the builtins for generating non-temporal memory operations,
but now I realized that they lack documentation. This patch adds some.

[rsmith]

docs/LanguageExtensions.rst

1802–1807

This seems to make the feature essentially useless, since you cannot guarantee that the address register is set up sufficiently far before the non-temporal load. Should the compiler not be required to insert the necessary barrier itself in this case?

[mzolotukhin]

docs/LanguageExtensions.rst

1802–1807

Yes, we can require targets to only use corresponding NT instructions when it's safe, and then remove this remark from the documentation. For ARM64 that would mean either not to emit LDNP at all, or conservatively emit barriers before each LDNP (which probably removes all performance benefits of using it) - that is, yes, non-temporal loads would be useless on this target.

But I think we want to keep the builtin for NT-load, as it's a generic feature, not ARM64 specific. It can be used on other targets - e.g. we can use this in x86 stream builtins, and hopefully simplify their current implementation. I don't know about non-temporal operations on other targets, but if there are others, they can use it too right out of the box.

[mzolotukhin]

• Remove paragraph about changing program behavior (since we shouldn't change it anyway).

[rsmith]

LGTM

docs/LanguageExtensions.rst

1784

to generate -> generation of

1799

would -> will

[mzolotukhin]

Thanks, committed in r247374 with the requested changes!

Michael

docs/LanguageExtensions.rst

 
 Also, loads and stores may be implicit in code written between the ``ldrex`` and
 ``strex``. Clang will not necessarily mitigate the effects of these either, so
 care should be exercised.
 
 For these reasons the higher level atomic primitives should be preferred where
 possible.
 
+Non-temporal load/store builtins
+--------------------------------
+
+Clang provides overloaded builtins allowing to generate non-temporal memory
+accesses.
+
+.. code-block:: c
+
+  T __builtin_nontemporal_load(T *addr);
+  void __builtin_nontemporal_store(T value, T *addr);
+
+The types ``T`` currently supported are:
+
+* Integer types.
+* Floating-point types.
+* Vector types.
+
+Note that the compiler does not guarantee that non-temporal loads or stores
+would be used.
+
 Non-standard C++11 Attributes
 =============================
 
 Clang's non-standard C++11 attributes live in the ``clang`` attribute
 namespace.
 
 Clang supports GCC's ``gnu`` attribute namespace. All GCC attributes which
 are accepted with the ``__attribute__((foo))`` syntax are also accepted as
 ``[[gnu::foo]]``. This only extends to attributes which are specified by GCC
 (see the list of `GCC function attributes
 <http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html>`_, `GCC variable
 attributes <http://gcc.gnu.org/onlinedocs/gcc/Variable-Attributes.html>`_, and
 `GCC type attributes
 <http://gcc.gnu.org/onlinedocs/gcc/Type-Attributes.html>`_). As with the GCC
 implementation, these attributes must appertain to the *declarator-id* in a