Document several clang-supported builtins

[serge-sans-paille]

Summary

Namely

builtin_alloca
builtin_alloca_with_align
builtin_call_with_static_chain
builtin_expect
builtin_expect_with_probablity
builtin_prefetch

[aaron.ballman]

Thanks for this new documentation! I'm by no means an expert in these builtins, so I had some questions on places we might want to expand things a bit more.

clang/docs/LanguageExtensions.rst

2183

2569–2570

2588

2617

2620

Should we mention that the probability should be specified as a value between [0.0, 1.0]?

2627–2628

2647–2648

Is there a way to specify both read and write at the same time? Or does write access imply read access? (If so, it'd be good to make that explicit.)

2649–2650

How do users know whether to pick 1 or 2? Should we list the values in table form with some prose describing when to use each value?

[aaron.ballman]

LGTM aside from a minor nit with duplicate punctuation.

clang/docs/LanguageExtensions.rst

2653

clang/docs/LanguageExtensions.rst

 Please note that Clang does not and will not support all of the GCC builtins
 for vector operations.  Instead of using builtins, you should use the functions
 defined in target-specific header files like ``<xmmintrin.h>``, which define
 portable wrappers for these.  Many of the Clang versions of these functions are
 implemented directly in terms of :ref:`extended vector support
 <langext-vectors>` instead of builtins, in order to reduce the number of
 builtins that we need to implement.
 
-.. _langext-__builtin_assume:
-
-``__builtin_assume``
-------------------------------
+``__builtin_alloca``
+--------------------
 
-``__builtin_assume`` is used to provide the optimizer with a boolean
-invariant that is defined to be true.
+``__builtin_alloca`` is used to dynamically allocate memory on the stack. Memory
+is automatically freed upon function termination.
 
 **Syntax**:
 
 .. code-block:: c++
 
-  __builtin_assume(bool)
+  __builtin_alloca(size_t n)
 
 **Example of Use**:
 
 .. code-block:: c++
 
-  int foo(int x) {
-    __builtin_assume(x != 0);
+  void init(float* data, size_t nbelems);
+  void process(float* data, size_t nbelems);
+  int foo(size_t n) {
+    auto mem = (float*)__builtin_alloca(n * sizeof(float));
+    init(mem, n);
+    process(mem, n);
+    /* mem is automatically freed at this point */
+  }
+
+**Description**:
+
+``__builtin_alloca`` is meant to be used to allocate a dynamic amount of memory
+on the stack. This amount is subject to stack allocation limits.
+
+Query for this feature with ``__has_builtin(__builtin_alloca)``.
+
+``__builtin_alloca_with_align``
+-------------------------------
 
-    // The optimizer may short-circuit this check using the invariant.
-    if (x == 0)
-      return do_something();
+``__builtin_alloca_with_align`` is used to dynamically allocate memory on the
+stack while controlling its alignment. Memory is automatically freed upon
+function termination.
 
-    return do_something_else();
+
+**Syntax**:
+
+.. code-block:: c++
+
+  __builtin_alloca_with_align(size_t n, size_t align)
+
+**Example of Use**:
+
+.. code-block:: c++
+
+  void init(float* data, size_t nbelems);
+  void process(float* data, size_t nbelems);
+  int foo(size_t n) {
+    auto mem = (float*)__builtin_alloca_with_align(
+                        n * sizeof(float),
+                        8 * alignof(float));
+    init(mem, n);
+    process(mem, n);
+    /* mem is automatically freed at this point */
   }
 
 **Description**:
 
-The boolean argument to this function is defined to be true. The optimizer may
-analyze the form of the expression provided as the argument and deduce from
-that information used to optimize the program. If the condition is violated
-during execution, the behavior is undefined. The argument itself is never
-evaluated, so any side effects of the expression will be discarded.
+``__builtin_alloca_with_align`` is meant to be used to allocate a dynamic amount of memory
+on the stack. It is similar to ``__builtin_alloca`` but accepts a second
+argument whose value is the alignment constraint, as a power of 2 in *bits*.
+
+Query for this feature with ``__has_builtin(__builtin_alloca_with_align)``.
 
-Query for this feature with ``__has_builtin(__builtin_assume)``.
+.. _langext-__builtin_assume:
+
+``__builtin_call_with_static_chain``
+------------------------------------
+
+``__builtin_call_with_static_chain`` is used to perform a static call while
+setting updating the static chain register.
+
+**Syntax**:
+
+.. code-block:: c++
+
+  T __builtin_call_with_static_chain(T expr, void* ptr)
+
+**Example of Use**:
+
+.. code-block:: c++
+
+  auto v = __builtin_call_with_static_chain(foo(3), foo);
+
+**Description**:
+
+This builtin returns ``expr`` after checking that ``expr`` is a non-member
+static call expression. The call to that expression is made while using ``ptr``
+as a function pointer stored in a dedicated register to implement *static chain*
+calling convention, as used by some language to implement closures or nested
+functions.
+
+Query for this feature with ``__has_builtin(__builtin_call_with_static_chain)``.
 
 ``__builtin_readcyclecounter``
 ------------------------------
 
 ``__builtin_readcyclecounter`` is used to access the cycle counter register (or
 a similar low-latency, high-accuracy clock) on those targets that support it.
 
 **Syntax**:

 
 **Description**:
 
 The ``__builtin_unpredictable()`` builtin is expected to be used with control
 flow conditions such as in ``if`` and ``switch`` statements.
 
 Query for this feature with ``__has_builtin(__builtin_unpredictable)``.
 
+
+``__builtin_expect``
+--------------------
+
+``__builtin_expect`` is used to indicate that an expression is expected (I;e.
+very likely to) take a given value.
+
+**Syntax**:
+
+.. code-block:: c++
+
+    long __builtin_expect(long expr, long val)
+
+**Example of use**:
+
+.. code-block:: c++
+
+  if (__builtin_expect(x, 0)) {
+     bar();
+  }
+
+**Description**:
+
+The ``__builtin_expect()`` builtin is expected to be used with control flow
+conditions such as in ``if`` and ``switch`` statements to help branch
+prediction. It means that its first argument ``expr`` is expected to take the
+value of its second argument ``val``. It always returns ``expr``.
+
+Query for this feature with ``__has_builtin(__builtin_expect)``.
+
+``__builtin_expect_with_probability``
+-------------------------------------
+
+``__builtin_expect_with_probability`` is similar to ``__builtin_expect`` but it
+takes a probability as third argument.
+
+**Syntax**:
+
+.. code-block:: c++
+
+    long __builtin_expect_with_probability(long expr, long val, double p)
+
+**Example of use**:
+
+.. code-block:: c++
+
+  if (__builtin_expect_with_probability(x, 0, .3)) {
+     bar();
+  }
+
+**Description**:
+
+The ``__builtin_expect_with_probability()`` builtin is expected to be used with control flow
+conditions such as in ``if`` and ``switch`` statements to help branch
+prediction. It means that its first argument ``expr`` is expected to take the
+value of its second argument ``val`` with probability ``p``. It always returns ``expr``.
+
+Query for this feature with ``__has_builtin(__builtin_expect_with_probability)``.
+
+``__builtin_prefetch``
+----------------------
+
+``__builtin_prefetch`` is used to communicate with the cache handler and bring
+data in cache before it gets used.
+
+**Syntax**:
+
+.. code-block:: c++
+
+    void __builtin_prefetch(const void *addr, int rw=0, int locality=3)
+
+**Example of use**:
+
+.. code-block:: c++
+
+    __builtin_prefetch(a + i);
+
+**Description**:
+
+The ``__builtin_prefetch(addr, rw, locality)`` builtin is expected to be used to
+avoid cache misses when the developper has a good understanding of which data
+are going to be used next. ``addr`` is the address that needs to be brought into
+the cache. ``rw`` indicates the expected access mode: ``0`` for *read* and ``1``
+for *write*. ``locality`` indicates the expected persistance of data in cache,
+from ``0`` which means that data can be discarded from cache after its next use
+to ``3`` which means that data is going to be resued a lot once in cache.
+
+Query for this feature with ``__has_builtin(__builtin_prefetch)``.
+
 ``__sync_swap``
 ---------------
 
 ``__sync_swap`` is used to atomically swap integers or pointers in memory.
 
 **Syntax**:
 
 .. code-block:: c++