add __builtin_unpredictable and convert to metadata

This patch depends on r246688 (D12341).

The goal is to make LLVM generate different code for these functions for a target that
has cheap branches (see PR23827 for more details):

int foo();

int normal(int x, int y, int z) {
   if (x != 0 && y != 0) return foo();
   return 1;
}

int crazy(int x, int y) {
   if (__builtin_unpredictable(x != 0 && y != 0)) return foo();
   return 1;
}

Differential Revision: http://reviews.llvm.org/D12458

llvm-svn: 246699

1 files changed, 27 insertions, 0 deletions

diff --git a/clang/docs/LanguageExtensions.rst b/clang/docs/LanguageExtensions.rst
index ca2e177b368..af235808cd4 100644
--- a/clang/docs/LanguageExtensions.rst
+++ b/clang/docs/LanguageExtensions.rst
@@ -1540,6 +1540,33 @@ takes no arguments and produces a void result.
 
 Query for this feature with ``__has_builtin(__builtin_unreachable)``.
 
+``__builtin_unpredictable``
+---------------------------
+
+``__builtin_unpredictable`` is used to indicate that a branch condition is
+unpredictable by hardware mechanisms such as branch prediction logic.
+
+**Syntax**:
+
+.. code-block:: c++
+
+    __builtin_unpredictable(long long)
+
+**Example of use**:
+
+.. code-block:: c++
+
+  if (__builtin_unpredictable(x > 0)) {
+     foo();
+  }
+
+**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)``.
+
 ``__sync_swap``
 ---------------