Sphinxify BranchWeightMetadata document.

llvm-svn: 158810

Author: isanbard

llvm/docs/BranchWeightMetadata.html

@@ -0,0 +1,118 @@
+.. _branch_weight:
+
+===========================
+LLVM Branch Weight Metadata
+===========================
+
+.. contents::
+   :local:
+
+Introduction
+============
+
+Branch Weight Metadata represents branch weights as its likeliness to be
+taken. Metadata is assigned to the ``TerminatorInst`` as a ``MDNode`` of the
+``MD_prof`` kind. The first operator is always a ``MDString`` node with the
+string "branch_weights". Number of operators depends on the terminator type.
+
+Branch weights might be fetch from the profiling file, or generated based on
+`__builtin_expect`_ instruction.
+
+All weights are represented as an unsigned 32-bit values, where higher value
+indicates greater chance to be taken.
+
+Supported Instructions
+======================
+
+``BranchInst``
+^^^^^^^^^^^^^^
+
+Metadata is only assign to the conditional branches. There are two extra
+operarands, for the true and the false branch.
+
+.. code-block:: llvm
+
+  !0 = metadata !{
+    metadata !"branch_weights",
+    i32 <TRUE_BRANCH_WEIGHT>,
+    i32 <FALSE_BRANCH_WEIGHT>
+  }
+
+``SwitchInst``
+^^^^^^^^^^^^^^
+
+Branch weights are assign to every case (including ``default`` case which is
+always case #0).
+
+.. code-block:: llvm
+
+  !0 = metadata !{
+    metadata !"branch_weights",
+    i32 <DEFAULT_BRANCH_WEIGHT>
+    [ , i32 <CASE_BRANCH_WEIGHT> ... ]
+  }
+
+``IndirectBrInst``
+^^^^^^^^^^^^^^^^^^
+
+Branch weights are assign to every destination.
+
+.. code-block:: llvm
+
+  !0 = metadata !{
+    metadata !"branch_weights",
+    i32 <LABEL_BRANCH_WEIGHT>
+    [ , i32 <LABEL_BRANCH_WEIGHT> ... ]
+  }
+
+Other
+^^^^^
+
+Other terminator instructions are not allowed to contain Branch Weight Metadata.
+
+.. _\__builtin_expect:
+
+Built-in ``expect`` Instructions
+================================
+
+``__builtin_expect(long exp, long c)`` instruction provides branch prediction
+information. The return value is the value of ``exp``.
+
+It is especially useful in conditional statements. Currently Clang supports two
+conditional statements:
+
+``if`` statement
+^^^^^^^^^^^^^^^^
+
+The ``exp`` parameter is the condition. The ``c`` parameter is the expected
+comparison value. If it is equal to 1 (true), the condition is likely to be
+true, in other case condition is likely to be false. For example:
+
+.. code-block:: c++
+
+  if (__builtin_expect(x > 0, 1)) {
+    // This block is likely to be taken.
+  }
+
+``switch`` statement
+^^^^^^^^^^^^^^^^^^^^
+
+The ``exp`` parameter is the value. The ``c`` parameter is the expected
+value. If the expected value doesn't show on the cases list, the ``default``
+case is assumed to be likely taken.
+
+.. code-block:: c++
+
+  switch (__builtin_expect(x, 5)) {
+  default: break;
+  case 0:  // ...
+  case 3:  // ...
+  case 5:  // This case is likely to be taken.
+  }
+
+CFG Modifications
+=================
+
+Branch Weight Metatada is not proof against CFG changes. If terminator operands'
+are changed some action should be taken. In other case some misoptimizations may
+occur due to incorrent branch prediction information.

llvm/docs/BranchWeightMetadata.rst

@@ -7,6 +7,7 @@ Subsystem Documentation
    :hidden:
 
    AliasAnalysis
+   BranchWeightMetadata
    LinkTimeOptimization
 
 * `Writing an LLVM Pass <WritingAnLLVMPass.html>`_
@@ -75,6 +76,6 @@ Subsystem Documentation
 
    How to debug JITed code with GDB.
 
-* `Branch Weight Metadata <BranchWeightMetadata.html>`_
+* :ref:`branch_weight`
 
    Provides information about Branch Prediction Information.