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      1 ===========================
      2 LLVM Branch Weight Metadata
      3 ===========================
      4 
      5 .. contents::
      6    :local:
      7 
      8 Introduction
      9 ============
     10 
     11 Branch Weight Metadata represents branch weights as its likeliness to be taken
     12 (see :doc:`BlockFrequencyTerminology`). Metadata is assigned to the
     13 ``TerminatorInst`` as a ``MDNode`` of the ``MD_prof`` kind. The first operator
     14 is always a ``MDString`` node with the string "branch_weights".  Number of
     15 operators depends on the terminator type.
     16 
     17 Branch weights might be fetch from the profiling file, or generated based on
     18 `__builtin_expect`_ instruction.
     19 
     20 All weights are represented as an unsigned 32-bit values, where higher value
     21 indicates greater chance to be taken.
     22 
     23 Supported Instructions
     24 ======================
     25 
     26 ``BranchInst``
     27 ^^^^^^^^^^^^^^
     28 
     29 Metadata is only assigned to the conditional branches. There are two extra
     30 operarands for the true and the false branch.
     31 
     32 .. code-block:: llvm
     33 
     34   !0 = metadata !{
     35     metadata !"branch_weights",
     36     i32 <TRUE_BRANCH_WEIGHT>,
     37     i32 <FALSE_BRANCH_WEIGHT>
     38   }
     39 
     40 ``SwitchInst``
     41 ^^^^^^^^^^^^^^
     42 
     43 Branch weights are assigned to every case (including the ``default`` case which
     44 is always case #0).
     45 
     46 .. code-block:: llvm
     47 
     48   !0 = metadata !{
     49     metadata !"branch_weights",
     50     i32 <DEFAULT_BRANCH_WEIGHT>
     51     [ , i32 <CASE_BRANCH_WEIGHT> ... ]
     52   }
     53 
     54 ``IndirectBrInst``
     55 ^^^^^^^^^^^^^^^^^^
     56 
     57 Branch weights are assigned to every destination.
     58 
     59 .. code-block:: llvm
     60 
     61   !0 = metadata !{
     62     metadata !"branch_weights",
     63     i32 <LABEL_BRANCH_WEIGHT>
     64     [ , i32 <LABEL_BRANCH_WEIGHT> ... ]
     65   }
     66 
     67 Other
     68 ^^^^^
     69 
     70 Other terminator instructions are not allowed to contain Branch Weight Metadata.
     71 
     72 .. _\__builtin_expect:
     73 
     74 Built-in ``expect`` Instructions
     75 ================================
     76 
     77 ``__builtin_expect(long exp, long c)`` instruction provides branch prediction
     78 information. The return value is the value of ``exp``.
     79 
     80 It is especially useful in conditional statements. Currently Clang supports two
     81 conditional statements:
     82 
     83 ``if`` statement
     84 ^^^^^^^^^^^^^^^^
     85 
     86 The ``exp`` parameter is the condition. The ``c`` parameter is the expected
     87 comparison value. If it is equal to 1 (true), the condition is likely to be
     88 true, in other case condition is likely to be false. For example:
     89 
     90 .. code-block:: c++
     91 
     92   if (__builtin_expect(x > 0, 1)) {
     93     // This block is likely to be taken.
     94   }
     95 
     96 ``switch`` statement
     97 ^^^^^^^^^^^^^^^^^^^^
     98 
     99 The ``exp`` parameter is the value. The ``c`` parameter is the expected
    100 value. If the expected value doesn't show on the cases list, the ``default``
    101 case is assumed to be likely taken.
    102 
    103 .. code-block:: c++
    104 
    105   switch (__builtin_expect(x, 5)) {
    106   default: break;
    107   case 0:  // ...
    108   case 3:  // ...
    109   case 5:  // This case is likely to be taken.
    110   }
    111 
    112 CFG Modifications
    113 =================
    114 
    115 Branch Weight Metatada is not proof against CFG changes. If terminator operands'
    116 are changed some action should be taken. In other case some misoptimizations may
    117 occur due to incorrent branch prediction information.
    118