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