mirror of
https://github.com/c64scene-ar/llvm-6502.git
synced 2025-10-31 08:16:47 +00:00
23a6033104
Documents terminology used in the forthcoming rewrite of BlockFrequencyInfo. <rdar://problem/14292693> git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206086 91177308-0d34-0410-b5e6-96231b3b80d8
118 lines
2.9 KiB
ReStructuredText
118 lines
2.9 KiB
ReStructuredText
===========================
|
|
LLVM Branch Weight Metadata
|
|
===========================
|
|
|
|
.. contents::
|
|
:local:
|
|
|
|
Introduction
|
|
============
|
|
|
|
Branch Weight Metadata represents branch weights as its likeliness to be taken
|
|
(see :doc:`BlockFrequencyTerminology`). 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 assigned 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 assigned to every case (including the ``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 assigned 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.
|