cobalt / cobalt / 25399f97090f8a86f56525c38486080cb2747250 / . / src / third_party / llvm-project / llvm / docs / BlockFrequencyTerminology.rst

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LLVM Block Frequency Terminology | |

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.. contents:: | |

:local: | |

Introduction | |

============ | |

Block Frequency is a metric for estimating the relative frequency of different | |

basic blocks. This document describes the terminology that the | |

``BlockFrequencyInfo`` and ``MachineBlockFrequencyInfo`` analysis passes use. | |

Branch Probability | |

================== | |

Blocks with multiple successors have probabilities associated with each | |

outgoing edge. These are called branch probabilities. For a given block, the | |

sum of its outgoing branch probabilities should be 1.0. | |

Branch Weight | |

============= | |

Rather than storing fractions on each edge, we store an integer weight. | |

Weights are relative to the other edges of a given predecessor block. The | |

branch probability associated with a given edge is its own weight divided by | |

the sum of the weights on the predecessor's outgoing edges. | |

For example, consider this IR: | |

.. code-block:: llvm | |

define void @foo() { | |

; ... | |

A: | |

br i1 %cond, label %B, label %C, !prof !0 | |

; ... | |

} | |

!0 = metadata !{metadata !"branch_weights", i32 7, i32 8} | |

and this simple graph representation:: | |

A -> B (edge-weight: 7) | |

A -> C (edge-weight: 8) | |

The probability of branching from block A to block B is 7/15, and the | |

probability of branching from block A to block C is 8/15. | |

See :doc:`BranchWeightMetadata` for details about the branch weight IR | |

representation. | |

Block Frequency | |

=============== | |

Block frequency is a relative metric that represents the number of times a | |

block executes. The ratio of a block frequency to the entry block frequency is | |

the expected number of times the block will execute per entry to the function. | |

Block frequency is the main output of the ``BlockFrequencyInfo`` and | |

``MachineBlockFrequencyInfo`` analysis passes. | |

Implementation: a series of DAGs | |

================================ | |

The implementation of the block frequency calculation analyses each loop, | |

bottom-up, ignoring backedges; i.e., as a DAG. After each loop is processed, | |

it's packaged up to act as a pseudo-node in its parent loop's (or the | |

function's) DAG analysis. | |

Block Mass | |

========== | |

For each DAG, the entry node is assigned a mass of ``UINT64_MAX`` and mass is | |

distributed to successors according to branch weights. Block Mass uses a | |

fixed-point representation where ``UINT64_MAX`` represents ``1.0`` and ``0`` | |

represents a number just above ``0.0``. | |

After mass is fully distributed, in any cut of the DAG that separates the exit | |

nodes from the entry node, the sum of the block masses of the nodes succeeded | |

by a cut edge should equal ``UINT64_MAX``. In other words, mass is conserved | |

as it "falls" through the DAG. | |

If a function's basic block graph is a DAG, then block masses are valid block | |

frequencies. This works poorly in practise though, since downstream users rely | |

on adding block frequencies together without hitting the maximum. | |

Loop Scale | |

========== | |

Loop scale is a metric that indicates how many times a loop iterates per entry. | |

As mass is distributed through the loop's DAG, the (otherwise ignored) backedge | |

mass is collected. This backedge mass is used to compute the exit frequency, | |

and thus the loop scale. | |

Implementation: Getting from mass and scale to frequency | |

======================================================== | |

After analysing the complete series of DAGs, each block has a mass (local to | |

its containing loop, if any), and each loop pseudo-node has a loop scale and | |

its own mass (from its parent's DAG). | |

We can get an initial frequency assignment (with entry frequency of 1.0) by | |

multiplying these masses and loop scales together. A given block's frequency | |

is the product of its mass, the mass of containing loops' pseudo nodes, and the | |

containing loops' loop scales. | |

Since downstream users need integers (not floating point), this initial | |

frequency assignment is shifted as necessary into the range of ``uint64_t``. | |

Block Bias | |

========== | |

Block bias is a proposed *absolute* metric to indicate a bias toward or away | |

from a given block during a function's execution. The idea is that bias can be | |

used in isolation to indicate whether a block is relatively hot or cold, or to | |

compare two blocks to indicate whether one is hotter or colder than the other. | |

The proposed calculation involves calculating a *reference* block frequency, | |

where: | |

* every branch weight is assumed to be 1 (i.e., every branch probability | |

distribution is even) and | |

* loop scales are ignored. | |

This reference frequency represents what the block frequency would be in an | |

unbiased graph. | |

The bias is the ratio of the block frequency to this reference block frequency. |