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[Doc] Document prof metadata in LangRef

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Summary:
Points to existing documentation for branch_weights and
function_entry_count, and adds an example for VP value profile metadata.

Reviewers: davidxl, reames

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34218

llvm-svn: 305475
This commit is contained in:
Teresa Johnson 2017-06-15 15:57:12 +00:00
parent e54d96df5f
commit 1eecb8a116
2 changed files with 80 additions and 0 deletions
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14
docs/BranchWeightMetadata.rst
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@ -64,6 +64,20 @@ Branch weights are assigned to every destination.
[ , i32 <LABEL_BRANCH_WEIGHT> ... ]
}
``CallInst``
^^^^^^^^^^^^^^^^^^
Calls may have branch weight metadata, containing the execution count of
the call. It is currently used in SamplePGO mode only, to augment the
block and entry counts which may not be accurate with sampling.
.. code-block:: none
!0 = metadata !{
metadata !"branch_weights",
i32 <CALL_BRANCH_WEIGHT>
}
Other
^^^^^

66
docs/LangRef.rst
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@ -5192,6 +5192,72 @@ Example:
!0 = !{i32* @a}
'``prof``' Metadata
^^^^^^^^^^^^^^^^^^^
The ``prof`` metadata is used to record profile data in the IR.
The first operand of the metadata node indicates the profile metadata
type. There are currently 3 types:
:ref:`branch_weights<prof_node_branch_weights>`,
:ref:`function_entry_count<prof_node_function_entry_count>`, and
:ref:`VP<prof_node_VP>`.
.. _prof_node_branch_weights:
branch_weights
""""""""""""""
Branch weight metadata attached to a branch, select, switch or call instruction
represents the likeliness of the associated branch being taken.
For more information, see :doc:`BranchWeightMetadata`.
.. _prof_node_function_entry_count:
function_entry_count
""""""""""""""""""""
Function entry count metadata can be attached to function definitions
to record the number of times the function is called. Used with BFI
information, it is also used to derive the basic block profile count.
For more information, see :doc:`BranchWeightMetadata`.
.. _prof_node_VP:
VP
""
VP (value profile) metadata can be attached to instructions that have
value profile information. Currently this is indirect calls (where it
records the hottest callees) and calls to memory intrinsics such as memcpy,
memmove, and memset (where it records the hottest byte lengths).
Each VP metadata node contains "VP" string, then a uint32_t value for the value
profiling kind, a uint64_t value for the total number of times the instruction
is executed, followed by uint64_t value and execution count pairs.
The value profiling kind is 0 for indirect call targets and 1 for memory
operations. For indirect call targets, each profile value is a hash
of the callee function name, and for memory operations each value is the
byte length.
Note that the value counts do not need to add up to the total count
listed in the third operand (in practice only the top hottest values
are tracked and reported).
Indirect call example:
.. code-block:: llvm
call void %f(), !prof !1
!1 = !{!"VP", i32 0, i64 1600, i64 7651369219802541373, i64 1030, i64 -4377547752858689819, i64 410}
Note that the VP type is 0 (the second operand), which indicates this is
an indirect call value profile data. The third operand indicates that the
indirect call executed 1600 times. The 4th and 6th operands give the
hashes of the 2 hottest target functions' names (this is the same hash used
to represent function names in the profile database), and the 5th and 7th
operands give the execution count that each of the respective prior target
functions was called.
Module Flags Metadata
=====================