[PATCH] D34218: [Doc] Document prof metadata in LangRef

[Teresa Johnson via Phabricator via llvm-commits]

tejohnson created this revision.

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


https://reviews.llvm.org/D34218

Files:
  docs/LangRef.rst


Index: docs/LangRef.rst
===================================================================
--- docs/LangRef.rst
+++ docs/LangRef.rst
@@ -5192,6 +5192,68 @@
     !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, 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 enable comparing different functions during inter-procedural analysis
+and optimization.
+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 memcpy calls (where it records the
+hottest memcpy 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.
+
+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
 =====================
 


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