[compiler-rt] [llvm] [docs][IRPGO]Document two binary formats for IRPGO profiles (PR #76105)

[David Li via llvm-commits]

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+=====================
+IRPGO Profile Format
+=====================
+
+.. contents::
+   :local:
+
+
+Overview
+==========
+
+IR-based instrumentation (IRPGO) and its context-sensitive variant (CS-IRPGO)
+inserts `llvm.instrprof.*` `code generator intrinsics <https://llvm.org/docs/LangRef.html#code-generator-intrinsics>`_
+in LLVM IR to generate profiles. This document describes two binary profile
+formats (raw and indexed) used by IR-based instrumentation.
+
+.. note::
+
+  Both the compiler-rt profiling infrastructure and profile format are general
+  and could support other use cases (e.g., coverage and temporal profiling).
+  This document will focus on IRPGO while briefly introducing other use cases
+  with pointers.
+
+Raw PGO Profile Format
+========================
+
+The raw PGO profile is generated by running the instrumented binary. It is a
+memory dump of the profile data.
+
+Two kinds of frequently used profile information are function's basic block
+counters and its (various flavors of) value profiles. A function's profiled
+information span across several sections in the profile.
+
+General Storage Layout
+-----------------------
+
+A raw profile for an executable [1]_ consists of a profile header and several
+sections. The storage layout is illustrated below. Generally, when raw profile
+is read into an memory buffer, the actual byte offset of a section is inferred
+from the section's order in the layout and size information of all sections
+ahead of it.
+
+::
+
+  +----+-----------------------+
+  |    |        Magic          |
+  |    +-----------------------+
+  |    |        Version        |
+  |    +-----------------------+
+  H    |   Size Info for       |
+  E    |      Section 1        |
+  A    +-----------------------+
+  D    |   Size Info for       |
+  E    |      Section 2        |
+  R    +-----------------------+
+  |    |          ...          |
+  |    +-----------------------+
+  |    |   Size Info for       |
+  |    |      Section N        |
+  +----+-----------------------+
+  P    |       Section 1       |
+  A    +-----------------------+
+  Y    |       Section 2       |
+  L    +-----------------------+
+  O    |          ...          |
+  A    +-----------------------+
+  D    |       Section N       |
+  +----+-----------------------+
+
+
+.. note::
+   Sections might be padded to meet platform-specific alignment requirements.
+   For simplicity, header fields and data sections solely for padding purpose
+   are omitted in the data layout graph above and the rest of this document.
+
+Header
+-------
+
+``Magic``
+  With the magic number, data consumer could detect profile format and
+  endianness of the data, and quickly tells whether/how to continue reading.
----------------
david-xl wrote:

remove 'quickly'

https://github.com/llvm/llvm-project/pull/76105