[llvm] [compiler-rt] [docs][IRPGO]Document two binary formats for instrumentation-based profiles, with a focus on IRPGO. (PR #76105)

[Fangrui Song via llvm-commits]

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+===================================
+Instrumentation Profile Format
+===================================
+
+.. contents::
+   :local:
+
+
+Overview
+=========
+
+Clang supports two types of profiling via instrumentation [1]_: frontend-based
+and IR-based, and both could support a variety of use cases [2]_ .
+This document describes two binary serialization formats (raw and indexed) to
+store instrumented profiles with a specific emphasis on IRPGO use case, in the
+sense that when specific header fields and payload sections have different ways
+of interpretation across use cases, the documentation is based on IRPGO.
+
+.. note::
+  Frontend-generated profiles are used together with coverage mapping for
+  `source based code coverage`_. The `coverage mapping format`_ is different from
+  profile format.
+
+.. _`source based code coverage`: https://clang.llvm.org/docs/SourceBasedCodeCoverage.html
+.. _`coverage mapping format`: https://llvm.org/docs/CoverageMappingFormat.html
+
+Raw Profile Format
+===================
+
+The raw profile is generated by running the instrumented binary. The raw profile
+data from an executable or a shared library [3]_ consists of a header and
+multiple sections, with each section as a memory dump. The profile raw data needs
----------------
MaskRay wrote:

raw profile data

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