[llvm] [ctx_profile] Profile reader and writer (PR #91859)

[Snehasish Kumar via llvm-commits]

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+//===- PGOCtxProfWriter.h - Contextual Profile Writer -----------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file declares a utility for writing a contextual profile to bitstream.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_PROFILEDATA_PGOCTXPROFWRITER_H_
+#define LLVM_PROFILEDATA_PGOCTXPROFWRITER_H_
+
+#include "llvm/Bitstream/BitstreamWriter.h"
+#include "llvm/ProfileData/CtxInstrContextNode.h"
+
+namespace llvm {
+enum PGOCtxProfileRecords { Invalid = 0, Version, Guid, CalleeIndex, Counters };
+
+enum PGOCtxProfileBlockIDs {
+  ProfileMetadataBlockID = 100,
+  ContextNodeBlockID = ProfileMetadataBlockID + 1
+};
+
+/// Write one or more ContextNodes to the provided raw_fd_stream.
+/// The caller must destroy the PGOCtxProfileWriter object before closing the
+/// stream.
+/// The design allows serializing a bunch of contexts embedded in some other
+/// file. The overall format is:
+///
+///  [... other data written to the stream...]
+///  SubBlock(ProfileMetadataBlockID)
+///   Version
+///   SubBlock(ContextNodeBlockID)
+///     [RECORDS]
+///     SubBlock(ContextNodeBlockID)
+///       [RECORDS]
+///       [... more SubBlocks]
+///     EndBlock
+///   EndBlock
+///
+/// The "RECORDS" are bitsream records. The IDs are in CtxProfileCodes (except)
+/// for Version, which is just for metadata). All contexts will have Guid and
+/// Counters, and all but the roots have CalleeIndex. The order in which the
+/// records appear does not matter, but they must precede any subcontexts,
+/// because that helps keep the reader code simpler.
+///
+/// Subblock containment captures the context->subcontext relationship. The
+/// "next()" relationship in the raw profile, between call targets of indirect
+/// calls, are just modeled as peer subblocks where the callee index is the
+/// same.
+///
+/// Versioning: the writer may produce additional records not known by the
+/// reader. The version number indicates a more structural change.
+/// The current version, in particular, is set up to expect optional extensions
+/// like value profiling - which would appear as additional records. For
+/// example, value profiling would produce a new record with a new record ID,
+/// containing the profiled values (much like the counters)
+class PGOCtxProfileWriter final {
+  SmallVector<char, 1 << 20> Buff;
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snehasish wrote:

Why not use a llvm::MemoryBuffer?

https://github.com/llvm/llvm-project/pull/91859