[llvm] [CAS] Add LLVMCAS library with InMemoryCAS implementation (PR #114096)

[Adrian Prantl via llvm-commits]

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+//===- BuiltinCASContext.h --------------------------------------*- 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CAS_BUILTINCASCONTEXT_H
+#define LLVM_CAS_BUILTINCASCONTEXT_H
+
+#include "llvm/CAS/CASID.h"
+#include "llvm/Support/BLAKE3.h"
+#include "llvm/Support/Error.h"
+
+namespace llvm::cas::builtin {
+
+/// Current hash type for the builtin CAS.
+///
+/// FIXME: This should be configurable via an enum to allow configuring the hash
+/// function. The enum should be sent into \a createInMemoryCAS() and \a
+/// createOnDiskCAS().
+///
+/// This is important (at least) for future-proofing, when we want to make new
+/// CAS instances use BLAKE7, but still know how to read/write BLAKE3.
+///
+/// Even just for BLAKE3, it would be useful to have these values:
+///
+///     BLAKE3     => 32B hash from BLAKE3
+///     BLAKE3_16B => 16B hash from BLAKE3 (truncated)
+///
+/// ... where BLAKE3_16 uses \a TruncatedBLAKE3<16>.
+///
+/// Motivation for a truncated hash is that it's cheaper to store. It's not
+/// clear if we always (or ever) need the full 32B, and for an ephemeral
+/// in-memory CAS, we almost certainly don't need it.
+///
+/// Note that the cost is linear in the number of objects for the builtin CAS,
+/// since we're using internal offsets and/or pointers as an optimization.
+///
+/// However, it's possible we'll want to hook up a local builtin CAS to, e.g.,
+/// a distributed generic hash map to use as an ActionCache. In that scenario,
+/// the transitive closure of the structured objects that are the results of
+/// the cached actions would need to be serialized into the map, something
+/// like:
+///
+///     "action:<schema>:<key>" -> "0123"
+///     "object:<schema>:0123"  -> "3,4567,89AB,CDEF,9,some data"
+///     "object:<schema>:4567"  -> ...
+///     "object:<schema>:89AB"  -> ...
+///     "object:<schema>:CDEF"  -> ...
+///
+/// These references would be full cost.
+using HasherT = BLAKE3;
+using HashType = decltype(HasherT::hash(std::declval<ArrayRef<uint8_t> &>()));
+
+class BuiltinCASContext : public CASContext {
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adrian-prantl wrote:

Doxygen comment?

https://github.com/llvm/llvm-project/pull/114096