[llvm] [Transforms] Introduce BuildBuiltins.h atomic helpers (PR #134455)

[via llvm-commits]

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+//===- BuildBuiltins.h - Utility builder for builtins ---------------------===//
+//
+// 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 implements some functions for lowering compiler builtins,
+// specifically for atomics. Currently, LLVM-IR has no representation of atomics
+// that can be used independent of its arguments:
+//
+// * The instructions load atomic, store atomic, atomicrmw, and cmpxchg can only
+//   be used with constant memory model, sync scope, data sizes (that must be
+//   power-of-2), volatile and weak property, and should not be used with data
+//   types that are untypically large which may slow down the compiler.
+//
+// * libcall (in GCC's case: libatomic; LLVM: Compiler-RT) functions work with
+//   any data size, but are slower. Specialized functions for a selected number
+//   of data sizes exist as well. They do not support sync scopes, the volatile
+//   or weakness property. These functions may be implemented using a lock and
+//   availability depends on the target triple (e.g. GPU devices cannot
+//   implement a global lock by design).
+//
+// Whe want to mimic Clang's behaviour:
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NimishMishra wrote:

Nit: "we"

https://github.com/llvm/llvm-project/pull/134455