[llvm] Support: Add vfs::OutputBackend and OutputFile to virtualize compiler outputs (PR #113363)

[Alexandre Ganea via llvm-commits]

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+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// This file contains the declarations of the HashingOutputBackend class, which
+/// is the VirtualOutputBackend that is only producing the hash for the output
+/// files.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_SUPPORT_HASHINGOUTPUTBACKEND_H
+#define LLVM_SUPPORT_HASHINGOUTPUTBACKEND_H
+
+#include "llvm/ADT/StringExtras.h"
+#include "llvm/ADT/StringMap.h"
+#include "llvm/Support/HashBuilder.h"
+#include "llvm/Support/VirtualOutputBackend.h"
+#include "llvm/Support/VirtualOutputConfig.h"
+#include "llvm/Support/raw_ostream.h"
+#include <mutex>
+
+namespace llvm::vfs {
+
+/// raw_pwrite_stream that writes to a hasher.
+template <typename HasherT>
+class HashingStream : public llvm::raw_pwrite_stream {
+private:
+  SmallVector<char> Buffer;
+  raw_svector_ostream OS;
+
+  using HashBuilderT = HashBuilder<HasherT, endianness::native>;
+  HashBuilderT Builder;
+
+  void write_impl(const char *Ptr, size_t Size) override {
+    OS.write(Ptr, Size);
+  }
+
+  void pwrite_impl(const char *Ptr, size_t Size, uint64_t Offset) override {
+    OS.pwrite(Ptr, Size, Offset);
+  }
+
+  uint64_t current_pos() const override { return OS.str().size(); }
+
+public:
+  HashingStream() : OS(Buffer) { SetUnbuffered(); }
+
+  auto final() {
+    Builder.update(OS.str());
+    return Builder.final();
+  }
+};
+
+template <typename HasherT> class HashingOutputFile;
+
+/// An output backend that only generates the hash for outputs.
+template <typename HasherT> class HashingOutputBackend : public OutputBackend {
+private:
+  friend class HashingOutputFile<HasherT>;
+  void addOutputFile(StringRef Path, StringRef Hash) {
+    std::lock_guard<std::mutex> Lock(OutputHashLock);
+    OutputHashes[Path] = std::string(Hash);
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aganea wrote:

`Hash.str()`?

https://github.com/llvm/llvm-project/pull/113363