llvm-project/clang-tools-extra/clangd/Preamble.h

//===--- Preamble.h - Reusing expensive parts of the AST ---------*- 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
//
//===----------------------------------------------------------------------===//
//
// The vast majority of code in a typical translation unit is in the headers
// included at the top of the file.
//
// The preamble optimization says that we can parse this code once, and reuse
// the result multiple times. The preamble is invalidated by changes to the
// code in the preamble region, to the compile command, or to files on disk.
//
// This is the most important optimization in clangd: it allows operations like
// code-completion to have sub-second latency. It is supported by the
// PrecompiledPreamble functionality in clang, which wraps the techniques used
// by PCH files, modules etc into a convenient interface.
//
//===----------------------------------------------------------------------===//
#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_PREAMBLE_H
#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_PREAMBLE_H

#include "CollectMacros.h"
#include "Compiler.h"
#include "Diagnostics.h"
#include "FS.h"
#include "Headers.h"
#include "index/CanonicalIncludes.h"
#include "support/Path.h"
#include "clang/Frontend/CompilerInvocation.h"
#include "clang/Frontend/PrecompiledPreamble.h"
#include "clang/Tooling/CompilationDatabase.h"
#include "llvm/ADT/StringRef.h"

#include <memory>
#include <string>
#include <vector>

namespace clang {
namespace clangd {

/// The parsed preamble and associated data.
///
/// As we must avoid re-parsing the preamble, any information that can only
/// be obtained during parsing must be eagerly captured and stored here.
struct PreambleData {
  PreambleData(const ParseInputs &Inputs, PrecompiledPreamble Preamble,
               std::vector<Diag> Diags, IncludeStructure Includes,
               MainFileMacros Macros,
               std::unique_ptr<PreambleFileStatusCache> StatCache,
               CanonicalIncludes CanonIncludes);

  // Version of the ParseInputs this preamble was built from.
  std::string Version;
  tooling::CompileCommand CompileCommand;
  PrecompiledPreamble Preamble;
  std::vector<Diag> Diags;
  // Processes like code completions and go-to-definitions will need #include
  // information, and their compile action skips preamble range.
  IncludeStructure Includes;
  // Macros defined in the preamble section of the main file.
  // Users care about headers vs main-file, not preamble vs non-preamble.
  // These should be treated as main-file entities e.g. for code completion.
  MainFileMacros Macros;
  // Cache of FS operations performed when building the preamble.
  // When reusing a preamble, this cache can be consumed to save IO.
  std::unique_ptr<PreambleFileStatusCache> StatCache;
  CanonicalIncludes CanonIncludes;
};

using PreambleParsedCallback =
    std::function<void(ASTContext &, std::shared_ptr<clang::Preprocessor>,
                       const CanonicalIncludes &)>;

/// Build a preamble for the new inputs unless an old one can be reused.
/// If \p PreambleCallback is set, it will be run on top of the AST while
/// building the preamble.
std::shared_ptr<const PreambleData>
buildPreamble(PathRef FileName, CompilerInvocation CI,
              const ParseInputs &Inputs, bool StoreInMemory,
              PreambleParsedCallback PreambleCallback);

/// Returns true if \p Preamble is reusable for \p Inputs. Note that it will
/// return true when some missing headers are now available.
/// FIXME: Should return more information about the delta between \p Preamble
/// and \p Inputs, e.g. new headers.
bool isPreambleCompatible(const PreambleData &Preamble,
                          const ParseInputs &Inputs, PathRef FileName,
                          const CompilerInvocation &CI);

/// Stores information required to parse a TU using a (possibly stale) Baseline
/// preamble. Later on this information can be injected into the main file by
/// updating compiler invocation with \c apply. This injected section
/// approximately reflects additions to the preamble in Modified contents, e.g.
/// new include directives.
class PreamblePatch {
public:
  /// \p Preamble is used verbatim.
  static PreamblePatch unmodified(const PreambleData &Preamble);
  /// Builds a patch that contains new PP directives introduced to the preamble
  /// section of \p Modified compared to \p Baseline.
  /// FIXME: This only handles include directives, we should at least handle
  /// define/undef.
  static PreamblePatch create(llvm::StringRef FileName,
                              const ParseInputs &Modified,
                              const PreambleData &Baseline);
  /// Adjusts CI (which compiles the modified inputs) to be used with the
  /// baseline preamble. This is done by inserting an artifical include to the
  /// \p CI that contains new directives calculated in create.
  void apply(CompilerInvocation &CI) const;

  /// Returns #include directives from the \c Modified preamble that were
  /// resolved using the \c Baseline preamble. This covers the new locations of
  /// inclusions that were moved around, but not inclusions of new files. Those
  /// will be recorded when parsing the main file: the includes in the injected
  /// section will be resolved back to their spelled positions in the main file
  /// using the presumed-location mechanism.
  std::vector<Inclusion> preambleIncludes() const;

  /// Returns textual patch contents.
  llvm::StringRef text() const { return PatchContents; }

private:
  PreamblePatch() = default;
  std::string PatchContents;
  std::string PatchFileName;
  /// Includes that are present in both \p Baseline and \p Modified. Used for
  /// patching includes of baseline preamble.
  std::vector<Inclusion> PreambleIncludes;
};

} // namespace clangd
} // namespace clang

#endif // LLVM_CLANG_TOOLS_EXTRA_CLANGD_PREAMBLE_H