EHScopeStack.h

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//===-- EHScopeStack.h - Stack for cleanup CIR generation -------*- 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
//
//===----------------------------------------------------------------------===//
//
// These classes should be the minimum interface required for other parts of
// CIR CodeGen to emit cleanups.  The implementation is in CIRGenCleanup.cpp and
// other implemenentation details that are not widely needed are in
// CIRGenCleanup.h.
//
// TODO(cir): this header should be shared between LLVM and CIR codegen.
//
//===----------------------------------------------------------------------===//
 
#ifndef CLANG_LIB_CIR_CODEGEN_EHSCOPESTACK_H
#define CLANG_LIB_CIR_CODEGEN_EHSCOPESTACK_H
 
#include "clang/CIR/Dialect/IR/CIRDialect.h"
#include "llvm/ADT/SmallVector.h"
 
namespace clang::CIRGen {
 
class CIRGenFunction;
 
enum CleanupKind : unsigned {
  /// Denotes a cleanup that should run when a scope is exited using exceptional
  /// control flow (a throw statement leading to stack unwinding, ).
  EHCleanup = 0x1,
 
  /// Denotes a cleanup that should run when a scope is exited using normal
  /// control flow (falling off the end of the scope, return, goto, ...).
  NormalCleanup = 0x2,
 
  NormalAndEHCleanup = EHCleanup | NormalCleanup,
 
  LifetimeMarker = 0x8,
  NormalEHLifetimeMarker = LifetimeMarker | NormalAndEHCleanup,
};
 
/// A stack of scopes which respond to exceptions, including cleanups
/// and catch blocks.
class EHScopeStack {
  friend class CIRGenFunction;
 
public:
  // TODO(ogcg): Switch to alignof(uint64_t) instead of 8
  enum { ScopeStackAlignment = 8 };
 
  /// A saved depth on the scope stack.  This is necessary because
  /// pushing scopes onto the stack invalidates iterators.
  class stable_iterator {
    friend class EHScopeStack;
 
    /// Offset from startOfData to endOfBuffer.
    ptrdiff_t size = -1;
 
    explicit stable_iterator(ptrdiff_t size) : size(size) {}
 
  public:
    static stable_iterator invalid() { return stable_iterator(-1); }
    stable_iterator() = default;
 
    bool isValid() const { return size >= 0; }
 
    /// Returns true if this scope encloses I.
    /// Returns false if I is invalid.
    /// This scope must be valid.
    bool encloses(stable_iterator other) const { return size <= other.size; }
 
    /// Returns true if this scope strictly encloses I: that is,
    /// if it encloses I and is not I.
    /// Returns false is I is invalid.
    /// This scope must be valid.
    bool strictlyEncloses(stable_iterator I) const { return size < I.size; }
 
    friend bool operator==(stable_iterator A, stable_iterator B) {
      return A.size == B.size;
    }
    friend bool operator!=(stable_iterator A, stable_iterator B) {
      return A.size != B.size;
    }
  };
 
  /// Information for lazily generating a cleanup.  Subclasses must be
  /// POD-like: cleanups will not be destructed, and they will be
  /// allocated on the cleanup stack and freely copied and moved
  /// around.
  ///
  /// Cleanup implementations should generally be declared in an
  /// anonymous namespace.
  class LLVM_MOVABLE_POLYMORPHIC_TYPE Cleanup {
    // Anchor the construction vtable.
    virtual void anchor();
 
  public:
    Cleanup(const Cleanup &) = default;
    Cleanup(Cleanup &&) {}
    Cleanup() = default;
 
    virtual ~Cleanup() = default;
 
    virtual bool isRedundantBeforeReturn() { return false; }
 
    /// Generation flags.
    class Flags {
      enum {
        F_IsForEH = 0x1,
        F_IsNormalCleanupKind = 0x2,
        F_IsEHCleanupKind = 0x4,
        F_HasExitSwitch = 0x8,
      };
      unsigned flags = 0;
 
    public:
      Flags() = default;
 
      /// isForEH - true if the current emission is for an EH cleanup.
      bool isForEHCleanup() const { return flags & F_IsForEH; }
      bool isForNormalCleanup() const { return !isForEHCleanup(); }
      void setIsForEHCleanup() { flags |= F_IsForEH; }
 
      bool isNormalCleanupKind() const { return flags & F_IsNormalCleanupKind; }
      void setIsNormalCleanupKind() { flags |= F_IsNormalCleanupKind; }
 
      /// isEHCleanupKind - true if the cleanup was pushed as an EH
      /// cleanup.
      bool isEHCleanupKind() const { return flags & F_IsEHCleanupKind; }
      void setIsEHCleanupKind() { flags |= F_IsEHCleanupKind; }
 
      bool hasExitSwitch() const { return flags & F_HasExitSwitch; }
      void setHasExitSwitch() { flags |= F_HasExitSwitch; }
    };
 
    /// Emit the cleanup.  For normal cleanups, this is run in the
    /// same EH context as when the cleanup was pushed, i.e. the
    /// immediately-enclosing context of the cleanup scope.  For
    /// EH cleanups, this is run in a terminate context.
    ///
    // \param flags cleanup kind.
    virtual void emit(CIRGenFunction &cgf, Flags flags) = 0;
  };
 
private:
  // The implementation for this class is in CIRGenCleanup.h and
  // CIRGenCleanup.cpp; the definition is here because it's used as a
  // member of CIRGenFunction.
 
  /// The start of the scope-stack buffer, i.e. the allocated pointer
  /// for the buffer.  All of these pointers are either simultaneously
  /// null or simultaneously valid.
  std::unique_ptr<char[]> startOfBuffer;
 
  /// The end of the buffer.
  char *endOfBuffer = nullptr;
 
  /// The first valid entry in the buffer.
  char *startOfData = nullptr;
 
  /// The innermost normal cleanup on the stack.
  stable_iterator innermostNormalCleanup = stable_end();
 
  /// The innermost EH scope on the stack.
  stable_iterator innermostEHScope = stable_end();
 
  /// The CGF this Stack belong to
  CIRGenFunction *cgf = nullptr;
 
  /// When true, pushCleanup() does not create a cir.cleanup.scope op for the
  /// pushed cleanup. This is set while emitting a loop's condition variable so
  /// its destructor can be captured on the stack and later emitted into the
  /// loop op's per-iteration cleanup region.
  bool capturingLoopConditionCleanups = false;
 
  // This class uses a custom allocator for maximum efficiency because cleanups
  // are allocated and freed very frequently. It's basically a bump pointer
  // allocator, but we can't use LLVM's BumpPtrAllocator because we use offsets
  // into the buffer as stable iterators.
  char *allocate(size_t size);
  void deallocate(size_t size);
 
  void *pushCleanup(CleanupKind kind, size_t dataSize);
 
public:
  EHScopeStack() = default;
  ~EHScopeStack() = default;
 
  /// Push a lazily-created cleanup on the stack.
  template <class T, class... As> void pushCleanup(CleanupKind kind, As... a) {
    static_assert(alignof(T) <= ScopeStackAlignment,
                  "Cleanup's alignment is too large.");
    void *buffer = pushCleanup(kind, sizeof(T));
    [[maybe_unused]] Cleanup *obj = new (buffer) T(a...);
  }
 
  /// Push a cleanup with non-constant storage requirements on the
  /// stack.  The cleanup type must provide an additional static method:
  ///   static size_t getExtraSize(size_t);
  /// The argument to this method will be the value N, which will also
  /// be passed as the first argument to the constructor.
  ///
  /// The data stored in the extra storage must obey the same
  /// restrictions as normal cleanup member data.
  ///
  /// The pointer returned from this method is valid until the cleanup
  /// stack is modified.
  template <class T, class... As>
  T *pushCleanupWithExtra(CleanupKind kind, size_t n, As... a) {
    static_assert(alignof(T) <= ScopeStackAlignment,
                  "Cleanup's alignment is too large.");
    void *buffer = pushCleanup(kind, sizeof(T) + T::getExtraSize(n));
    return new (buffer) T(n, a...);
  }
 
  /// Push a cleanup by copying a serialized cleanup object from the
  /// LifetimeExtendedCleanupStack onto the EH scope stack. This is used when
  /// a full-expression's RunCleanupsScope exits: cleanups that were deferred
  /// for lifetime extension (e.g. destroying a temporary bound to a local
  /// reference) are promoted from the byte buffer to the enclosing scope's
  /// EH stack so they run when that scope ends.
  ///
  /// The memcpy is safe because Cleanup subclasses are required to be POD-like
  /// (see the Cleanup class comment), and the vtable pointer is part of the
  /// copied bytes, so the clone dispatches to the correct emit() override.
  void pushCopyOfCleanup(CleanupKind kind, const void *cleanup, size_t size) {
    void *buffer = pushCleanup(kind, size);
    std::memcpy(buffer, cleanup, size);
  }
 
  void setCGF(CIRGenFunction *inCGF) { cgf = inCGF; }
 
  bool isCapturingLoopConditionCleanups() const {
    return capturingLoopConditionCleanups;
  }
  void setCapturingLoopConditionCleanups(bool value) {
    capturingLoopConditionCleanups = value;
  }
 
  /// Pops a cleanup scope off the stack.  This is private to CIRGenCleanup.cpp.
  void popCleanup();
 
  /// Determines whether the exception-scopes stack is empty.
  bool empty() const { return startOfData == endOfBuffer; }
 
  bool requiresCatchOrCleanup() const;
 
  /// Determines whether there are any normal cleanups on the stack.
  bool hasNormalCleanups() const {
    return innermostNormalCleanup != stable_end();
  }
 
  /// Returns the innermost normal cleanup on the stack, or
  /// stable_end() if there are no normal cleanups.
  stable_iterator getInnermostNormalCleanup() const {
    return innermostNormalCleanup;
  }
  stable_iterator getInnermostActiveNormalCleanup() const;
 
  stable_iterator getInnermostEHScope() const { return innermostEHScope; }
 
  /// An unstable reference to a scope-stack depth.  Invalidated by
  /// pushes but not pops.
  class iterator;
 
  /// Returns an iterator pointing to the innermost EH scope.
  iterator begin() const;
 
  /// Returns an iterator pointing to the outermost EH scope.
  iterator end() const;
 
  /// Create a stable reference to the top of the EH stack.  The
  /// returned reference is valid until that scope is popped off the
  /// stack.
  stable_iterator stable_begin() const {
    return stable_iterator(endOfBuffer - startOfData);
  }
 
  /// Create a stable reference to the bottom of the EH stack.
  static stable_iterator stable_end() { return stable_iterator(0); }
 
  /// Turn a stable reference to a scope depth into a unstable pointer
  /// to the EH stack.
  iterator find(stable_iterator savePoint) const;
};
 
} // namespace clang::CIRGen
 
#endif // CLANG_LIB_CIR_CODEGEN_EHSCOPESTACK_H