src/jump-target.h

// Copyright 2008 the V8 project authors. All rights reserved.
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#ifndef V8_JUMP_TARGET_H_
#define V8_JUMP_TARGET_H_

#include "virtual-frame.h"

namespace v8 { namespace internal {

// -------------------------------------------------------------------------
// Jump targets
//
// A jump target is an abstraction of a basic-block entry in generated
// code.  It collects all the virtual frames reaching the block by
// forward jumps and pairs them with labels for the merge code along
// all forward-reaching paths.  When bound, an expected frame for the
// block is determined and code is generated to merge to the expected
// frame.  For backward jumps, the merge code is generated at the edge
// leaving the predecessor block.
//
// A jump target must have been reached via control flow (either by
// jumping, branching, or falling through) at the time it is bound.
// In particular, this means that at least one of the control-flow
// graph edges reaching the target must be a forward edge.

class JumpTarget : public Malloced {  // Shadows are dynamically allocated.
 public:
  // Forward-only jump targets can only be reached by forward CFG edges.
  enum Directionality { FORWARD_ONLY, BIDIRECTIONAL };

  // Construct a jump target with a given code generator used to generate
  // code and to provide access to a current frame.
  explicit JumpTarget(CodeGenerator* cgen,
                      Directionality direction = FORWARD_ONLY);

  // Construct a jump target without a code generator.  A code
  // generator must be supplied before using the jump target as a
  // label.  This is useful, eg, when break targets are embedded in
  // AST nodes.
  JumpTarget();

  // Supply a code generator and directionality to an already
  // constructed jump target.  This function expects to be given a
  // non-null code generator, and to be called only when the code
  // generator is not yet set.
  virtual void Initialize(CodeGenerator* cgen,
                          Directionality direction = FORWARD_ONLY);

  virtual ~JumpTarget() { Unuse(); }

  // Treat the jump target as a fresh one.  The state is reset and
  // pointed-to virtual frames are deallocated.  There should be no
  // dangling jumps to the target.
  void Unuse();

  // Reset the internal state of this jump target.  Pointed-to virtual
  // frames are not deallocated and dangling jumps to the target are
  // left dangling.
  void Reset();

  // Accessors.
  CodeGenerator* code_generator() const { return cgen_; }

  Label* entry_label() { return &entry_label_; }

  VirtualFrame* entry_frame() const { return entry_frame_; }
  void set_entry_frame(VirtualFrame* frame) {
    entry_frame_ = frame;
  }

  void make_bidirectional() { direction_ = BIDIRECTIONAL; }

  // Predicates testing the state of the encapsulated label.
  bool is_bound() const { return is_bound_; }
  bool is_linked() const { return is_linked_; }
  bool is_unused() const { return !is_bound() && !is_linked(); }

  // Emit a jump to the target.  There must be a current frame at the
  // jump and there will be no current frame after the jump.
  virtual void Jump();
  void Jump(Result* arg);
  void Jump(Result* arg0, Result* arg1);
  void Jump(Result* arg0, Result* arg1, Result* arg2);

  // Emit a conditional branch to the target.  There must be a current
  // frame at the branch.  The current frame will fall through to the
  // code after the branch.
  virtual void Branch(Condition cc, Hint hint = no_hint);
  void Branch(Condition cc, Result* arg, Hint hint = no_hint);
  void Branch(Condition cc, Result* arg0, Result* arg1, Hint hint = no_hint);
  void Branch(Condition cc,
              Result* arg0,
              Result* arg1,
              Result* arg2,
              Hint hint = no_hint);
  void Branch(Condition cc,
              Result* arg0,
              Result* arg1,
              Result* arg2,
              Result* arg3,
              Hint hint = no_hint);

  // Bind a jump target.  If there is no current frame at the binding
  // site, there must be at least one frame reaching via a forward
  // jump.
  //
  // The number of mergable elements is a number of frame elements
  // counting from the top down which must be "mergable" (not
  // constants or copies) in the entry frame at the jump target.
  // Backward jumps to the target must contain the same constants and
  // sharing as the entry frame, except for the mergable elements.
  //
  // A mergable elements argument of kAllElements indicates that all
  // frame elements must be mergable.  Mergable elements are ignored
  // completely for forward-only jump targets.
  virtual void Bind(int mergable_elements = kAllElements);
  void Bind(Result* arg, int mergable_elements = kAllElements);
  void Bind(Result* arg0, Result* arg1, int mergable_elements = kAllElements);
  void Bind(Result* arg0,
            Result* arg1,
            Result* arg2,
            int mergable_elements = kAllElements);
  void Bind(Result* arg0,
            Result* arg1,
            Result* arg2,
            Result* arg3,
            int mergable_elements = kAllElements);

  // Emit a call to a jump target.  There must be a current frame at
  // the call.  The frame at the target is the same as the current
  // frame except for an extra return address on top of it.  The frame
  // after the call is the same as the frame before the call.
  void Call();

  static const int kAllElements = -1;  // Not a valid number of elements.

 protected:
  // The code generator gives access to its current frame.
  CodeGenerator* cgen_;

  // Used to emit code.
  MacroAssembler* masm_;

  // Directionality flag set at initialization time.
  Directionality direction_;

  // A list of frames reaching this block via forward jumps.
  List<VirtualFrame*> reaching_frames_;

  // A parallel list of labels for merge code.
  List<Label> merge_labels_;

  // The frame used on entry to the block and expected at backward
  // jumps to the block.  Set when the jump target is bound, but may
  // or may not be set for forward-only blocks.
  VirtualFrame* entry_frame_;

  // The actual entry label of the block.
  Label entry_label_;

  // A target is bound if its Bind member function has been called.
  // It is linked if it is not bound but its Jump, Branch, or Call
  // member functions have been called.
  bool is_bound_;
  bool is_linked_;

 private:
  // Add a virtual frame reaching this labeled block via a forward
  // jump, and a fresh label for its merge code.
  void AddReachingFrame(VirtualFrame* frame);

  // Choose an element from a pair of frame elements to be in the
  // expected frame.  Return null if they are incompatible.
  FrameElement* Combine(FrameElement* left, FrameElement* right);

  // Compute a frame to use for entry to this block.  Mergable
  // elements is as described for the Bind function.
  void ComputeEntryFrame(int mergable_elements);

  DISALLOW_COPY_AND_ASSIGN(JumpTarget);
};


// -------------------------------------------------------------------------
// Break targets
//
// A break target is a jump target that can be used to break out of a
// statement that keeps extra state on the stack (eg, for/in or
// try/finally).  They know the expected stack height at the target
// and will drop state from nested statements as part of merging.
//
// Break targets are used for return, break, and continue targets.

class BreakTarget : public JumpTarget {
 public:
  // Construct a break target without a code generator.  A code
  // generator must be supplied before using the break target as a
  // label.  This is useful, eg, when break targets are embedded in AST
  // nodes.
  BreakTarget() {}

  // Supply a code generator, expected expression stack height, and
  // directionality to an already constructed break target.  This
  // function expects to be given a non-null code generator, and to be
  // called only when the code generator is not yet set.
  virtual void Initialize(CodeGenerator* cgen,
                          Directionality direction = FORWARD_ONLY);

  // Copy the state of this break target to the destination.  The
  // lists of forward-reaching frames and merge-point labels are
  // copied.  All virtual frame pointers are copied, not the
  // pointed-to frames.  The previous state of the destination is
  // overwritten, without deallocating pointed-to virtual frames.
  void CopyTo(BreakTarget* destination);

  // Emit a jump to the target.  There must be a current frame at the
  // jump and there will be no current frame after the jump.
  virtual void Jump();

  // Emit a conditional branch to the target.  There must be a current
  // frame at the branch.  The current frame will fall through to the
  // code after the branch.
  virtual void Branch(Condition cc, Hint hint = no_hint);

  // Bind a break target.  If there is no current frame at the binding
  // site, there must be at least one frame reaching via a forward
  // jump.
  virtual void Bind(int mergable_elements = kAllElements);

  // Setter for expected height.
  void set_expected_height(int expected) { expected_height_ = expected; }

 private:
  // The expected height of the expression stack where the target will
  // be bound, statically known at initialization time.
  int expected_height_;

  DISALLOW_COPY_AND_ASSIGN(BreakTarget);
};


// -------------------------------------------------------------------------
// Shadow break targets
//
// A shadow break target represents a break target that is temporarily
// shadowed by another one (represented by the original during
// shadowing).  They are used to catch jumps to labels in certain
// contexts, e.g. try blocks.  After shadowing ends, the formerly
// shadowed target is again represented by the original and the
// ShadowTarget can be used as a jump target in its own right,
// representing the formerly shadowing target.

class ShadowTarget : public BreakTarget {
 public:
  // Construct a shadow jump target.  After construction the shadow
  // target object holds the state of the original target, and the
  // original target is actually a fresh one that intercepts control
  // flow intended for the shadowed one.
  explicit ShadowTarget(BreakTarget* shadowed);

  virtual ~ShadowTarget() {
    ASSERT(!is_shadowing_);
  }

  // End shadowing.  After shadowing ends, the original jump target
  // again gives access to the formerly shadowed target and the shadow
  // target object gives access to the formerly shadowing target.
  void StopShadowing();

  // During shadowing, the currently shadowing target.  After
  // shadowing, the target that was shadowed.
  BreakTarget* other_target() const { return other_target_; }

 private:
  // During shadowing, the currently shadowing target.  After
  // shadowing, the target that was shadowed.
  BreakTarget* other_target_;

#ifdef DEBUG
  bool is_shadowing_;
#endif

  DISALLOW_COPY_AND_ASSIGN(ShadowTarget);
};


} }  // namespace v8::internal

#endif  // V8_JUMP_TARGET_H_