xref: /external/v8/src/ia32/virtual-frame-ia32.cc

// Copyright 2009 the V8 project authors. All rights reserved.
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
//     * Redistributions of source code must retain the above copyright
//       notice, this list of conditions and the following disclaimer.
//     * Redistributions in binary form must reproduce the above
//       copyright notice, this list of conditions and the following
//       disclaimer in the documentation and/or other materials provided
//       with the distribution.
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//       contributors may be used to endorse or promote products derived
//       from this software without specific prior written permission.
//
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// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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#include "v8.h"

#include "codegen-inl.h"
#include "register-allocator-inl.h"
#include "scopes.h"

namespace v8 {
namespace internal {

#define __ ACCESS_MASM(masm())

// -------------------------------------------------------------------------
// VirtualFrame implementation.

// On entry to a function, the virtual frame already contains the receiver,
// the parameters, and a return address.  All frame elements are in memory.
VirtualFrame::VirtualFrame()
    : elements_(parameter_count() + local_count() + kPreallocatedElements),
      stack_pointer_(parameter_count() + 1) {  // 0-based index of TOS.
  for (int i = 0; i <= stack_pointer_; i++) {
    elements_.Add(FrameElement::MemoryElement(NumberInfo::kUnknown));
  }
  for (int i = 0; i < RegisterAllocator::kNumRegisters; i++) {
    register_locations_[i] = kIllegalIndex;
  }
}


void VirtualFrame::SyncElementBelowStackPointer(int index) {
  // Emit code to write elements below the stack pointer to their
  // (already allocated) stack address.
  ASSERT(index <= stack_pointer_);
  FrameElement element = elements_[index];
  ASSERT(!element.is_synced());
  switch (element.type()) {
    case FrameElement::INVALID:
      break;

    case FrameElement::MEMORY:
      // This function should not be called with synced elements.
      // (memory elements are always synced).
      UNREACHABLE();
      break;

    case FrameElement::REGISTER:
      __ mov(Operand(ebp, fp_relative(index)), element.reg());
      break;

    case FrameElement::CONSTANT:
      if (cgen()->IsUnsafeSmi(element.handle())) {
        cgen()->StoreUnsafeSmiToLocal(fp_relative(index), element.handle());
      } else {
        __ Set(Operand(ebp, fp_relative(index)),
               Immediate(element.handle()));
      }
      break;

    case FrameElement::COPY: {
      int backing_index = element.index();
      FrameElement backing_element = elements_[backing_index];
      if (backing_element.is_memory()) {
        Result temp = cgen()->allocator()->Allocate();
        ASSERT(temp.is_valid());
        __ mov(temp.reg(), Operand(ebp, fp_relative(backing_index)));
        __ mov(Operand(ebp, fp_relative(index)), temp.reg());
      } else {
        ASSERT(backing_element.is_register());
        __ mov(Operand(ebp, fp_relative(index)), backing_element.reg());
      }
      break;
    }
  }
  elements_[index].set_sync();
}


void VirtualFrame::SyncElementByPushing(int index) {
  // Sync an element of the frame that is just above the stack pointer
  // by pushing it.
  ASSERT(index == stack_pointer_ + 1);
  stack_pointer_++;
  FrameElement element = elements_[index];

  switch (element.type()) {
    case FrameElement::INVALID:
      __ push(Immediate(Smi::FromInt(0)));
      break;

    case FrameElement::MEMORY:
      // No memory elements exist above the stack pointer.
      UNREACHABLE();
      break;

    case FrameElement::REGISTER:
      __ push(element.reg());
      break;

    case FrameElement::CONSTANT:
      if (cgen()->IsUnsafeSmi(element.handle())) {
       cgen()->PushUnsafeSmi(element.handle());
      } else {
        __ push(Immediate(element.handle()));
      }
      break;

    case FrameElement::COPY: {
      int backing_index = element.index();
      FrameElement backing = elements_[backing_index];
      ASSERT(backing.is_memory() || backing.is_register());
      if (backing.is_memory()) {
        __ push(Operand(ebp, fp_relative(backing_index)));
      } else {
        __ push(backing.reg());
      }
      break;
    }
  }
  elements_[index].set_sync();
}


// Clear the dirty bits for the range of elements in
// [min(stack_pointer_ + 1,begin), end].
void VirtualFrame::SyncRange(int begin, int end) {
  ASSERT(begin >= 0);
  ASSERT(end < element_count());
  // Sync elements below the range if they have not been materialized
  // on the stack.
  int start = Min(begin, stack_pointer_ + 1);

  // Emit normal push instructions for elements above stack pointer
  // and use mov instructions if we are below stack pointer.
  for (int i = start; i <= end; i++) {
    if (!elements_[i].is_synced()) {
      if (i <= stack_pointer_) {
        SyncElementBelowStackPointer(i);
      } else {
        SyncElementByPushing(i);
      }
    }
  }
}


void VirtualFrame::MakeMergable() {
  for (int i = 0; i < element_count(); i++) {
    FrameElement element = elements_[i];

    // All number type information is reset to unknown for a mergable frame
    // because of incoming back edges.
    if (element.is_constant() || element.is_copy()) {
      if (element.is_synced()) {
        // Just spill.
        elements_[i] = FrameElement::MemoryElement(NumberInfo::kUnknown);
      } else {
        // Allocate to a register.
        FrameElement backing_element;  // Invalid if not a copy.
        if (element.is_copy()) {
          backing_element = elements_[element.index()];
        }
        Result fresh = cgen()->allocator()->Allocate();
        ASSERT(fresh.is_valid());  // A register was spilled if all were in use.
        elements_[i] =
            FrameElement::RegisterElement(fresh.reg(),
                                          FrameElement::NOT_SYNCED,
                                          NumberInfo::kUnknown);
        Use(fresh.reg(), i);

        // Emit a move.
        if (element.is_constant()) {
          if (cgen()->IsUnsafeSmi(element.handle())) {
            cgen()->MoveUnsafeSmi(fresh.reg(), element.handle());
          } else {
            __ Set(fresh.reg(), Immediate(element.handle()));
          }
        } else {
          ASSERT(element.is_copy());
          // Copies are only backed by register or memory locations.
          if (backing_element.is_register()) {
            // The backing store may have been spilled by allocating,
            // but that's OK.  If it was, the value is right where we
            // want it.
            if (!fresh.reg().is(backing_element.reg())) {
              __ mov(fresh.reg(), backing_element.reg());
            }
          } else {
            ASSERT(backing_element.is_memory());
            __ mov(fresh.reg(), Operand(ebp, fp_relative(element.index())));
          }
        }
      }
      // No need to set the copied flag --- there are no copies.
    } else {
      // Clear the copy flag of non-constant, non-copy elements.
      // They cannot be copied because copies are not allowed.
      // The copy flag is not relied on before the end of this loop,
      // including when registers are spilled.
      elements_[i].clear_copied();
      elements_[i].set_number_info(NumberInfo::kUnknown);
    }
  }
}


void VirtualFrame::MergeTo(VirtualFrame* expected) {
  Comment cmnt(masm(), "[ Merge frame");
  // We should always be merging the code generator's current frame to an
  // expected frame.
  ASSERT(cgen()->frame() == this);

  // Adjust the stack pointer upward (toward the top of the virtual
  // frame) if necessary.
  if (stack_pointer_ < expected->stack_pointer_) {
    int difference = expected->stack_pointer_ - stack_pointer_;
    stack_pointer_ = expected->stack_pointer_;
    __ sub(Operand(esp), Immediate(difference * kPointerSize));
  }

  MergeMoveRegistersToMemory(expected);
  MergeMoveRegistersToRegisters(expected);
  MergeMoveMemoryToRegisters(expected);

  // Adjust the stack pointer downward if necessary.
  if (stack_pointer_ > expected->stack_pointer_) {
    int difference = stack_pointer_ - expected->stack_pointer_;
    stack_pointer_ = expected->stack_pointer_;
    __ add(Operand(esp), Immediate(difference * kPointerSize));
  }

  // At this point, the frames should be identical.
  ASSERT(Equals(expected));
}


void VirtualFrame::MergeMoveRegistersToMemory(VirtualFrame* expected) {
  ASSERT(stack_pointer_ >= expected->stack_pointer_);

  // Move registers, constants, and copies to memory.  Perform moves
  // from the top downward in the frame in order to leave the backing
  // stores of copies in registers.
  //
  // Moving memory-backed copies to memory requires a spare register
  // for the memory-to-memory moves.  Since we are performing a merge,
  // we use esi (which is already saved in the frame).  We keep track
  // of the index of the frame element esi is caching or kIllegalIndex
  // if esi has not been disturbed.
  int esi_caches = kIllegalIndex;
  for (int i = element_count() - 1; i >= 0; i--) {
    FrameElement target = expected->elements_[i];
    if (target.is_register()) continue;  // Handle registers later.
    if (target.is_memory()) {
      FrameElement source = elements_[i];
      switch (source.type()) {
        case FrameElement::INVALID:
          // Not a legal merge move.
          UNREACHABLE();
          break;

        case FrameElement::MEMORY:
          // Already in place.
          break;

        case FrameElement::REGISTER:
          Unuse(source.reg());
          if (!source.is_synced()) {
            __ mov(Operand(ebp, fp_relative(i)), source.reg());
          }
          break;

        case FrameElement::CONSTANT:
          if (!source.is_synced()) {
            if (cgen()->IsUnsafeSmi(source.handle())) {
              esi_caches = i;
              cgen()->MoveUnsafeSmi(esi, source.handle());
              __ mov(Operand(ebp, fp_relative(i)), esi);
            } else {
              __ Set(Operand(ebp, fp_relative(i)), Immediate(source.handle()));
            }
          }
          break;

        case FrameElement::COPY:
          if (!source.is_synced()) {
            int backing_index = source.index();
            FrameElement backing_element = elements_[backing_index];
            if (backing_element.is_memory()) {
              // If we have to spill a register, we spill esi.
              if (esi_caches != backing_index) {
                esi_caches = backing_index;
                __ mov(esi, Operand(ebp, fp_relative(backing_index)));
              }
              __ mov(Operand(ebp, fp_relative(i)), esi);
            } else {
              ASSERT(backing_element.is_register());
              __ mov(Operand(ebp, fp_relative(i)), backing_element.reg());
            }
          }
          break;
      }
    }
    elements_[i] = target;
  }

  if (esi_caches != kIllegalIndex) {
    __ mov(esi, Operand(ebp, fp_relative(context_index())));
  }
}


void VirtualFrame::MergeMoveRegistersToRegisters(VirtualFrame* expected) {
  // We have already done X-to-memory moves.
  ASSERT(stack_pointer_ >= expected->stack_pointer_);

  for (int i = 0; i < RegisterAllocator::kNumRegisters; i++) {
    // Move the right value into register i if it is currently in a register.
    int index = expected->register_location(i);
    int use_index = register_location(i);
    // Skip if register i is unused in the target or else if source is
    // not a register (this is not a register-to-register move).
    if (index == kIllegalIndex || !elements_[index].is_register()) continue;

    Register target = RegisterAllocator::ToRegister(i);
    Register source = elements_[index].reg();
    if (index != use_index) {
      if (use_index == kIllegalIndex) {  // Target is currently unused.
        // Copy contents of source from source to target.
        // Set frame element register to target.
        Use(target, index);
        Unuse(source);
        __ mov(target, source);
      } else {
        // Exchange contents of registers source and target.
        // Nothing except the register backing use_index has changed.
        elements_[use_index].set_reg(source);
        set_register_location(target, index);
        set_register_location(source, use_index);
        __ xchg(source, target);
      }
    }

    if (!elements_[index].is_synced() &&
        expected->elements_[index].is_synced()) {
      __ mov(Operand(ebp, fp_relative(index)), target);
    }
    elements_[index] = expected->elements_[index];
  }
}


void VirtualFrame::MergeMoveMemoryToRegisters(VirtualFrame* expected) {
  // Move memory, constants, and copies to registers.  This is the
  // final step and since it is not done from the bottom up, but in
  // register code order, we have special code to ensure that the backing
  // elements of copies are in their correct locations when we
  // encounter the copies.
  for (int i = 0; i < RegisterAllocator::kNumRegisters; i++) {
    int index = expected->register_location(i);
    if (index != kIllegalIndex) {
      FrameElement source = elements_[index];
      FrameElement target = expected->elements_[index];
      Register target_reg = RegisterAllocator::ToRegister(i);
      ASSERT(target.reg().is(target_reg));
      switch (source.type()) {
        case FrameElement::INVALID:  // Fall through.
          UNREACHABLE();
          break;
        case FrameElement::REGISTER:
          ASSERT(source.Equals(target));
          // Go to next iteration.  Skips Use(target_reg) and syncing
          // below.  It is safe to skip syncing because a target
          // register frame element would only be synced if all source
          // elements were.
          continue;
          break;
        case FrameElement::MEMORY:
          ASSERT(index <= stack_pointer_);
          __ mov(target_reg, Operand(ebp, fp_relative(index)));
          break;

        case FrameElement::CONSTANT:
          if (cgen()->IsUnsafeSmi(source.handle())) {
            cgen()->MoveUnsafeSmi(target_reg, source.handle());
          } else {
           __ Set(target_reg, Immediate(source.handle()));
          }
          break;

        case FrameElement::COPY: {
          int backing_index = source.index();
          FrameElement backing = elements_[backing_index];
          ASSERT(backing.is_memory() || backing.is_register());
          if (backing.is_memory()) {
            ASSERT(backing_index <= stack_pointer_);
            // Code optimization if backing store should also move
            // to a register: move backing store to its register first.
            if (expected->elements_[backing_index].is_register()) {
              FrameElement new_backing = expected->elements_[backing_index];
              Register new_backing_reg = new_backing.reg();
              ASSERT(!is_used(new_backing_reg));
              elements_[backing_index] = new_backing;
              Use(new_backing_reg, backing_index);
              __ mov(new_backing_reg,
                     Operand(ebp, fp_relative(backing_index)));
              __ mov(target_reg, new_backing_reg);
            } else {
              __ mov(target_reg, Operand(ebp, fp_relative(backing_index)));
            }
          } else {
            __ mov(target_reg, backing.reg());
          }
        }
      }
      // Ensure the proper sync state.
      if (target.is_synced() && !source.is_synced()) {
        __ mov(Operand(ebp, fp_relative(index)), target_reg);
      }
      Use(target_reg, index);
      elements_[index] = target;
    }
  }
}


void VirtualFrame::Enter() {
  // Registers live on entry: esp, ebp, esi, edi.
  Comment cmnt(masm(), "[ Enter JS frame");

#ifdef DEBUG
  if (FLAG_debug_code) {
    // Verify that edi contains a JS function.  The following code
    // relies on eax being available for use.
    __ test(edi, Immediate(kSmiTagMask));
    __ Check(not_zero,
             "VirtualFrame::Enter - edi is not a function (smi check).");
    __ CmpObjectType(edi, JS_FUNCTION_TYPE, eax);
    __ Check(equal,
             "VirtualFrame::Enter - edi is not a function (map check).");
  }
#endif

  EmitPush(ebp);

  __ mov(ebp, Operand(esp));

  // Store the context in the frame.  The context is kept in esi and a
  // copy is stored in the frame.  The external reference to esi
  // remains.
  EmitPush(esi);

  // Store the function in the frame.  The frame owns the register
  // reference now (ie, it can keep it in edi or spill it later).
  Push(edi);
  SyncElementAt(element_count() - 1);
  cgen()->allocator()->Unuse(edi);
}


void VirtualFrame::Exit() {
  Comment cmnt(masm(), "[ Exit JS frame");
  // Record the location of the JS exit code for patching when setting
  // break point.
  __ RecordJSReturn();

  // Avoid using the leave instruction here, because it is too
  // short. We need the return sequence to be a least the size of a
  // call instruction to support patching the exit code in the
  // debugger. See VisitReturnStatement for the full return sequence.
  __ mov(esp, Operand(ebp));
  stack_pointer_ = frame_pointer();
  for (int i = element_count() - 1; i > stack_pointer_; i--) {
    FrameElement last = elements_.RemoveLast();
    if (last.is_register()) {
      Unuse(last.reg());
    }
  }

  EmitPop(ebp);
}


void VirtualFrame::AllocateStackSlots() {
  int count = local_count();
  if (count > 0) {
    Comment cmnt(masm(), "[ Allocate space for locals");
    // The locals are initialized to a constant (the undefined value), but
    // we sync them with the actual frame to allocate space for spilling
    // them later.  First sync everything above the stack pointer so we can
    // use pushes to allocate and initialize the locals.
    SyncRange(stack_pointer_ + 1, element_count() - 1);
    Handle<Object> undefined = Factory::undefined_value();
    FrameElement initial_value =
        FrameElement::ConstantElement(undefined, FrameElement::SYNCED);
    if (count == 1) {
      __ push(Immediate(undefined));
    } else if (count < kLocalVarBound) {
      // For less locals the unrolled loop is more compact.
      Result temp = cgen()->allocator()->Allocate();
      ASSERT(temp.is_valid());
      __ Set(temp.reg(), Immediate(undefined));
      for (int i = 0; i < count; i++) {
        __ push(temp.reg());
      }
    } else {
      // For more locals a loop in generated code is more compact.
      Label alloc_locals_loop;
      Result cnt = cgen()->allocator()->Allocate();
      Result tmp = cgen()->allocator()->Allocate();
      ASSERT(cnt.is_valid());
      ASSERT(tmp.is_valid());
      __ mov(cnt.reg(), Immediate(count));
      __ mov(tmp.reg(), Immediate(undefined));
      __ bind(&alloc_locals_loop);
      __ push(tmp.reg());
      __ dec(cnt.reg());
      __ j(not_zero, &alloc_locals_loop);
    }
    for (int i = 0; i < count; i++) {
      elements_.Add(initial_value);
      stack_pointer_++;
    }
  }
}


void VirtualFrame::SaveContextRegister() {
  ASSERT(elements_[context_index()].is_memory());
  __ mov(Operand(ebp, fp_relative(context_index())), esi);
}


void VirtualFrame::RestoreContextRegister() {
  ASSERT(elements_[context_index()].is_memory());
  __ mov(esi, Operand(ebp, fp_relative(context_index())));
}


void VirtualFrame::PushReceiverSlotAddress() {
  Result temp = cgen()->allocator()->Allocate();
  ASSERT(temp.is_valid());
  __ lea(temp.reg(), ParameterAt(-1));
  Push(&temp);
}


int VirtualFrame::InvalidateFrameSlotAt(int index) {
  FrameElement original = elements_[index];

  // Is this element the backing store of any copies?
  int new_backing_index = kIllegalIndex;
  if (original.is_copied()) {
    // Verify it is copied, and find first copy.
    for (int i = index + 1; i < element_count(); i++) {
      if (elements_[i].is_copy() && elements_[i].index() == index) {
        new_backing_index = i;
        break;
      }
    }
  }

  if (new_backing_index == kIllegalIndex) {
    // No copies found, return kIllegalIndex.
    if (original.is_register()) {
      Unuse(original.reg());
    }
    elements_[index] = FrameElement::InvalidElement();
    return kIllegalIndex;
  }

  // This is the backing store of copies.
  Register backing_reg;
  if (original.is_memory()) {
    Result fresh = cgen()->allocator()->Allocate();
    ASSERT(fresh.is_valid());
    Use(fresh.reg(), new_backing_index);
    backing_reg = fresh.reg();
    __ mov(backing_reg, Operand(ebp, fp_relative(index)));
  } else {
    // The original was in a register.
    backing_reg = original.reg();
    set_register_location(backing_reg, new_backing_index);
  }
  // Invalidate the element at index.
  elements_[index] = FrameElement::InvalidElement();
  // Set the new backing element.
  if (elements_[new_backing_index].is_synced()) {
    elements_[new_backing_index] =
        FrameElement::RegisterElement(backing_reg,
                                      FrameElement::SYNCED,
                                      original.number_info());
  } else {
    elements_[new_backing_index] =
        FrameElement::RegisterElement(backing_reg,
                                      FrameElement::NOT_SYNCED,
                                      original.number_info());
  }
  // Update the other copies.
  for (int i = new_backing_index + 1; i < element_count(); i++) {
    if (elements_[i].is_copy() && elements_[i].index() == index) {
      elements_[i].set_index(new_backing_index);
      elements_[new_backing_index].set_copied();
    }
  }
  return new_backing_index;
}


void VirtualFrame::TakeFrameSlotAt(int index) {
  ASSERT(index >= 0);
  ASSERT(index <= element_count());
  FrameElement original = elements_[index];
  int new_backing_store_index = InvalidateFrameSlotAt(index);
  if (new_backing_store_index != kIllegalIndex) {
    elements_.Add(CopyElementAt(new_backing_store_index));
    return;
  }

  switch (original.type()) {
    case FrameElement::MEMORY: {
      // Emit code to load the original element's data into a register.
      // Push that register as a FrameElement on top of the frame.
      Result fresh = cgen()->allocator()->Allocate();
      ASSERT(fresh.is_valid());
      FrameElement new_element =
          FrameElement::RegisterElement(fresh.reg(),
                                        FrameElement::NOT_SYNCED,
                                        original.number_info());
      Use(fresh.reg(), element_count());
      elements_.Add(new_element);
      __ mov(fresh.reg(), Operand(ebp, fp_relative(index)));
      break;
    }
    case FrameElement::REGISTER:
      Use(original.reg(), element_count());
      // Fall through.
    case FrameElement::CONSTANT:
    case FrameElement::COPY:
      original.clear_sync();
      elements_.Add(original);
      break;
    case FrameElement::INVALID:
      UNREACHABLE();
      break;
  }
}


void VirtualFrame::StoreToFrameSlotAt(int index) {
  // Store the value on top of the frame to the virtual frame slot at
  // a given index.  The value on top of the frame is left in place.
  // This is a duplicating operation, so it can create copies.
  ASSERT(index >= 0);
  ASSERT(index < element_count());

  int top_index = element_count() - 1;
  FrameElement top = elements_[top_index];
  FrameElement original = elements_[index];
  if (top.is_copy() && top.index() == index) return;
  ASSERT(top.is_valid());

  InvalidateFrameSlotAt(index);

  // InvalidateFrameSlotAt can potentially change any frame element, due
  // to spilling registers to allocate temporaries in order to preserve
  // the copy-on-write semantics of aliased elements.  Reload top from
  // the frame.
  top = elements_[top_index];

  if (top.is_copy()) {
    // There are two cases based on the relative positions of the
    // stored-to slot and the backing slot of the top element.
    int backing_index = top.index();
    ASSERT(backing_index != index);
    if (backing_index < index) {
      // 1. The top element is a copy of a slot below the stored-to
      // slot.  The stored-to slot becomes an unsynced copy of that
      // same backing slot.
      elements_[index] = CopyElementAt(backing_index);
    } else {
      // 2. The top element is a copy of a slot above the stored-to
      // slot.  The stored-to slot becomes the new (unsynced) backing
      // slot and both the top element and the element at the former
      // backing slot become copies of it.  The sync state of the top
      // and former backing elements is preserved.
      FrameElement backing_element = elements_[backing_index];
      ASSERT(backing_element.is_memory() || backing_element.is_register());
      if (backing_element.is_memory()) {
        // Because sets of copies are canonicalized to be backed by
        // their lowest frame element, and because memory frame
        // elements are backed by the corresponding stack address, we
        // have to move the actual value down in the stack.
        //
        // TODO(209): considering allocating the stored-to slot to the
        // temp register.  Alternatively, allow copies to appear in
        // any order in the frame and lazily move the value down to
        // the slot.
        Result temp = cgen()->allocator()->Allocate();
        ASSERT(temp.is_valid());
        __ mov(temp.reg(), Operand(ebp, fp_relative(backing_index)));
        __ mov(Operand(ebp, fp_relative(index)), temp.reg());
      } else {
        set_register_location(backing_element.reg(), index);
        if (backing_element.is_synced()) {
          // If the element is a register, we will not actually move
          // anything on the stack but only update the virtual frame
          // element.
          backing_element.clear_sync();
        }
      }
      elements_[index] = backing_element;

      // The old backing element becomes a copy of the new backing
      // element.
      FrameElement new_element = CopyElementAt(index);
      elements_[backing_index] = new_element;
      if (backing_element.is_synced()) {
        elements_[backing_index].set_sync();
      }

      // All the copies of the old backing element (including the top
      // element) become copies of the new backing element.
      for (int i = backing_index + 1; i < element_count(); i++) {
        if (elements_[i].is_copy() && elements_[i].index() == backing_index) {
          elements_[i].set_index(index);
        }
      }
    }
    return;
  }

  // Move the top element to the stored-to slot and replace it (the
  // top element) with a copy.
  elements_[index] = top;
  if (top.is_memory()) {
    // TODO(209): consider allocating the stored-to slot to the temp
    // register.  Alternatively, allow copies to appear in any order
    // in the frame and lazily move the value down to the slot.
    FrameElement new_top = CopyElementAt(index);
    new_top.set_sync();
    elements_[top_index] = new_top;

    // The sync state of the former top element is correct (synced).
    // Emit code to move the value down in the frame.
    Result temp = cgen()->allocator()->Allocate();
    ASSERT(temp.is_valid());
    __ mov(temp.reg(), Operand(esp, 0));
    __ mov(Operand(ebp, fp_relative(index)), temp.reg());
  } else if (top.is_register()) {
    set_register_location(top.reg(), index);
    // The stored-to slot has the (unsynced) register reference and
    // the top element becomes a copy.  The sync state of the top is
    // preserved.
    FrameElement new_top = CopyElementAt(index);
    if (top.is_synced()) {
      new_top.set_sync();
      elements_[index].clear_sync();
    }
    elements_[top_index] = new_top;
  } else {
    // The stored-to slot holds the same value as the top but
    // unsynced.  (We do not have copies of constants yet.)
    ASSERT(top.is_constant());
    elements_[index].clear_sync();
  }
}


void VirtualFrame::PushTryHandler(HandlerType type) {
  ASSERT(cgen()->HasValidEntryRegisters());
  // Grow the expression stack by handler size less one (the return
  // address is already pushed by a call instruction).
  Adjust(kHandlerSize - 1);
  __ PushTryHandler(IN_JAVASCRIPT, type);
}


Result VirtualFrame::RawCallStub(CodeStub* stub) {
  ASSERT(cgen()->HasValidEntryRegisters());
  __ CallStub(stub);
  Result result = cgen()->allocator()->Allocate(eax);
  ASSERT(result.is_valid());
  return result;
}


Result VirtualFrame::CallStub(CodeStub* stub, Result* arg) {
  PrepareForCall(0, 0);
  arg->ToRegister(eax);
  arg->Unuse();
  return RawCallStub(stub);
}


Result VirtualFrame::CallStub(CodeStub* stub, Result* arg0, Result* arg1) {
  PrepareForCall(0, 0);

  if (arg0->is_register() && arg0->reg().is(eax)) {
    if (arg1->is_register() && arg1->reg().is(edx)) {
      // Wrong registers.
      __ xchg(eax, edx);
    } else {
      // Register edx is free for arg0, which frees eax for arg1.
      arg0->ToRegister(edx);
      arg1->ToRegister(eax);
    }
  } else {
    // Register eax is free for arg1, which guarantees edx is free for
    // arg0.
    arg1->ToRegister(eax);
    arg0->ToRegister(edx);
  }

  arg0->Unuse();
  arg1->Unuse();
  return RawCallStub(stub);
}


Result VirtualFrame::CallRuntime(Runtime::Function* f, int arg_count) {
  PrepareForCall(arg_count, arg_count);
  ASSERT(cgen()->HasValidEntryRegisters());
  __ CallRuntime(f, arg_count);
  Result result = cgen()->allocator()->Allocate(eax);
  ASSERT(result.is_valid());
  return result;
}


Result VirtualFrame::CallRuntime(Runtime::FunctionId id, int arg_count) {
  PrepareForCall(arg_count, arg_count);
  ASSERT(cgen()->HasValidEntryRegisters());
  __ CallRuntime(id, arg_count);
  Result result = cgen()->allocator()->Allocate(eax);
  ASSERT(result.is_valid());
  return result;
}


#ifdef ENABLE_DEBUGGER_SUPPORT
void VirtualFrame::DebugBreak() {
  PrepareForCall(0, 0);
  ASSERT(cgen()->HasValidEntryRegisters());
  __ DebugBreak();
  Result result = cgen()->allocator()->Allocate(eax);
  ASSERT(result.is_valid());
}
#endif


Result VirtualFrame::InvokeBuiltin(Builtins::JavaScript id,
                                   InvokeFlag flag,
                                   int arg_count) {
  PrepareForCall(arg_count, arg_count);
  ASSERT(cgen()->HasValidEntryRegisters());
  __ InvokeBuiltin(id, flag);
  Result result = cgen()->allocator()->Allocate(eax);
  ASSERT(result.is_valid());
  return result;
}


Result VirtualFrame::RawCallCodeObject(Handle<Code> code,
                                       RelocInfo::Mode rmode) {
  ASSERT(cgen()->HasValidEntryRegisters());
  __ call(code, rmode);
  Result result = cgen()->allocator()->Allocate(eax);
  ASSERT(result.is_valid());
  return result;
}


Result VirtualFrame::CallLoadIC(RelocInfo::Mode mode) {
  // Name and receiver are on the top of the frame.  The IC expects
  // name in ecx and receiver in eax.
  Handle<Code> ic(Builtins::builtin(Builtins::LoadIC_Initialize));
  Result name = Pop();
  Result receiver = Pop();
  PrepareForCall(0, 0);  // No stack arguments.
  // Move results to the right registers:
  if (name.is_register() && name.reg().is(eax)) {
    if (receiver.is_register() && receiver.reg().is(ecx)) {
      // Wrong registers.
      __ xchg(eax, ecx);
    } else {
      // Register ecx is free for name, which frees eax for receiver.
      name.ToRegister(ecx);
      receiver.ToRegister(eax);
    }
  } else {
    // Register eax is free for receiver, which frees ecx for name.
    receiver.ToRegister(eax);
    name.ToRegister(ecx);
  }
  name.Unuse();
  receiver.Unuse();
  return RawCallCodeObject(ic, mode);
}


Result VirtualFrame::CallKeyedLoadIC(RelocInfo::Mode mode) {
  // Key and receiver are on top of the frame. Put them in eax and edx.
  Result key = Pop();
  Result receiver = Pop();
  PrepareForCall(0, 0);

  if (!key.is_register() || !key.reg().is(edx)) {
    // Register edx is available for receiver.
    receiver.ToRegister(edx);
    key.ToRegister(eax);
  } else if (!receiver.is_register() || !receiver.reg().is(eax)) {
    // Register eax is available for key.
    key.ToRegister(eax);
    receiver.ToRegister(edx);
  } else {
    __ xchg(edx, eax);
  }
  key.Unuse();
  receiver.Unuse();

  Handle<Code> ic(Builtins::builtin(Builtins::KeyedLoadIC_Initialize));
  return RawCallCodeObject(ic, mode);
}


Result VirtualFrame::CallStoreIC(Handle<String> name, bool is_contextual) {
  // Value and (if not contextual) receiver are on top of the frame.
  //  The IC expects name in ecx, value in eax, and receiver in edx.
  Handle<Code> ic(Builtins::builtin(Builtins::StoreIC_Initialize));
  Result value = Pop();
  if (is_contextual) {
    PrepareForCall(0, 0);
    value.ToRegister(eax);
    __ mov(edx, Operand(esi, Context::SlotOffset(Context::GLOBAL_INDEX)));
    __ mov(ecx, name);
  } else {
    Result receiver = Pop();
    PrepareForCall(0, 0);

    if (value.is_register() && value.reg().is(edx)) {
      if (receiver.is_register() && receiver.reg().is(eax)) {
        // Wrong registers.
        __ xchg(eax, edx);
      } else {
        // Register eax is free for value, which frees edx for receiver.
        value.ToRegister(eax);
        receiver.ToRegister(edx);
      }
    } else {
      // Register edx is free for receiver, which guarantees eax is free for
      // value.
      receiver.ToRegister(edx);
      value.ToRegister(eax);
    }
  }
  __ mov(ecx, name);
  value.Unuse();
  return RawCallCodeObject(ic, RelocInfo::CODE_TARGET);
}


Result VirtualFrame::CallKeyedStoreIC() {
  // Value, key, and receiver are on the top of the frame.  The IC
  // expects value in eax and key and receiver on the stack.  It does
  // not drop the key and receiver.
  Handle<Code> ic(Builtins::builtin(Builtins::KeyedStoreIC_Initialize));
  Result value = Pop();
  PrepareForCall(2, 0);  // Two stack args, neither callee-dropped.
  value.ToRegister(eax);
  value.Unuse();
  return RawCallCodeObject(ic, RelocInfo::CODE_TARGET);
}


Result VirtualFrame::CallCallIC(RelocInfo::Mode mode,
                                int arg_count,
                                int loop_nesting) {
  // Function name, arguments, and receiver are on top of the frame.
  // The IC expects the name in ecx and the rest on the stack and
  // drops them all.
  InLoopFlag in_loop = loop_nesting > 0 ? IN_LOOP : NOT_IN_LOOP;
  Handle<Code> ic = cgen()->ComputeCallInitialize(arg_count, in_loop);
  // Spill args, receiver, and function.  The call will drop args and
  // receiver.
  Result name = Pop();
  PrepareForCall(arg_count + 1, arg_count + 1);  // Arguments + receiver.
  name.ToRegister(ecx);
  name.Unuse();
  return RawCallCodeObject(ic, mode);
}


Result VirtualFrame::CallConstructor(int arg_count) {
  // Arguments, receiver, and function are on top of the frame.  The
  // IC expects arg count in eax, function in edi, and the arguments
  // and receiver on the stack.
  Handle<Code> ic(Builtins::builtin(Builtins::JSConstructCall));
  // Duplicate the function before preparing the frame.
  PushElementAt(arg_count + 1);
  Result function = Pop();
  PrepareForCall(arg_count + 1, arg_count + 1);  // Spill args and receiver.
  function.ToRegister(edi);

  // Constructors are called with the number of arguments in register
  // eax for now. Another option would be to have separate construct
  // call trampolines per different arguments counts encountered.
  Result num_args = cgen()->allocator()->Allocate(eax);
  ASSERT(num_args.is_valid());
  __ Set(num_args.reg(), Immediate(arg_count));

  function.Unuse();
  num_args.Unuse();
  return RawCallCodeObject(ic, RelocInfo::CONSTRUCT_CALL);
}


void VirtualFrame::Drop(int count) {
  ASSERT(count >= 0);
  ASSERT(height() >= count);
  int num_virtual_elements = (element_count() - 1) - stack_pointer_;

  // Emit code to lower the stack pointer if necessary.
  if (num_virtual_elements < count) {
    int num_dropped = count - num_virtual_elements;
    stack_pointer_ -= num_dropped;
    __ add(Operand(esp), Immediate(num_dropped * kPointerSize));
  }

  // Discard elements from the virtual frame and free any registers.
  for (int i = 0; i < count; i++) {
    FrameElement dropped = elements_.RemoveLast();
    if (dropped.is_register()) {
      Unuse(dropped.reg());
    }
  }
}


Result VirtualFrame::Pop() {
  FrameElement element = elements_.RemoveLast();
  int index = element_count();
  ASSERT(element.is_valid());

  // Get number type information of the result.
  NumberInfo::Type info;
  if (!element.is_copy()) {
    info = element.number_info();
  } else {
    info = elements_[element.index()].number_info();
  }

  bool pop_needed = (stack_pointer_ == index);
  if (pop_needed) {
    stack_pointer_--;
    if (element.is_memory()) {
      Result temp = cgen()->allocator()->Allocate();
      ASSERT(temp.is_valid());
      __ pop(temp.reg());
      temp.set_number_info(info);
      return temp;
    }

    __ add(Operand(esp), Immediate(kPointerSize));
  }
  ASSERT(!element.is_memory());

  // The top element is a register, constant, or a copy.  Unuse
  // registers and follow copies to their backing store.
  if (element.is_register()) {
    Unuse(element.reg());
  } else if (element.is_copy()) {
    ASSERT(element.index() < index);
    index = element.index();
    element = elements_[index];
  }
  ASSERT(!element.is_copy());

  // The element is memory, a register, or a constant.
  if (element.is_memory()) {
    // Memory elements could only be the backing store of a copy.
    // Allocate the original to a register.
    ASSERT(index <= stack_pointer_);
    Result temp = cgen()->allocator()->Allocate();
    ASSERT(temp.is_valid());
    Use(temp.reg(), index);
    FrameElement new_element =
        FrameElement::RegisterElement(temp.reg(),
                                      FrameElement::SYNCED,
                                      element.number_info());
    // Preserve the copy flag on the element.
    if (element.is_copied()) new_element.set_copied();
    elements_[index] = new_element;
    __ mov(temp.reg(), Operand(ebp, fp_relative(index)));
    return Result(temp.reg(), info);
  } else if (element.is_register()) {
    return Result(element.reg(), info);
  } else {
    ASSERT(element.is_constant());
    return Result(element.handle());
  }
}


void VirtualFrame::EmitPop(Register reg) {
  ASSERT(stack_pointer_ == element_count() - 1);
  stack_pointer_--;
  elements_.RemoveLast();
  __ pop(reg);
}


void VirtualFrame::EmitPop(Operand operand) {
  ASSERT(stack_pointer_ == element_count() - 1);
  stack_pointer_--;
  elements_.RemoveLast();
  __ pop(operand);
}


void VirtualFrame::EmitPush(Register reg, NumberInfo::Type info) {
  ASSERT(stack_pointer_ == element_count() - 1);
  elements_.Add(FrameElement::MemoryElement(info));
  stack_pointer_++;
  __ push(reg);
}


void VirtualFrame::EmitPush(Operand operand, NumberInfo::Type info) {
  ASSERT(stack_pointer_ == element_count() - 1);
  elements_.Add(FrameElement::MemoryElement(info));
  stack_pointer_++;
  __ push(operand);
}


void VirtualFrame::EmitPush(Immediate immediate, NumberInfo::Type info) {
  ASSERT(stack_pointer_ == element_count() - 1);
  elements_.Add(FrameElement::MemoryElement(info));
  stack_pointer_++;
  __ push(immediate);
}


void VirtualFrame::Push(Expression* expr) {
  ASSERT(expr->IsTrivial());

  Literal* lit = expr->AsLiteral();
  if (lit != NULL) {
    Push(lit->handle());
    return;
  }

  VariableProxy* proxy = expr->AsVariableProxy();
  if (proxy != NULL && proxy->is_this()) {
    PushParameterAt(-1);
    return;
  }

  UNREACHABLE();
}


#undef __

} }  // namespace v8::internal