Removed unsafe optimization in RecordWrite.
Optimization was only unsafe if new-space was in the low half of memory and an object could be located in the top half at an addressed that only differ from a new-space address by the high bit. Review URL: http://codereview.chromium.org/159784 git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2608 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
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@ -226,7 +226,9 @@ enum ScaleFactor {
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times_1 = 0,
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times_2 = 1,
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times_4 = 2,
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times_8 = 3
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times_8 = 3,
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times_pointer_size = times_4,
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times_half_pointer_size = times_2
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};
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@ -146,43 +146,30 @@ void MacroAssembler::RecordWrite(Register object, int offset,
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// for the remembered set bits.
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Label done;
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// This optimization cannot survive serialization and deserialization,
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// so we disable as long as serialization can take place.
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int32_t new_space_start =
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reinterpret_cast<int32_t>(ExternalReference::new_space_start().address());
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if (Serializer::enabled() || new_space_start < 0) {
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// Cannot do smart bit-twiddling. Need to do two consecutive checks.
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// Check for Smi first.
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test(value, Immediate(kSmiTagMask));
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j(zero, &done);
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// Test that the object address is not in the new space. We cannot
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// set remembered set bits in the new space.
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// Skip barrier if writing a smi.
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ASSERT_EQ(0, kSmiTag);
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test(value, Immediate(kSmiTagMask));
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j(zero, &done);
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if (Serializer::enabled()) {
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// Can't do arithmetic on external references if it might get serialized.
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mov(value, Operand(object));
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and_(value, Heap::NewSpaceMask());
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cmp(Operand(value), Immediate(ExternalReference::new_space_start()));
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j(equal, &done);
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} else {
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// move the value SmiTag into the sign bit
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shl(value, 31);
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// combine the object with value SmiTag
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or_(value, Operand(object));
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// remove the uninteresing bits inside the page
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and_(value, Heap::NewSpaceMask() | (1 << 31));
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// xor has two effects:
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// - if the value was a smi, then the result will be negative
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// - if the object is pointing into new space area the page bits will
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// all be zero
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xor_(value, new_space_start | (1 << 31));
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// Check for both conditions in one branch
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j(less_equal, &done);
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int32_t new_space_start = reinterpret_cast<int32_t>(
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ExternalReference::new_space_start().address());
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lea(value, Operand(object, -new_space_start));
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and_(value, Heap::NewSpaceMask());
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j(equal, &done);
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}
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if ((offset > 0) && (offset < Page::kMaxHeapObjectSize)) {
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// Compute the bit offset in the remembered set, leave it in 'value'.
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mov(value, Operand(object));
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lea(value, Operand(object, offset));
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and_(value, Page::kPageAlignmentMask);
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add(Operand(value), Immediate(offset));
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shr(value, kObjectAlignmentBits);
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shr(value, kPointerSizeLog2);
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// Compute the page address from the heap object pointer, leave it in
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// 'object'.
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@ -192,7 +179,7 @@ void MacroAssembler::RecordWrite(Register object, int offset,
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// to limit code size. We should probably evaluate this decision by
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// measuring the performance of an equivalent implementation using
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// "simpler" instructions
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bts(Operand(object, 0), value);
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bts(Operand(object, Page::kRSetOffset), value);
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} else {
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Register dst = scratch;
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if (offset != 0) {
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@ -201,7 +188,9 @@ void MacroAssembler::RecordWrite(Register object, int offset,
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// array access: calculate the destination address in the same manner as
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// KeyedStoreIC::GenerateGeneric. Multiply a smi by 2 to get an offset
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// into an array of words.
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lea(dst, Operand(object, dst, times_2,
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ASSERT_EQ(1, kSmiTagSize);
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ASSERT_EQ(0, kSmiTag);
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lea(dst, Operand(object, dst, times_half_pointer_size,
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FixedArray::kHeaderSize - kHeapObjectTag));
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}
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// If we are already generating a shared stub, not inlining the
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