Revision: 2608 Author: [email protected] Date: Mon Aug 3 06:17:34 2009 Log: 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 http://code.google.com/p/v8/source/detail?r=2608 Modified: /branches/bleeding_edge/src/ia32/assembler-ia32.h /branches/bleeding_edge/src/ia32/macro-assembler-ia32.cc ======================================= --- /branches/bleeding_edge/src/ia32/assembler-ia32.h Thu Jul 16 21:57:17 2009 +++ /branches/bleeding_edge/src/ia32/assembler-ia32.h Mon Aug 3 06:17:34 2009 @@ -226,7 +226,9 @@ times_1 = 0, times_2 = 1, times_4 = 2, - times_8 = 3 + times_8 = 3, + times_pointer_size = times_4, + times_half_pointer_size = times_2 }; ======================================= --- /branches/bleeding_edge/src/ia32/macro-assembler-ia32.cc Fri Jul 17 05:12:24 2009 +++ /branches/bleeding_edge/src/ia32/macro-assembler-ia32.cc Mon Aug 3 06:17:34 2009 @@ -146,43 +146,30 @@ // for the remembered set bits. Label done; - // This optimization cannot survive serialization and deserialization, - // so we disable as long as serialization can take place. - int32_t new_space_start = - reinterpret_cast<int32_t>(ExternalReference::new_space_start().address()); - if (Serializer::enabled() || new_space_start < 0) { - // Cannot do smart bit-twiddling. Need to do two consecutive checks. - // Check for Smi first. - test(value, Immediate(kSmiTagMask)); - j(zero, &done); - // Test that the object address is not in the new space. We cannot - // set remembered set bits in the new space. + // Skip barrier if writing a smi. + ASSERT_EQ(0, kSmiTag); + test(value, Immediate(kSmiTagMask)); + j(zero, &done); + + if (Serializer::enabled()) { + // Can't do arithmetic on external references if it might get serialized. mov(value, Operand(object)); and_(value, Heap::NewSpaceMask()); cmp(Operand(value), Immediate(ExternalReference::new_space_start())); j(equal, &done); } else { - // move the value SmiTag into the sign bit - shl(value, 31); - // combine the object with value SmiTag - or_(value, Operand(object)); - // remove the uninteresing bits inside the page - and_(value, Heap::NewSpaceMask() | (1 << 31)); - // xor has two effects: - // - if the value was a smi, then the result will be negative - // - if the object is pointing into new space area the page bits will - // all be zero - xor_(value, new_space_start | (1 << 31)); - // Check for both conditions in one branch - j(less_equal, &done); + int32_t new_space_start = reinterpret_cast<int32_t>( + ExternalReference::new_space_start().address()); + lea(value, Operand(object, -new_space_start)); + and_(value, Heap::NewSpaceMask()); + j(equal, &done); } if ((offset > 0) && (offset < Page::kMaxHeapObjectSize)) { // Compute the bit offset in the remembered set, leave it in 'value'. - mov(value, Operand(object)); + lea(value, Operand(object, offset)); and_(value, Page::kPageAlignmentMask); - add(Operand(value), Immediate(offset)); - shr(value, kObjectAlignmentBits); + shr(value, kPointerSizeLog2); // Compute the page address from the heap object pointer, leave it in // 'object'. @@ -192,7 +179,7 @@ // to limit code size. We should probably evaluate this decision by // measuring the performance of an equivalent implementation using // "simpler" instructions - bts(Operand(object, 0), value); + bts(Operand(object, Page::kRSetOffset), value); } else { Register dst = scratch; if (offset != 0) { @@ -201,7 +188,9 @@ // array access: calculate the destination address in the same manner as // KeyedStoreIC::GenerateGeneric. Multiply a smi by 2 to get an offset // into an array of words. - lea(dst, Operand(object, dst, times_2, + ASSERT_EQ(1, kSmiTagSize); + ASSERT_EQ(0, kSmiTag); + lea(dst, Operand(object, dst, times_half_pointer_size, FixedArray::kHeaderSize - kHeapObjectTag)); } // If we are already generating a shared stub, not inlining the --~--~---------~--~----~------------~-------~--~----~ v8-dev mailing list [email protected] http://groups.google.com/group/v8-dev -~----------~----~----~----~------~----~------~--~---
