Improve bit tests on IA32.

src/ia32/macro-assembler-ia32.cc

 // Copyright 2011 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
@@ skipping 55 matching lines @@
   // Compute the page start address from the heap object pointer, and reuse
   // the 'object' register for it.
   and_(object, ~Page::kPageAlignmentMask);
 
   // Compute number of region covering addr. See Page::GetRegionNumberForAddress
   // method for more details.
   and_(addr, Page::kPageAlignmentMask);
   shr(addr, Page::kRegionSizeLog2);
 
   // Set dirty mark for region.
-  bts(Operand(object, Page::kDirtyFlagOffset), addr);
+  // Bit tests with a memory operand should be avoided on Intel processors,
+  // as they usually have long latency and multiple uops. We load the bit base
+  // operand to a register at first and store it back after bit set.
+  mov(scratch, Operand(object, Page::kDirtyFlagOffset));
+  bts(Operand(scratch), addr);
+  mov(Operand(object, Page::kDirtyFlagOffset), scratch);
 }
 
 
 void MacroAssembler::ClampDoubleToUint8(XMMRegister input_reg,
                                         XMMRegister scratch_reg,
                                         Register result_reg) {
   Label done;
   ExternalReference zero_ref = ExternalReference::address_of_zero();
   movdbl(scratch_reg, Operand::StaticVariable(zero_ref));
   Set(result_reg, Immediate(0));
@@ skipping 1996 matching lines @@
 
   // Check that the code was patched as expected.
   ASSERT(masm_.pc_ == address_ + size_);
   ASSERT(masm_.reloc_info_writer.pos() == address_ + size_ + Assembler::kGap);
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_IA32