Improve bit tests on IA32.

Bit tests with a memory operand should be avoided on Intel processors.
On Atom bts with a memory operand has 10~11 cycles latency and on Core 2
it has 5~6 cycles latency, and also produces multiple uops.
If the bit base operand is a register the latency is 1 cycle instead.
This CL improves several cases from Kraken 1.1 benchmark on Atom. For example,
12% for imaging-desaturate, 8% for audio-fft, and 5% for audio-beat-detection.

Review URL: http://codereview.chromium.org//7048003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7987 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/ia32/macro-assembler-ia32.cc

@@ -73,7 +73,12 @@ void MacroAssembler::RecordWriteHelper(Register object,
   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);
 }