Implement bitLength intrinsic on x64.

runtime/vm/intrinsifier_x64.cc

Index: runtime/vm/intrinsifier_x64.cc
===================================================================
--- runtime/vm/intrinsifier_x64.cc	(revision 43118)
+++ runtime/vm/intrinsifier_x64.cc	(working copy)
@@ -794,7 +794,18 @@
 
 
 void Intrinsifier::Smi_bitLength(Assembler* assembler) {
-  // TODO(sra): Implement using bsrq.
+  ASSERT(kSmiTagShift == 1);
+  __ movq(RAX, Address(RSP, + 1 * kWordSize));  // Index.
+  // XOR with sign bit to complement bits if value is negative.
+  __ movq(RCX, RAX);
+  __ sarq(RCX, Immediate(63));  // All 0 or all 1.

[Lasse Reichstein Nielsen, 2015/01/25 11:26:53]

If you shift RAX instead of RCX here, would the dependecy depth drop by one?

[srdjan, 2015/01/26 18:43:18]

Thanks for the comment. In theory yes, but with Intel's register renaming, it
should not matter, AFAIK.
Will change it once I tackle MIPS and ARM implementations as well.

+  __ xorq(RAX, RCX);
+  // BSR does not write the destination register if source is zero.  Put a 1 in
+  // the Smi tag bit to ensure BSR writes to destination register.
+  __ orq(RAX, Immediate(kSmiTagMask));
+  __ bsrq(RAX, RAX);
+  __ SmiTag(RAX);
+  __ ret();
 }