Implement bitLength intrinsic on x64.

runtime/vm/intrinsifier_x64.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"  // Needed here to get TARGET_ARCH_X64.
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/intrinsifier.h"
 
 #include "vm/assembler.h"
@@ skipping 776 matching lines @@
 // Argument is Smi (receiver).
 void Intrinsifier::Smi_bitNegate(Assembler* assembler) {
   __ movq(RAX, Address(RSP, + 1 * kWordSize));  // Index.
   __ notq(RAX);
   __ andq(RAX, Immediate(~kSmiTagMask));  // Remove inverted smi-tag.
   __ ret();
 }
 
 
 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();
 }
 
 
 void Intrinsifier::Bigint_setNeg(Assembler* assembler) {
   __ movq(RAX, Address(RSP, + 1 * kWordSize));
   __ movq(RCX, Address(RSP, + 2 * kWordSize));
   __ StoreIntoObject(RCX, FieldAddress(RCX, Bigint::neg_offset()), RAX, false);
   __ ret();
 }
 
@@ skipping 1201 matching lines @@
   // Set return value to Isolate::current_tag_.
   __ movq(RAX, Address(RBX, Isolate::current_tag_offset()));
   __ ret();
 }
 
 #undef __
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_X64