Esoteric bit operations.

runtime/vm/intrinsifier_ia32.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.
 //
 // The intrinsic code below is executed before a method has built its frame.
 // The return address is on the stack and the arguments below it.
 // Registers EDX (arguments descriptor) and ECX (function) must be preserved.
 // Each intrinsification method returns true if the corresponding
 // Dart method was intrinsified.
 
@@ skipping 1069 matching lines @@
 
 // Argument is Smi (receiver).
 void Intrinsifier::Smi_bitNegate(Assembler* assembler) {
   __ movl(EAX, Address(ESP, + 1 * kWordSize));  // Index.
   __ notl(EAX);
   __ andl(EAX, Immediate(~kSmiTagMask));  // Remove inverted smi-tag.
   __ ret();
 }
 
 
+void Intrinsifier::Smi_bitLength(Assembler* assembler) {

[srdjan, 2013/09/06 15:35:34]

How does optimized code compare, is intrinsified method needed at all?

[sra1, 2013/09/06 22:11:57]

It appears to be worthwhile.
The release build is 6x faster than an instrinsifier that always falls through.
It is also about 1.8x faster than implementing _Smi.bitLength as a sequence of
bit operations and tests in Dart.

+  ASSERT(kSmiTagShift == 1);
+  __ movl(EAX, Address(ESP, + 1 * kWordSize));  // Index.
+  // XOR with sign bit to complement bits if value is negative.
+  __ movl(ECX, EAX);
+  __ sarl(ECX, Immediate(31));  // All 0 or all 1.
+  __ xorl(EAX, ECX);
+  // 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.
+  __ orl(EAX, Immediate(kSmiTagMask));
+  __ bsrl(EAX, EAX);
+  __ SmiTag(EAX);
+  __ ret();
+}
+
+
 // Check if the last argument is a double, jump to label 'is_smi' if smi
 // (easy to convert to double), otherwise jump to label 'not_double_smi',
 // Returns the last argument in EAX.
 static void TestLastArgumentIsDouble(Assembler* assembler,
                                      Label* is_smi,
                                      Label* not_double_smi) {
   __ movl(EAX, Address(ESP, + 1 * kWordSize));
   __ testl(EAX, Immediate(kSmiTagMask));
   __ j(ZERO, is_smi, Assembler::kNearJump);  // Jump if Smi.
   __ CompareClassId(EAX, kDoubleCid, EBX);
@@ skipping 617 matching lines @@
   __ Bind(&ok);
   __ ret();
 
   __ Bind(&fall_through);
 }
 
 #undef __
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_IA32