Esoteric bit operations.

runtime/vm/intrinsifier_ia32.cc

Index: runtime/vm/intrinsifier_ia32.cc
diff --git a/runtime/vm/intrinsifier_ia32.cc b/runtime/vm/intrinsifier_ia32.cc
index 251ba782474e3e31120821250e107478322d08db..5f59d6facb17c15d5ad2330429dd65b6f8d91732 100644
--- a/runtime/vm/intrinsifier_ia32.cc
+++ b/runtime/vm/intrinsifier_ia32.cc
@@ -1087,6 +1087,22 @@ void Intrinsifier::Smi_bitNegate(Assembler* assembler) {
 }
 
 
+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.