A64: Port LSeqStringSetChar optimizations from r16707 and r17521.

src/a64/macro-assembler-a64.cc

Index: src/a64/macro-assembler-a64.cc
diff --git a/src/a64/macro-assembler-a64.cc b/src/a64/macro-assembler-a64.cc
index 7a245ad65ac776592afb2ad7e7e4a33ec65fd6f0..24c7689f7409b794912f8628f60a58ed8514db72 100644
--- a/src/a64/macro-assembler-a64.cc
+++ b/src/a64/macro-assembler-a64.cc
@@ -3621,29 +3621,34 @@ void MacroAssembler::IndexFromHash(Register hash, Register index) {
 
 void MacroAssembler::EmitSeqStringSetCharCheck(Register string,
                                                Register index,
+                                               Register scratch,
                                                uint32_t encoding_mask) {
-  Register scratch = __ Tmp1();
   ASSERT(!AreAliased(string, index, scratch));
 
-  AssertSmi(index);
-
   // Check that string is an object.
   ThrowIfSmi(string, kNonObject);
 
   // Check that string has an appropriate map.
   Ldr(scratch, FieldMemOperand(string, HeapObject::kMapOffset));
   Ldrb(scratch, FieldMemOperand(scratch, Map::kInstanceTypeOffset));
+
   And(scratch, scratch, kStringRepresentationMask | kStringEncodingMask);
   Cmp(scratch, encoding_mask);
   ThrowIf(ne, kUnexpectedStringType);
 
-  // Check that the index points inside the string.
+  // The index is assumed to be untagged coming in, tag it to compare with the
+  // string length without using a temp register, it is restored at the end of
+  // this function.
+  SmiTag(index);
+
   Ldr(scratch, FieldMemOperand(string, String::kLengthOffset));
   Cmp(index, scratch);

[baptiste.afsa1, 2014/02/06 14:52:07]

You can avoid to Tag/Untag the index by untagging scratch here just for the Cmp.
Here it will be something similar to:
  Cmp(index, Operand::UntagSmi(scratch))

It will make use of shifted register and will avoid adding extra instructions.

   ThrowIf(ge, kIndexIsTooLarge);
 
   Cmp(index, Operand(Smi::FromInt(0)));
   ThrowIf(lt, kIndexIsNegative);
+
+  SmiUntag(index);
 }