A64: Port LSeqStringSetChar optimizations from r16707 and r17521.

src/a64/macro-assembler-a64.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
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 // modification, are permitted provided that the following conditions are
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 //
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@@ skipping 3603 matching lines @@
   // We want the smi-tagged index in key.  kArrayIndexValueMask has zeros in
   // the low kHashShift bits.
   STATIC_ASSERT(kSmiTag == 0);
   Ubfx(hash, hash, String::kHashShift, String::kArrayIndexValueBits);
   SmiTag(index, hash);
 }
 
 
 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);
 }
 
 
 void MacroAssembler::CheckAccessGlobalProxy(Register holder_reg,
                                             Register scratch,
                                             Label* miss) {
   // TODO(jbramley): Sort out the uses of Tmp0() and Tmp1() in this function.
   // The ARM version takes two scratch registers, and that should be enough for
   // all of the checks.
 
@@ skipping 1173 matching lines @@
   }
 }
 
 
 #undef __
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_A64